Homemade wireless voice transmitter for telephone

Homemade wireless voice transmitter for telephone

Recently, I found a copy of the 2002 edition of the Electronic Newspaper. I was inspired by an article called "The Simplest Recording Phone" and assembled a wireless telephone monitor. I would like to share it with you. If there is anything wrong, please point it out.

Features: This circuit has a simple structure, uses few components, is low cost, and is easy to assemble. It uses FM to transmit signals wirelessly and then uses an ordinary FM radio to receive (working range ≥ 15m), and can stay on the telephone line for a long time.

1. Working Principle
This circuit is connected in series with the telephone line. When the phone is on-hook, the telephone line voltage (hereinafter referred to as line voltage) is above 50V. At this time, the relay is energized, the charging circuit is turned on, and the battery is charged. When the phone is off-hook, the line voltage drops to about 10V. At this time, the relay coil stops working, the charging circuit is disconnected, and the FM transmission circuit is turned on. The transmission circuit transmits the incoming call signal of the phone (Note: The function of transformer B is to sense the audio signal on the line and supply the signal to the transmission circuit).

2. Component Selection
(1) The transformer should be a push-pull output transformer of a pocket radio.

(2) The relay should be an electromagnetic relay with double contacts, 12V operating voltage, small size, high sensitivity, and the smallest possible coil operating current.

(3) The transistor is a high-frequency, low-power tube C9018.

(4) Inductor T is made by winding 4 turns of φ0.6 mm enameled wire on a ballpoint pen core. Other components are selected according to the values ​​in the circuit diagram.

3. Debugging
1) Adjust the turn pitch of the inductor coil T so that the transmitting frequency is between 88MHz and 108MHz, and avoid the local FM radio station.

2) Fine-tune R3 to make the transistor operating point at a reasonable position.

3) Adjust R5 so that the transformer output signal strength is within the appropriate range.

4) Adjust R6 so that the electromagnetic relay can just close when the phone is on hook.

5) Adjust R7 so ​​that the voltage across the battery is around 2V during charging (Note: R7 should be readjusted each time the rechargeable battery is replaced to ensure the charging voltage is appropriate).

When experimenting at home, I found that after connecting this machine in series to the telephone line, the telephone volume and the response speed when picking up the phone decreased, but the call quality was not affected. In addition, the greater the working current of the relay, the more the telephone volume decreased, and even became silent.