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 signaling. The operating frequency is determined by a resonator, F1. The following types may be used:
UK: Type 82528 for 418 MHz;
USA: Type 81530 for 315 MHz;
Holland, Belgium, Germany: 1)Type 82554 for 433.92 MHz;
France: Type 82523 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 component overlay. Also on this side of the PCB is a short wire link (use silver-plated wire) that connects stripline inductor L to the positive supply track that m 1 ran in parallel with it. The position of the wire link depends on the transmit frequency. The lower the frequency, the more inductance is required, that is, the further the link has’ to be ‘moved’ towards the edge of the PCB. Some experimenting may be necessary to find the best position. Start by setting the trimmer, C3, about mid-way, and fit the wire link about ‘half way’ on the strip line. Monitor the received signal, and adjust C3 until a maximum is found. If you can not find a maximum, try moving the wire link either towards the transistor (smaller inductance) or towards the PCB edge (greater inductance). The best position is that at which C3 ‘peaks’ when set about mid-way.
Since we are dealing with a UHF circuit, it goes without saying that all component terminals must be kept as short as possible. The transmitter must be housed in a plastic enclosure to enable it to radiate.
Note that radio regulations apply to the possession and use of this transmitter. In the United Kingdom, a license must be obtained from the Department of Trade and Industry.
Parts list
Resistors:
R1 = 150 Ω
R2 = 8.2 kΩ
R3 = 100 kΩ
R4 = 6.8 kΩ
R5 = 3.9 kΩ
R6 = 22 kΩ
Capacitors:
C1= 5.6 pF
C2 = 0.68 pF
C3 = 0 pF
C4 = 470 pF
Semiconductors:
T1 = BFR91
D1, D2 = BB405
D3 = LED, red, 5 mm
Miscellaneous:
FI1 = ceramic resonator (see text) Silver-plated wire, 0.8 mm dia.
