I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --

I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.

So --
My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.

But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?

Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?

THANKS

On Tue, 4 Jan 2022 07:44:21 -0800 (PST), mkr5000 <mikerbgr@gmail.com>
wrote:

>I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
>
>I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
>
>So --
>My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
>
>But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
>have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
>
>Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
>done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>
>THANKS

You could sense the AC line current with a split-core CT.

https://www.digikey.com/en/products/detail/seeed-technology-co-ltd/101990059/5775191

Slit open the AC line cord outer jacket and slip the CT over the hot
or neutral wire. No need to expose any conductors. Or hack an
extension cord to do the same thing.

Scope the output tx/rx and decide what to do next. That one makes 0.1
volt per amp so there will be a pretty healthy signal to work with.

You could build a box with a plug and an outlet, in which case you
could slip an AC wire through a small toroidal inductor of something.
Similar idea. You could get a lot of signal voltage that way, enough
to drive an SSR directly with no local power supply.

An RF detector shouldn't be hard either.

--

I yam what I yam - Popeye

On 04/01/2022 15:44, mkr5000 wrote:

<snipped>
>
> Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>
> THANKS

A reasonably sensitive reed switch inside a solenoid with a few tens of
turns should do it. This sort of thing...

https://www.electroschematics.com/reed-switch-as-a-current-monitor/

--
Cheers
Clive

On 04/01/2022 3:44 pm, mkr5000 wrote:
> I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
>
> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
>
> So --
> My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
>
> But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
>
> Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>
> THANKS

Couple things occur to me:

If the mode is constant envelope (like RTTY or FM) then the current draw
or RF output will be kinda constant during the over, if SSB or CW then
the detectable signal will fluctuate with modulation.

12V 100W is 8.3 amps, assuming 100% efficiency but I reckon most rigs
are 50% or so which means the RX idle current of <2A should be very easy
to distinguish from 15 amps TX. Does your power supply have an ammeter
display, many do and it can be instructive?

Reed contacts can be used to sense cable current (and much smaller than
what you are considering) but hysteresis effect is very large and having
closed on TX current may not release if the RX current is not vastly lower.

Sensing voltage drop in the power cable is possible, sketch attached
shows one way that has been used. (LM301 is great at sensing right up to
positive rail, uA741 less so but usable. Or reconfigure to use later
parts sensing in the negative return). Use ohms law and knowledge of the
power cable wiring gauge to determine how far apart the sense pickoff
taps should be, I reckon 5 milli-ohms resistance may be enough?

<https://www.dropbox.com/s/c7kqeqkaxux75lz/current_sense_rig.pdf?raw=1>

piglet

On Tuesday, January 4, 2022 at 11:44:14 AM UTC-5, erichp...@hotmail.com wrote:
> On 04/01/2022 3:44 pm, mkr5000 wrote:
> > I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
> >
> > I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
> >
> > So --
> > My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
> >
> > But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> > have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
> >
> > Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> > done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
> >
> > THANKS
> Couple things occur to me:
>
> If the mode is constant envelope (like RTTY or FM) then the current draw
> or RF output will be kinda constant during the over, if SSB or CW then
> the detectable signal will fluctuate with modulation.
>
> 12V 100W is 8.3 amps, assuming 100% efficiency but I reckon most rigs
> are 50% or so which means the RX idle current of <2A should be very easy
> to distinguish from 15 amps TX. Does your power supply have an ammeter
> display, many do and it can be instructive?
>
> Reed contacts can be used to sense cable current (and much smaller than
> what you are considering) but hysteresis effect is very large and having
> closed on TX current may not release if the RX current is not vastly lower.
>
> Sensing voltage drop in the power cable is possible, sketch attached
> shows one way that has been used. (LM301 is great at sensing right up to
> positive rail, uA741 less so but usable. Or reconfigure to use later
> parts sensing in the negative return). Use ohms law and knowledge of the
> power cable wiring gauge to determine how far apart the sense pickoff
> taps should be, I reckon 5 milli-ohms resistance may be enough?
>
>
> <https://www.dropbox.com/s/c7kqeqkaxux75lz/current_sense_rig.pdf?raw=1>
>
> piglet
Great !
At least I know I'm on the right track. I like starting with the reed switch idea and if I have to work up from there. The switch would have dry contacts and be passive which is pretty cool. Inductance has never been one of my strong points so what is a good place to start for a coil. Thing is, I really need something that is completely passive and works on the AC field alone.

