Telephone FM “Button” Transmitter Circuit

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This is the original telephone button transmitter circuit of John S. Wilson Jr. that was used in the transmitter button of a telephone headset attached to the old T1 Carbon element. The positive lead was soldered to the positive ring of the T1 and the negative lead was soldered to the negative lead.

The original element was unscrewed from the phone and dropped out and the new modified T1 dropped back in.

By all accounts it is still useful since it can be attached within a lot of phone buttons that have not gone digital.

At the same time it can be engineered and adapted to the new digital phones with a little ingenuity. The polarity is absolute Positive to the transmit button and negative to the negative button. It is powered by the terminals that feed the transmit button and detects the voltage changes, thus transmitting the audio signal.

L1 is # 20 Magnet wire. For the frequency range of 88 to 108 MHz wind 5 tight turns on the smooth end if a 1/8 inch drill bit. 108 to 147 MHz 3 Turns. 149 to 170 MHz 2 turns. Adjust the 5 to 50 pF capacitor to your desired frequency.

Range is roughly 50 to 75 feet without an antenna. Adding a 1 inch antenna to the collector of Q1 increases the distance. Both sides of the conversation is transmitted when the phone is off the hook - or in operation.
Telephone FM “Button” Transmitter Circuit