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 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.
Different recorders may have different polarities in their REMOTE sockets. To allow for this a PCB mounted switch has been added to the board which will reverse the polarity of the REMOTE switch just by switching it. When the Kit is completed place the Kit and your tape recorder next to the phone. Plug in the 2 sockets (MIC & REMOTE) into the recorder. Attach one alligator clip to each phone line (you may have to remove some plastic or covering from the phone line cables to get a good electrical connection.) Put in a casette tape and push 'play'. Provided the Kit has been put together correctly either of two things will happen: the tape in the recorder will start to play or it will not.