On Tuesday, January 4, 2022 at 12:04:39 PM UTC-5, mkr5000 wrote:
> On Tuesday, January 4, 2022 at 11:44:14 AM UTC-5, erichp...@hotmail.com wrote:
> > On 04/01/2022 3:44 pm, mkr5000 wrote:
> > > I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
> > >
> > > I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either..
> > >
> > > So --
> > > My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output).. Plan on experimenting with it -- don't have the parts yet.
> > >
> > > But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> > > have never toyed with these sensors or instrumentation amplifiers etc.. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
> > >
> > > Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> > > done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore).. Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
> > >
> > > THANKS
> > Couple things occur to me:
> >
> > If the mode is constant envelope (like RTTY or FM) then the current draw
> > or RF output will be kinda constant during the over, if SSB or CW then
> > the detectable signal will fluctuate with modulation.
> >
> > 12V 100W is 8.3 amps, assuming 100% efficiency but I reckon most rigs
> > are 50% or so which means the RX idle current of <2A should be very easy
> > to distinguish from 15 amps TX. Does your power supply have an ammeter
> > display, many do and it can be instructive?
> >
> > Reed contacts can be used to sense cable current (and much smaller than
> > what you are considering) but hysteresis effect is very large and having
> > closed on TX current may not release if the RX current is not vastly lower.
> >
> > Sensing voltage drop in the power cable is possible, sketch attached
> > shows one way that has been used. (LM301 is great at sensing right up to
> > positive rail, uA741 less so but usable. Or reconfigure to use later
> > parts sensing in the negative return). Use ohms law and knowledge of the
> > power cable wiring gauge to determine how far apart the sense pickoff
> > taps should be, I reckon 5 milli-ohms resistance may be enough?
> >
> >
> > <https://www.dropbox.com/s/c7kqeqkaxux75lz/current_sense_rig.pdf?raw=1>
> >
> > piglet
> Great !
> At least I know I'm on the right track. I like starting with the reed switch idea and if I have to work up from there. The switch would have dry contacts and be passive which is pretty cool. Inductance has never been one of my strong points so what is a good place to start for a coil. Thing is, I really need something that is completely passive and works on the AC field alone.
I don't want to do anything "in circuit".

On 04/01/2022 17:06, mkr5000 wrote:
> On Tuesday, January 4, 2022 at 12:04:39 PM UTC-5, mkr5000 wrote:
>> On Tuesday, January 4, 2022 at 11:44:14 AM UTC-5, erichp...@hotmail.com wrote:
>>> On 04/01/2022 3:44 pm, mkr5000 wrote:
>>>> I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
>>>>
>>>> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
>>>>
>>>> So --
>>>> My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
>>>>
>>>> But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
>>>> have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
>>>>
>>>> Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
>>>> done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>>>>
>>>> THANKS
>>> Couple things occur to me:
>>>
>>> If the mode is constant envelope (like RTTY or FM) then the current draw
>>> or RF output will be kinda constant during the over, if SSB or CW then
>>> the detectable signal will fluctuate with modulation.
>>>
>>> 12V 100W is 8.3 amps, assuming 100% efficiency but I reckon most rigs
>>> are 50% or so which means the RX idle current of <2A should be very easy
>>> to distinguish from 15 amps TX. Does your power supply have an ammeter
>>> display, many do and it can be instructive?
>>>
>>> Reed contacts can be used to sense cable current (and much smaller than
>>> what you are considering) but hysteresis effect is very large and having
>>> closed on TX current may not release if the RX current is not vastly lower.
>>>
>>> Sensing voltage drop in the power cable is possible, sketch attached
>>> shows one way that has been used. (LM301 is great at sensing right up to
>>> positive rail, uA741 less so but usable. Or reconfigure to use later
>>> parts sensing in the negative return). Use ohms law and knowledge of the
>>> power cable wiring gauge to determine how far apart the sense pickoff
>>> taps should be, I reckon 5 milli-ohms resistance may be enough?
>>>
>>>
>>> <https://www.dropbox.com/s/c7kqeqkaxux75lz/current_sense_rig.pdf?raw=1>
>>>
>>> piglet
>> Great !
>> At least I know I'm on the right track. I like starting with the reed switch idea and if I have to work up from there. The switch would have dry contacts and be passive which is pretty cool. Inductance has never been one of my strong points so what is a good place to start for a coil. Thing is, I really need something that is completely passive and works on the AC field alone.
> I don't want to do anything "in circuit".
>

