I've been able to understand most of what's going on in the schematics (5V power supply with the Zener, potential dividers in front of the transistors), but there are a few things that are eluding me. Primarily I am confused as to what the function of the 1N4148 signal diodes (D1, D2) are in the receiver module. Is D2 just there for the voltage drop? What purpose does D1 serve, being that it's simply connected between a voltage source and ground?
Thanks for any input on this.