This circuit will allow you to record both sides of your telephone conversations for future reference. For commercial orders placed over the phone such a recording device is invaluable
This circuit allows you to construct a electronic switch which will automatically turn on your tape recorder when you pick up the handset of your telephone. Note that your tape recorder must have both a MIC socket and a REMOTE socket on it so that circuit can plug into the recorder and control it. Cheaper tape recorders tend to leave off these external plug sockets (although if you know where to look they may still be accessed inside the tape unit on the circuit board.) Telephone recorder switch is designed to work for the newer 1.5V and 3V tape recorders as well as the usual 6V or 12V ones. The voltage of the normal telephone line is between 40 to 60 volts (depending on country and telephone system.) When you pick up the handset of the telephone the voltage falls to between 6 and 12 volts. It is this drop in voltage which is used to control the tape recorder through the REMOTE connector. When the line voltage is high the base of the BC548 is pulled high so the transistor is turned on. This pulls the gate of the FET down to less than 1 volt. This shuts off the FET. (N channel enhancement mode FET’s need drain bias positive and a positive gate to turn on.) When the line voltage falls (that is, the handpiece is picked up) the BC548 must turn off; adjust the trimpot if it does not. So the FET gate potential rises to the 10 volts set by the zener diode. This turns the FET on to high efficiency conduction mode
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