No problem. Look for reed switches with a long body - so you can wind
enough turns of wire onto it - and with lower Ampere-Turns values (fewer
turns of wire will be needed to activate).

No real inductance involved here, just wind from 1 to 20 turns (wild
guess range) of wire around the reed.

Some gottchas to beware of: the reeds could react fast enough to buzz at
100/120Hz, the power supply input current could be so spiky even at low
load that getting reliable discrimination needs experimental fine
tuning, reed contacts love to weld themselves so do not shunt with a
capacitor (unless via a series resistor) and use well within ratings.

piglet

mkr5000 wrote:
> I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
>
> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
>
> So --
> My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
>
> But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
>
> Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>
> THANKS
>

If you have a SWR bridge in line, you could tap off the forward power
detector.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Tuesday, 4 January 2022 at 17:23:20 UTC+1, jla...@highlandsniptechnology..com wrote:
> On Tue, 4 Jan 2022 07:44:21 -0800 (PST), mkr5000 <mike...@gmail.com>
> wrote:
> >I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
> >
> >I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
> >
> >So --
> >My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
> >
> >But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> >have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
> >
> >Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> >done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
> >
> >THANKS
> You could sense the AC line current with a split-core CT.
>
> https://www.digikey.com/en/products/detail/seeed-technology-co-ltd/101990059/5775191
>
> Slit open the AC line cord outer jacket and slip the CT over the hot
> or neutral wire. No need to expose any conductors. Or hack an
> extension cord to do the same thing.
>
> Scope the output tx/rx and decide what to do next. That one makes 0.1
> volt per amp so there will be a pretty healthy signal to work with.
>
> You could build a box with a plug and an outlet, in which case you
> could slip an AC wire through a small toroidal inductor of something.
> Similar idea. You could get a lot of signal voltage that way, enough
> to drive an SSR directly with no local power supply.
>
> An RF detector shouldn't be hard either.
>
>
>
> --
>
> I yam what I yam - Popeye
When using a reed relay to react on AC current, adding 2 shorts (a small length of copper tube over the reed relay) on both sides of the coil will generate a more stable field. The same mechanism can be seen in AC relays, where part of the core is surrounded by a copper short.
BR, Wim

On Tuesday, January 4, 2022 at 2:52:43 PM UTC-5, wim...@gmail.com wrote:
> On Tuesday, 4 January 2022 at 17:23:20 UTC+1, jla...@highlandsniptechnology.com wrote:
> > On Tue, 4 Jan 2022 07:44:21 -0800 (PST), mkr5000 <mike...@gmail.com>
> > wrote:
> > >I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
> > >
> > >I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air.. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
> > >
> > >So --
> > >My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
> > >
> > >But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> > >have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
> > >
> > >Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> > >done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
> > >
> > >THANKS
> > You could sense the AC line current with a split-core CT.
> >
> > https://www.digikey.com/en/products/detail/seeed-technology-co-ltd/101990059/5775191
> >
> > Slit open the AC line cord outer jacket and slip the CT over the hot
> > or neutral wire. No need to expose any conductors. Or hack an
> > extension cord to do the same thing.
> >
> > Scope the output tx/rx and decide what to do next. That one makes 0.1
> > volt per amp so there will be a pretty healthy signal to work with.
> >
> > You could build a box with a plug and an outlet, in which case you
> > could slip an AC wire through a small toroidal inductor of something.
> > Similar idea. You could get a lot of signal voltage that way, enough
> > to drive an SSR directly with no local power supply.
> >
> > An RF detector shouldn't be hard either.
> >
> >
> >
> > --
> >
> > I yam what I yam - Popeye
> When using a reed relay to react on AC current, adding 2 shorts (a small length of copper tube over the reed relay) on both sides of the coil will generate a more stable field. The same mechanism can be seen in AC relays, where part of the core is surrounded by a copper short.
> BR, Wim
Ok. Will try the reed first but also will get the parts for the split coil sensor I've seen. I like the idea of a snap on module and any circuit could also be powered from +5v I would have available. Don't like the cost of those sensors though.

