What is the chip ID? Tune the unit to 10 KHz off of your desired center frequency and then look at the voltage on pin 11 with and without carrier received.

 

Chip is TA2003, pin 11 is audio out.

 

How many units are required? Are there any other Unicom receivers close by? Are you close by, or will receiver be close by, an airport?

What kind of test equipment do you have.

What happens if someone transmits on 123.1 or 122.9..........and the LED lights? Is that bad?

AND, is turning on a LED, really the only thing you want to do.

Everything matters.

What are all the conditions?

Why are you doing what you are doing?

 

Chip is TA2003, pin 11 is audio out.

Depending on exactly how the FM demod is done internally.....

With no signal input, I would expect pin 11 to be at some nominal voltage. With a carrier on frequency, I would expect the voltage to be the same as with no signal in. As the input frequency varies above and below the tuned frequency, the voltage on pin 11 should vary above and below the no signal level. If it is working like this and you tune the chip say 10 KHz away from the desired center frequency, then the voltage at pin 11 should shift when the carrier is seen. Connect up a comparator and you should be good to go.

 

Thanks.
Two units; yes, other receivers in control tower and aircraft on tarmac or in flight nearby. Located at tarmac runways, ground level, solar+battery powered.
Amber audio analyzer, oscilloscopes, Motorola R2600 spectrum analyzer, Bearcat scanner, multimeters...
Transmission on other frequencies and LED coming on; an inconvenience but not hazardous or bad at all.
LED in optoisolator is the only pertinent to this thread, energizes other non-RF circuits

Tuning steps readout of the test receiver much larger than 10KHz: 100KHz. Not matching 25KHz airband channeling.


Broadcast FM receiver kit after modification - works.