With an Op Amp and a few components a cheap FM digital receiver can be made.
- $1 FM receiver
- Dual Comparator (LM2903)
- Switching Diode (1N4148)
- Potentiometer (22k)
- 150pF Cap
- 10kΩ resistor
- 4.7kΩ resistor
- 4 pin header
Step 1: The Circuit
Step 2: Assembly
The pin-out is not crucial I went
Step 3: Tuning
I kept the reset button for a manual override of sorts.
Again I suggest using a relay as opposed to transistor. It will add weight and cost but relieve some headache.(I will keep trying to improve the transistor though)
I used an FM receiver antenna, it would not reliably pick up my broadcast without it. Hooks to the inner of the 3.5mm jack leads.
Step 4: Specs
This board is 8.7g(.3oz) and 58.3mm(2.3in) tall with header 32.5mm(1.3in) wide 35.8(1.4in) with knob and only 13.3mm(.5in) thick.
I wanted it light and small for flying RC applications.
Range is dependent on transmitter power. With a store bought FM transmitter I got 10m easy.
Step 5: Trouble-Shooting and Expansion
I included two alternate circuits for demodulating digital data over audio.(if the comparator gives you to much trouble)
The best test instrument to see if it is tuning is an 8Ω speaker with a 2k2Ω resistor on it. It allows you to hear the station and does not inhibit the circuit. It may require adjustment of the pot when adding or removing it.(also causes circuit to become untuned). LEDs will work but are misleading with audio frequency noise(it blinks but you can not see it)
If it is longer range or a transceiver you are after use a pair of two way radios(not cheap walkie-talkies unless you would like to modify the 6PST switch for MCU control). The same dual comparator circuit should work with 2-ways(for receive only), I used a transformer and Schmitt. I want to make a converter module, but it would not be self contained.(and would be bulky)