Let me ask this --
put the reed switch inside a smaller lengthwise piece of copper tubing, maybe a touch of silicone also. copper would help keep any chance of breaking the glass down also. and then wrap that assembly on the cord with maybe some 20 g solid hookup wire. 12 -15 turns or so?

On Tuesday, January 4, 2022 at 3:20:02 PM UTC-5, mkr5000 wrote:
> On Tuesday, January 4, 2022 at 2:52:43 PM UTC-5, wim...@gmail.com wrote:
> > On Tuesday, 4 January 2022 at 17:23:20 UTC+1, jla...@highlandsniptechnology.com wrote:
> > > On Tue, 4 Jan 2022 07:44:21 -0800 (PST), mkr5000 <mike...@gmail.com>
> > > wrote:
> > > >I'm going to just come right out and ask for some assistance on this.. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
> > > >
> > > >I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
> > > >
> > > >So --
> > > >My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
> > > >
> > > >But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> > > >have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
> > > >
> > > >Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> > > >done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
> > > >
> > > >THANKS
> > > You could sense the AC line current with a split-core CT.
> > >
> > > https://www.digikey.com/en/products/detail/seeed-technology-co-ltd/101990059/5775191
> > >
> > > Slit open the AC line cord outer jacket and slip the CT over the hot
> > > or neutral wire. No need to expose any conductors. Or hack an
> > > extension cord to do the same thing.
> > >
> > > Scope the output tx/rx and decide what to do next. That one makes 0.1
> > > volt per amp so there will be a pretty healthy signal to work with.
> > >
> > > You could build a box with a plug and an outlet, in which case you
> > > could slip an AC wire through a small toroidal inductor of something.
> > > Similar idea. You could get a lot of signal voltage that way, enough
> > > to drive an SSR directly with no local power supply.
> > >
> > > An RF detector shouldn't be hard either.
> > >
> > >
> > >
> > > --
> > >
> > > I yam what I yam - Popeye
> > When using a reed relay to react on AC current, adding 2 shorts (a small length of copper tube over the reed relay) on both sides of the coil will generate a more stable field. The same mechanism can be seen in AC relays, where part of the core is surrounded by a copper short.
> > BR, Wim
> Ok. Will try the reed first but also will get the parts for the split coil sensor I've seen. I like the idea of a snap on module and any circuit could also be powered from +5v I would have available. Don't like the cost of those sensors though.
>
> Let me ask this --
> put the reed switch inside a smaller lengthwise piece of copper tubing, maybe a touch of silicone also. copper would help keep any chance of breaking the glass down also. and then wrap that assembly on the cord with maybe some 20 g solid hookup wire. 12 -15 turns or so?
and the reed switch AT rating ? the lowest I see is 6 to 10 on digikey -- is that sensitive enough?

In article <0c9f53fe-89c9-4340-84dc-732bf116163fn@googlegroups.com>,
mikerbgr@gmail.com says...
>
> > >
> > > I yam what I yam - Popeye
> > When using a reed relay to react on AC current, adding 2 shorts (a small length of copper tube over the reed relay) on both sides of the coil will generate a more stable field. The same mechanism can be seen in AC relays, where part of the core is surrounded by a copper short.
> > BR, Wim
> Ok. Will try the reed first but also will get the parts for the split coil sensor I've seen. I like the idea of a snap on module and any circuit could also be powered from +5v I would have available. Don't like the cost of those sensors though.
>
> Let me ask this --
> put the reed switch inside a smaller lengthwise piece of copper tubing, maybe a touch of silicone also. copper would help keep any chance of breaking the glass down also. and then wrap that assembly on the cord with maybe some 20 g solid hookup wire. 12 -15 turns or so?

