Telephone FM “Button” Transmitter Circuit
This is the original VFO FM 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
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