Introduction

 This project is capable of barring STD, ISD and Mobile calls. Further this project cuts the local calls after desired interval. Before disconnecting the system will given a beep sound which will enable the customer to continue after giving start pulse.

The circuit here can be used in PCOs for displaying the actual bill. The comparative disadvantages of the presented circuit are as follows: 1. The calculator used along with this circuit is required to be switched ‘on’ manually before making a call. 2. Certain manual entries have to be made in the calculator; for example, for a pulse rate of Rs 1.26, number 1.26 is to be entered after switching ‘on’ the calculator followed by pressing of ‘+’ button twice. However, possibility exists for automating these two functions by using additional circuitry.

In telephony, on-hook condition is represented by existance of 48V to 52V across the line. Similarly, the off-hook condition is represented by the line voltage dropping to a level of 8V to 10V (depending upon the length of the local lead line (local loop) from telephone exchange to the subscriber’s premises as well as upon the impedance of telephone instrument). Handset is normally lifted either for dialing or in response to a ring. 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 (phaselocked 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 p e r m a n e n t l y capacitively coupled to ground via capacitor C2. As soon as call matures, 16kHz tone pulses would be pumped into the telephone line by the telephone exchange at suitable intervals. This interval depends on the pulse rate of the place called and also the time of the day and whether it’s 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 f the pulse. The output of IC2 is coupled via transistor T3 to optocoupler IC3. The output of this optocoupler is used to ridge the ‘=’ button on a calculator (such as Taksun make), which has the effect of pressing the ‘=’ button of the calculator.

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