Try using a Hall effect device. Maybe something with an Arduino like
this:

https://create.arduino.cc/projecthub/SurtrTech/interfacing-hall-effect-
sensor-with-arduino-ee3bbe

mkr5000 wrote:
==============
>
> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the
> output of an amateur radio xmtr when it's on the air.
>

** For what purpose ??

Any any practical design, the actual purpose is * everything *.
Why omit it from your post?

...... Phil

On 1/4/22 10:44 AM, mkr5000 wrote:
> I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
>
> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
>
> So --
> My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
>
> But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
>
> Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>
> THANKS
>

Asking for a "relay contact" seems to imply OP wants a binary OK/over
current detection, there's a simple over-current detect for a DC bus on
Jim Thompson's legacy site:

<https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/CurrentSense.pdf>

It looks straightforward enough to adapt to higher voltages and currents
and also optically isolate if that's what's desired.

On Tuesday, January 4, 2022 at 6:51:51 PM UTC-5, bitrex wrote:
> On 1/4/22 10:44 AM, mkr5000 wrote:
> > I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
> >
> > I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
> >
> > So --
> > My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
> >
> > But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
> > have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
> >
> > Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
> > done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
> >
> > THANKS
> >
> Asking for a "relay contact" seems to imply OP wants a binary OK/over
> current detection, there's a simple over-current detect for a DC bus on
> Jim Thompson's legacy site:
>
> <https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/CurrentSense.pdf>
>
> It looks straightforward enough to adapt to higher voltages and currents
> and also optically isolate if that's what's desired.

well I guess I could have stated it this way --
output of an amateur radio xmtr to KNOW when it's on the air.
how bout that as a purpose?

or is it? ....hmm

On 1/4/22 7:08 PM, mkr5000 wrote:
> On Tuesday, January 4, 2022 at 6:51:51 PM UTC-5, bitrex wrote:
>> On 1/4/22 10:44 AM, mkr5000 wrote:
>>> I'm going to just come right out and ask for some assistance on this. There are still "areas" in electronics I have never had any experience "tinkering" with in all my years as an "advanced" hobbyist. You guys are walking encyclopedias on so much. This is something I've never messed with --
>>>
>>> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the output of an amateur radio xmtr when it's on the air. Simple enough and would be easy to get in there and look for the PTT contacts and diy something but I don't want to do that. Most (if not all) of these xcvrs don't have a self contained relay to perform the function either.
>>>
>>> So --
>>> My immediate first thought was to sense the RF with a FSM and I may still breadboard something but I'm guessing as well shielded most of these new radios are, it's the actual antenna rf that would probably be the best source and I had better be pretty darn close to it. (100 watts or so output). Plan on experimenting with it -- don't have the parts yet.
>>>
>>> But then it also occured to me that I may be able to make something that would just sense the current being drawn from the power supply and have looked into some of these split coil sensors etc (and asc712?). After all, in receive mode current draw from the supply would be next to nothing and transmit for 100 watts would go up to .83 amps or so. Pat myself on the back anyway for the creative thinking I guess :) But -- like I say, I
>>> have never toyed with these sensors or instrumentation amplifiers etc. I see some off the shelf split coil ones (or maybe something diy?) and it looks like an AD623 is a good choice for amplifying millivolt signals? Anyway -- I just wondered what advice or ideas you guys may have and if it's possible, what the simplest lowest parts count approach would be?
>>>
>>> Another thing I though of this morning and another thing I never messed with much are magnetic reed switches. I kinda doubt this can be
>>> done but wonder if they make them sensitive enough to somehow react to that current draw from the supply with the right type of coil. By the way, the power supply of course has the AC mains wire but also the DC output on another cable. (Most ham xcvrs don't have their own power supply anymore). Long post I know but hopefully can get some input. Maybe .83 amps is too negligible?
>>>
>>> THANKS
>>>
>> Asking for a "relay contact" seems to imply OP wants a binary OK/over
>> current detection, there's a simple over-current detect for a DC bus on
>> Jim Thompson's legacy site:
>>
>> <https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/CurrentSense.pdf>
>>
>> It looks straightforward enough to adapt to higher voltages and currents
>> and also optically isolate if that's what's desired.
>
> well I guess I could have stated it this way --
> output of an amateur radio xmtr to KNOW when it's on the air.
> how bout that as a purpose?

