This low-power UHF FM transmitter is intended for remote control applications such as garage doors and wireless alarm systems . It is a single transistor design that operates at a frequency reserved for low-power wireless signalling. The operating frequency is determined by a resonator, F11. The following types may be used: UK: Type R2528 for 418 MHz; USA: Type Rl53O for 315 MHz; Holland, Belgium, Germany: Type R2554 for 433;92 MHZ: France: Type R2523 for 224.5 MHZ. The transmitter is frequency-modulated by an audio (or digital) signal applied to the junction of a varactors D1, D2 via R3. The varactors effectively change the shunt capacitance of the resonator as a function of the modulation signal, which results in FM. The design may be changed to produce AM (amplitude-modulation) by omitting components D1, D2, R2, R3 and R6, and interconnecting points ‘A’ and ‘B‘. Transistor T1 is fitted at the solder side of the printed circuit board, as indicated by the dashed outline on the compo...