The circuit presented here is a very useful add-on device to be connected to the telephone lines to count and display the number of incoming calls received in the absence of the subscriber.
The circuit can be fabricated using low-cost and easily available components. It uses a popular decade counter ic, a timer ic, and a few other discrete components.
The circuit may be divided into three sections: (a) ring detector, (b) call counter, and (c) timer and relay controller. The ring detector circuit detects the incoming ring signals. The timer circuit is used to control a relay, which controls the switching of ring signals to the ring detector section. The call counter is used to count the number of incoming calls.
To count the number of calls (not the number of rings), the response of the counter is limited to the first ring pulse only. In other words, it responds to the initial ring pulse and ignores subsequent ring pulses which repeat till the calling subscriber or the telephone exchange cuts them off.
When power switch S1 is turned on, the circuit gets 9V supply from the battery and the reset indicator led1 lights up. If led1 does not glow initially, press S2 until it glows. Now, in this standby mode, transistor T1 is in nonconducting state. But transistor T2 is forward biased via resistor R3. As a result pin 14 of IC1 is at a low potential.
When the telephone rings, ac voltage of about 75 volts appears across the input terminals of the circuit. During positive half cycles of the ring signal, D1 starts conducting and transistor T1 gets forward biased.
When transistor T1 conducts, transistor T2 gets reverse biased. Consequently, pin 14 of IC1 gets a short positive pulse through R4 and led 2 starts glowing, indicating call number 1.
Simultaneously IC2 (which is wired as a monostable flip-flop) is triggered by the ring detector circuit and the relay gets energised. As a result the circuit is disconnected from the telephone lines for a predetermined period (decided by resistor R7 and capacitor C5 values).
At the end of the mono time period, the circuit automatically returns to monitor the telephone line for the next call. This operation cycle is repeated with each succeeding call.
This circuit is capable of indicating up to 9 calls which are received in the absence of subscriber. It can be extended for more calls by cascading the required number of CD4017B ICs.