Improved micro FM transmitters

Jan 23, 2023 (Wireless communication and power)

Here are the full schematics for a (slightly improved) FM bug and beacon with a class-C output stage described in the last post.

Bug:

Beacon:

The improvements consist of 2 modified resistor values, the emitter resistor on the oscillator stage (Not much power is needed for the output stage), and the bias resistor for the amplifier.

Together these changes allow the device to run 1.5x as long with almost no effect on output power, and run down to 2V with a reduced output..

Construction advice

The most important thing is to pack components closely together. Perfboards/Matrixboards are a good way to create a compact and resilient layout.

The coils are 6 turns of insulated wire around a 3mm diameter form, and should be adjustable if a trimmer cap is not used. (able to be compressed/stretched out to change frequency). If a non adjustable coil is used, or more accurate tuning is desired (to hit an empty channel in an area with lots of stations), a variable/trimmer capacitor cab replace the fixed capacitor parallel to the coils.

Adjustment

First tune a FM radio to an empty channel (failing that, the one with the weakest signal), then adjust the coil (or the capacitor) in the middle of the schematic to set the frequency. Once it is set right you should hear loud feedback (with the bugs) or a tone in the radio. The other coil should now be adjusted to get maximum output.