Telephone Switch For Phone Recorder Project

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Introduction To Phone Recorder
This phone recorder project will enable you to record both sides of your telephone conversations. One constructed, this switch will allows you to 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 this device can plug into the recorder and control it. This circuit is designed to work for the newer 1.5V and 3V tape recorders as well as the usual 6V or 12V ones.

Phone Recorder Circuit Description

The circuit falls into two parts and these can be easily seen in the diagram. On the left are the connections to each telephone line and to the MIC socket of the tape recorder. The diode and capacitors ensure that no DC voltages pass through to the input of the MIC while the RC network clips large transients. On the right is the circuit which detects when the handset has been lifted and which then turns on the FET. The trim pot adjusts the voltage level of this circuit.
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 handset is picked up) the BC548 will 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. Different recorders may have different polarities in their REMOTE sockets. To allow for this a PCB mounted switch has to be added to the board which will reverse the polarity of the REMOTE switch just by switching it.

Once completed, place your tape recorder next to the phone. Plug in the 2 sockets (MIC & REMOTE) into the recorder. Put in a casette tape and push 'play'. If the device has been put together correctly either of two things will happen: the tape in the recorder will start to play or it will not.
1. If it does not play then pick up the phone handset. If the tape now starts to play then the device is working. Put the handset down, depress the play and record buttons and the tape will now record when the handset is raised.
2. If the tape plays then either of two things need adjustment:
a) move the position of the trimpot across its range of positions and see if this stops the playing. If it does then lift the handset to see if the playing starts. It should. The device is ready for use.
b) if adjustment of the trimpot does nothing then the REMOTE switch needs to be switched to the other position. Do this and repeat the steps as outlined above.The device should now work.