Well if it's drawing "next to nothing" power from the DC supply when not
transmitting and then a quantifiable more than next to nothing whenever
it's transmitting you could try setting the trip point somewhere between
next to nothing and more than next to nothing and see if that does ya I
think, some hysteresis would help and that topology has some.

> or is it? ....hmm
>

Is it? Am I? Should I?

<https://youtu.be/vLdIo2fEX1A?t=1812>

Mark Kholber = mkr5000 wrote:
=============================
>
> well I guess I could have stated it this way --
> output of an amateur radio xmtr to KNOW when it's on the air.
> how bout that as a purpose?

** For what purpose ??

Any any practical design, the actual purpose is * everything *.
Why omit it from your post? Over and over.

Think you are lying about something.
Time to own up with the full, unabridged story.

What model ham radio?
Whose ham radio ? Yours or not.
Why is there NO indication right now ?

...... Phil

On 05.01.22 2:38, Phil Allison wrote:
> Mark Kholber = mkr5000 wrote:
> =============================
>>
>> well I guess I could have stated it this way --
>> output of an amateur radio xmtr to KNOW when it's on the air.
>> how bout that as a purpose?
>
> ** For what purpose ??
>
> Any any practical design, the actual purpose is * everything *.
> Why omit it from your post? Over and over.
>
> Think you are lying about something.
> Time to own up with the full, unabridged story.
>
> What model ham radio?
> Whose ham radio ? Yours or not.
> Why is there NO indication right now ?
>
>
> ..... Phil
>
Or, why ask for a solution for igniting an explosion device.............

On 1/4/22 9:00 PM, Sjouke Burry wrote:
> On 05.01.22 2:38, Phil Allison wrote:
>> Mark Kholber  =  mkr5000 wrote:
>> =============================
>>>
>>> well I guess I could have stated it this way --
>>> output of an amateur radio xmtr to KNOW when it's on the air.
>>> how bout that as a purpose?
>>
>>   ** For what purpose ??
>>
>> Any any practical design, the actual purpose is * everything *.
>> Why omit it from your post?  Over and over.
>>
>> Think you are lying about something.
>> Time to own up with the full, unabridged story.
>>
>> What model ham radio?
>> Whose ham radio ?   Yours or not.
>> Why is there NO indication right now ?
>>
>>
>> .....   Phil
>>
> Or, why ask for a solution for igniting an explosion device.............

Acme alarm clock mounted to a bundle of dynamite not good enough for
people these days? It was good enough for Wile E. Coyote.

On a sunny day (Tue, 4 Jan 2022 14:43:59 -0800 (PST)) it happened Phil Allison
<pallison49@gmail.com> wrote in
<66ad22c9-e1fb-44de-8bec-630abd8d155bn@googlegroups.com>:

> mkr5000 wrote:
>=============>
>
>> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the
>> output of an amateur radio xmtr when it's on the air.
>>
>
>** For what purpose ??
>
>Any any practical design, the actual purpose is * everything *.
>Why omit it from your post?

Most ham transmitters have a mike with a push button
Hang an optocoupler or relay on that contact and be done with it
It is just babble talk he does?

Jan Panteltje wrote:
>
> >> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the
> >> output of an amateur radio xmtr when it's on the air.
> >>
> >
> >** For what purpose ??
> >
> >Any any practical design, the actual purpose is * everything *.
> >Why omit it from your post?
> Most ham transmitters have a mike with a push button
> Hang an optocoupler or relay on that contact and be done with it
> It is just babble talk he does?

** Yep.

Hard to believe any ham rig has no visible indication of being in Tx mode.
I smell fish.

