This is probably the simplest reciever you can build. It outputs a DC signal (or audio) when it detects a radio signal with the right frequency. It can be used in a remote control or even a slow serial data link. To trim the reciever, do like this:
(1) make sure your transmitter is off
(2) turn the pot (P1) just until no current flows through Q2.
(3) switch the transmitter on and adjust Cx untill maximum current flows through Q2. If no current flows, go to point 1.
(4) your reciever is now ready to use
The first thing you will probably notice is that the reciever has no antenna! Instead it uses the coil used for tuning as an antenna.
The coil can be wound on a 55mm long 12mm dia ferrite rod.
You can use 0.3mm insulated copper wire wrapped about 55 turns around the ferrite rod.
The inductance is about 270 uH if wound as above.
The reciever can easily be modified for outputing audio through a loudspeaker. Just connect a loudspeaker between + and output, and substitute Q2 for a less sensitive power NPN transistor.
P1 is now used to set the volume.
If the speaker scrapes, You may need to add a resistor between the base of Q2 and the collector of Q1. The value of this resistor may vary, and a little experimenting is therefore needed. a good value to start with is 1k ohm.
IC1 is a ZN414 which cost only a few dollars
