Low-Cost PCO Billing Meter

In the circuit shown in Fig. 1, when the handset is off-hook, the optocoupler MCT2E (IC1) conducts and forward biases transistor T1, which, in turn, forward biases transistor T2 and energises relay RL1. In energised condition of relay, the upper set of relay contacts connects the positive supply rail to PLL (phase-locked loop) IC2 (LM567) pin 4, while the lower set of relay contacts couples the positive telephone lead to input pin 3 of LM567 via capacitor C1 and resistor R3. The negative telephone lead is permanently capacitively coupled via capacitor C2. This interval depends on the pulse rate of the place called and also the time of the day and whether its a working-day or holiday. On receipt of 16kHz pulse, output pin 8 of IC LM567 (which is tuned for centre frequency of 16 kHz) goes low for the duration of the pulse. Th
Low-Cost PCO Billing Meter - schematic

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