gmrs/frs activated relay

[lockedupinsc]

i am trying to find a way to trigger a 12volt relay from the low voltage speaker leads on a gmrs/frs radio. this way when i press the push to talk on a different gmrs radio set on the correct channel i will be wirelessly triggering a relay. i plan to use if possible for a r/c car type project. the idea being that it will have a very good range. if anybody out there knows how or if i can do this please let me know. thanks

 

[Gary B]

It is done in two stages. First generate a voltage on your speaker lead. It is then passed through a bridge to produce DC that biases a medium power transistor on. The transistor then provides the current to activate the relay. The calculations can be quite straight forward. Divide the current required by the relay by the gain of the transistor used to get the amount of base current required. Use Ohm’s law to get the maximum size of the resister between the base and the bridge output (R= Bridge output voltage/base current). You can do the same calculation again using the maximum safe current the transistor can handle to get the minimum size of the base bias resister. I usually select a value that is about in the middle of the two values. A pull down resister between the base and emitter will make it more stable and efficient by ensuring that the transistor remains fully off when there is no signal present. I usually use something about 10 times the size of the base resister.

 

[MikeMl]

Except that remote control is not permitted on FRS channels.

 

[lockedupinsc]

thanks for the help gary b. is it possible to get a circuit diagram to go with your post.

 

[Gary B]

No, I'm not in a position to generate them and you will learn more doing it yourself.