...... Phil

On a sunny day (Tue, 4 Jan 2022 22:15:45 -0800 (PST)) it happened Phil Allison
<pallison49@gmail.com> wrote in
<b04d1234-ec02-451d-9b1b-27ad2a41b2e6n@googlegroups.com>:

>Jan Panteltje wrote:
>>
>> >> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the
>> >> output of an amateur radio xmtr when it's on the air.
>> >>
>> >
>> >** For what purpose ??
>> >
>> >Any any practical design, the actual purpose is * everything *.
>> >Why omit it from your post?
>> Most ham transmitters have a mike with a push button
>> Hang an optocoupler or relay on that contact and be done with it
>> It is just babble talk he does?
>
>** Yep.
>
> Hard to believe any ham rig has no visible indication of being in Tx mode.
> I smell fish.

My first big ham radio transmitter was about 250 W or more (tubes) and I could walk around in the garden with a neon light
that would light up when transmitting.
Now there is your indicator!
Of course I had no license back then...
But made a QSO anyways,

On 1/5/22 1:01 AM, Jan Panteltje wrote:
> On a sunny day (Tue, 4 Jan 2022 14:43:59 -0800 (PST)) it happened Phil Allison
> <pallison49@gmail.com> wrote in
> <66ad22c9-e1fb-44de-8bec-630abd8d155bn@googlegroups.com>:
>
>> mkr5000 wrote:
>> =============>
>>
>>> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the
>>> output of an amateur radio xmtr when it's on the air.
>>>
>>
>> ** For what purpose ??
>>
>> Any any practical design, the actual purpose is * everything *.
>> Why omit it from your post?
>
> Most ham transmitters have a mike with a push button
> Hang an optocoupler or relay on that contact and be done with it
> It is just babble talk he does?
>

Sounds great if it's for something like triggering a babble-timer or a
fancy neon "ON AIR" sign to impress the fam. God help him if it's for
something related to the safety of himself or others. It is unfortunate
he didn't specify cuz maybe he run off to build a THERAC 25 from ham
radio parts.

On a sunny day (Wed, 5 Jan 2022 01:50:59 -0500) it happened bitrex
<user@example.net> wrote in <ndbBJ.74583$cW6.56727@fx08.iad>:

>On 1/5/22 1:01 AM, Jan Panteltje wrote:
>> On a sunny day (Tue, 4 Jan 2022 14:43:59 -0800 (PST)) it happened Phil Allison
>> <pallison49@gmail.com> wrote in
>> <66ad22c9-e1fb-44de-8bec-630abd8d155bn@googlegroups.com>:
>>
>>> mkr5000 wrote:
>>> =============>
>>>
>>>> I've been wanting to somehow get a relay contact or a simple opto npn output to react to the
>>>> output of an amateur radio xmtr when it's on the air.
>>>>
>>>
>>> ** For what purpose ??
>>>
>>> Any any practical design, the actual purpose is * everything *.
>>> Why omit it from your post?
>>
>> Most ham transmitters have a mike with a push button
>> Hang an optocoupler or relay on that contact and be done with it
>> It is just babble talk he does?
>>
>
>Sounds great if it's for something like triggering a babble-timer or a
>fancy neon "ON AIR" sign to impress the fam. God help him if it's for
>something related to the safety of himself or others. It is unfortunate
>he didn't specify cuz maybe he run off to build a THERAC 25 from ham
>radio parts.

Yea, whats-it-called Darwin Awards.
To add to the not so simple solutions I once made an AC current meter using an
old audio tapehead:
http://panteltje.com/pub/play_back_head_current_sensor_img_1153.jpg
opamp... gives mains AC current.
mains wire glued in front of the headgap.

Jan Panteltje wrote:
================
> > >
> >Sounds great if it's for something like triggering a babble-timer or a
> >fancy neon "ON AIR" sign to impress the fam. God help him if it's for
> >something related to the safety of himself or others. It is unfortunate
> >he didn't specify cuz maybe he run off to build a THERAC 25 from ham
> >radio parts.

** See: https://en.wikipedia.org/wiki/Therac-25

>
> Yea, whats-it-called Darwin Awards.
> To add to the not so simple solutions I once made an AC current meter using an
> old audio tapehead:
> http://panteltje.com/pub/play_back_head_current_sensor_img_1153.jpg
> opamp... gives mains AC current.
> mains wire glued in front of the headgap.
>

** If you just added a second tape head - a few inches from the first -
you could tell which way the electrons were moving.

..... Phil