Remote Circuit Diagrams

UHF Remote Control Transmitter Schematic Circuit Diagram

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, and Germany: 1) Type 82554 for 433.92 MHz;
  • France: Type 82523 for 224.5 MHz

Frequency Modulation in the Transmitter

The transmitter undergoes frequency modulation through an audio (or digital) signal applied to the junction of varactors D1 and D2 via R3. Varactors play a crucial role by altering the shunt capacitance of the resonator in response to the modulation signal, resulting in frequency modulation (FM). The design can be modified to produce amplitude modulation (AM) by omitting components D1, D2, R2, R3, and R6, and connecting points ‘A’ and ‘B’. Notably, transistor T1 is positioned on the solder side of the printed circuit board, as outlined by the dashed marking on the component overlay. Additionally, a short wire link, using silver-plated wire, connects stripline inductor L to the positive supply track running parallel to it.

Adjusting the Wire Link for Transmit Frequency

The placement of the wire link plays a pivotal role in determining the transmit frequency. As the frequency decreases, more inductance is required, necessitating the movement of the link towards the PCB edge. Experimentation may be needed to identify the optimal position. Begin by setting trimmer C3 midway and placing the wire link approximately halfway on the strip line. Monitor the received signal and adjust C3 until the maximum signal is achieved. If no maximum is found, try adjusting the wire link either towards the transistor (smaller inductance) or towards the PCB edge (greater inductance). The ideal position is where C3 peaks when set around the midway point.

UHF remote control transmitter Schematic diagram

Optimizing Component Connections for UHF Circuit

Given the UHF nature of the circuit, it is imperative to maintain all component terminals as short as possible. This precautionary measure ensures efficient performance in the UHF frequency range. Additionally, to facilitate proper radiation, the transmitter necessitates placement within a plastic enclosure.

Regulatory Considerations for Transmitter Operation

It is essential to recognize that radio regulations govern the possession and utilization of this transmitter. In the United Kingdom, compliance with these regulations mandates obtaining a license from the Department of Trade and Industry. Adherence to regulatory protocols is crucial to ensure legal and responsible use of the transmitter.

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.

UHF remote control transmitter Schematic diagram

UHF remote control transmitter Schematic diagram

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