This circuit adds two extra channels to any 2~4 channel transmitter which generates PPM modulation (I used it to expand a Hitec Focus 4 to 6 channels). You must find where the transmitter's encoder output goes to the RF modulator, cut the wire or pcb trace, then insert this curcuit into the PPM signal stream.
The extra channels may be controlled by a two-axis gimbol, variable knobs, or switches. Switched channels are implemented using resistors to provide set analog voltage inputs.
The PIC12F615 runs at 8MHz, providing fine proportional control and high resolution output.