Since in some countries, it may be illegal to connect the circuit to the public telephone network, to check with your local telephone manager before building this circuit.
In most telephone networks, a direct voltage of 48-60 V exists at terminals a and b. When the call signal comes in, a 50-60 V alternating current at a frequency of 25-27 Hz is superimposed on the direct voltage. This open-circuit voltage is more than halved when the receiver is taken from the hook (which closes the current loop). The current in the loop is 20-100 mA.
The preset circuit starts a timer when it detects the call signal. The timer energizes relay Re1 and this switches on lamp La1. After the last ring, the timer remains on for a short time, but then returns to its quiescent state, whereupon the lamp goes out. When, however, the hook, the timer is triggered permanently, so that the lamp remains on during the telephone conversation and for some time afterward. Furthermore, the lamp may be switched on and off manually by S1 is pressed.
One part of the circuit is connected in parallel and the other in series, with telephone terminals a and b. The Parallel– connected part, which detects the call signal, is isolated from the direct voltage by C3. Because of this, no current flows through the LED of optoisolator 1C2 so that the phototransistor is off. The second optoisolator, which is in series with the a and b lines, also carries no current since the circuit is broken by the cradle switch. The call signal, however, is applied to IC2 via C3. Consequently, a current flows through the LED and the transistor is on. The current is limited by RI, while diodes D2 and D3 chop the peaks off both the half waves to prevent any damage to the optoisolator.
The phototransistor provides sufficient base voltage to T1 to switch on this transistor. Capacitor C4 discharges, whereupon the potential at pins 2 and 6 of timer- IC4a drops to earth, which triggers the timer. Its out changes state (goes high), whereupon T2 is switched on so that the relay is energized. The relay contact connects one of the mains lines (in the UK, the live) to lamp La1. The time constant of the timer has been arranged so that e the longest intervals in the call can signal be bridged. If the receiver is not lifted, and the last call signal has passed, the 1 remains on for the duration of the time constant, and then the circuit returns to the quiescent state.
When the receiver is lifted off the how current flows through the LED in IC3, whereupon the phototransistor in optoisolator, and consequently switched on. The timer is triggered as the telephone conversation lasts. When the receiver is replaced into the cradle, the lamp stays on for a short while determined by the setting of P1.
The second timer in the NE556. 1C4 is used for manual operation of the lamp, Before it is triggered, half the supply voltage exists at pins 8 and 12. The output (pin 9) is low so that C7 cannot be charged. When S1 is pressed, the trigger voltage briefly drops below the threshold, whereupon the output goes high. Since, owing to the time constant R9C7, the potential across C7 rises only gradually. the level at the trigger input (pin 8) remains below the upper threshold. When the switch is operated again, the full supply voltage is across C7, so that the level at the trigger input exceeds the upper threshold. The timer output then goes low again and the lamp goes out.
The power supply is a standard design but could be replaced by a suitable 12-V mains adaptor. The lamp could then be a 12-V car type. From a safety point of view. that is not a bad alternative.