Cobra 25ltd and 29Ltd and other 4 pin mic radios

[dss56]

Has someone ever added an external relay to a 4 pin radio and activated by the mic?
Need to control something with an external relay keyed by the mic.

This will be in a small plastic box with the radio mic wiring and was wondering if any of the pins can control the relay.
I know pin 1 is ground >pin 2 is audio> pin 3 is transmit >and pin 4 is receive.

If you was able to do this how and what pins did you use?

thanks

[Grinder74]

You may need to build a keying circuit if you use the mic's wiring.

[dss56]

Yes i know that and that is what im asking if someone has done this or made one.
It has to be a simple build just need one that will work

thanks

[MDYoungblood]

It would be easier to make a keying circuit inside the radio, then the relay has a power source and some shielding from RF, then just add a RCA jack on the back to connect the external devise.

3's

Greg

[dss56]

Thanks but trying to make so I can swap to another 4 pin radio using a 12 volt external power supply. so when I key the relay will either give me normally open or normally closed option.

thanks

[MDYoungblood]

I think it could be a pretty simple circuit, it will need power (probably 12dcv) from an external source. I would use "Normally Open" relay. Now is this to key a grounding circuit or a powered circuit on the devise you want to key up? If so it is a little more involved, the circuit will need some diodes inline so current goes in one direction, might add some ferrite beads for RF protection, and I would build it in a metal box over a plastic one.

3's

Greg

[dss56]

Well the box is plastic and it will control a grounded circuit, meaning the circuit needs to see a ground.
So on the relay I will do a common side to ground and a normally open to do the grounding so when relay is keyed the common and N/O will see ground when keyed.

Still need the mic side for keying seems pin 3 has about 8volts on it un keyed on a cobra 29 and 25s. But when keyed
seems to go low to ground dont under stand why I see 8volts at pin 3 un keyed. Well Ill keep trying maybe a blocking diode on pin 3 to block the 8 volts.

So on relay external 12 volts on 1 side of relay and on the other side from pin 3 seems to work but the 8 volts sitting there un keyed has me baffled.

Pin 3 seems to be is the transmit pin> pin 1 Ground> pin 2 Audio> pin 4 receive on most all 4 pin radios.

So ill try to make something up with 12 v external and pin 3 with a blocking diode to block the 8 volts.

thanks for some suggestions.

[MDYoungblood]

If you see voltage on pin 3 then the radio you have might be using a electret element microphone, when the PTT is keyed, it is suppose to short the switch contact to ground, curious. Only reason I suggested a metal box is to shield the circuit from stray RF.

3's

Greg

[dss56]

The circuit will not be affected by any rf it just needs to see ground and no audio to be affected by any stray rf power. There should be no stray rf power at the mic. Just the voltage I see again has me puzzled.
So later today I will put some more time and see if a blocking diode will block the 8volts on pin 3 and when mic is keyed
it will go to ground. Im sure there is a way as on ebay many external boxes hooked to the mic jack that will key up a noise toy or so echo box they are using the mic jack to key a relay i think.

Well later tonight I should have an answer I hope.

thanks