Telephone voice signal wireless transmitter

The telephone wireless generator circuit is simple and practical, and can transmit the conversation content of both parties into the air, which can be received by the wireless receiver. The circuit principle is shown in the attached figure.

The transmitter consists of two parts: an automatic switch and an FM transmitter. The automatic switch part consists of R1~R3, a trimmer potentiometer VR1, transistors T1, T2, a voltage regulator D2 and a diode D1. R1 and VR1 form a voltage divider. When the DC voltage of the telephone line is 48V, the output voltage of VR1 is 0~32V. The conversion of the circuit working state depends on the breakdown voltage of the voltage regulator D2 (here 24V) and the conduction voltage of T1 (0.7V). Therefore, when the output of VR1 is adjusted to 24.7v, D2 will break down and T1 will be turned on. At this time, the collector potential of T1 will decrease and T2 will be turned off. If the handle of the telephone is picked up, the voltage of the telephone line drops to about 11V, T1 is turned off, T2 is turned on, the DC path of T3 is formed, and the circuit starts to oscillate.

The low-power FM signal transmitter is composed of transistor T3, coil L1, etc. T2 acts as an electronic switch (on/off). The audio signal on the telephone line modulates the oscillation frequency along the transistor T2 and the series resistor R3, and the modulated high-frequency signal is sent to the antenna for transmission. As long as the FM receiver is tuned to the transmission frequency of the FM transmitter, the conversation between the two parties can be heard.

L1 is made of SWG36 enameled wire with 45 turns wound on a 1MΩ, 1W resistor. L2 is a hollow coil with a diameter of 12mm and is made of SWG21 enameled wire with 3 turns wound on it.

The DC power supply of this circuit is taken from the telephone line, and no external power supply is required. Since the circuit consumes very little power, it does not affect the normal operation of the telephone. After the transmitter is debugged, it can be installed in a 3.75×5cm box.