line. Protection diodes D10 and D11 limit the incoming signal on the phone line (between +12.7 and - 0.7 V) so that potentially damaging voltages like the 90 Vp-p ‘Ring’ signal does not blow up Q6! So far we have covered most of the circuitry on the board with exception of the power supply. VR1 and the surrounding parts C3, C4 and the polarity protection diode D1 form a simple voltage regulator to supply a steady 5 VDC to the ICs on the board. By using this scheme, we can operate the unit with a single 12 VDC (positive tip on J5) source and have access to the multiple voltages needed for the unit to operate. DCI1 7