The circuit “Automatic Time Indicator for Telephone” presented here provides an audible warning 2.5 minutes after dialing the telephone. The circuit does not make use of telephone line for its operation.
The circuit consists of three 555 ICs. It can be divided into three parts- touch switch using IC1, timer using IC2 and oscillator using IC3.
The on touch plate is in the form of a wire, the insulation of which is attached to the stopper on the telephone dial.
The off touch plate is in the form of a conducting silver paper strip which is stuck on the body of the telephone such that a similar strip on the handset touches it when the handset is kept in position.
While dialing a number, the finger touches the stopper and the switch becomes ‘on’. Pin 3 of IC1 goes high, thus giving supply to IC2 which is trigger.
The timer triggers falsely when the supply is giving and pin 3 of IC2 goes high. After preset time interval, the voltage drops to ground level and the oscillator becomes operative.
The circuit will not work on battery since the grounding of touch plates is involved. Use a 9-volt regulated supply. The line and neutral terminals should be connected properly. Otherwise, the touch switch action will be momentary. The wire connecting the on touch plate to the PCB can be long but that for the off plate should not be longer then 4-5 cms.
The siren can be stopped by keeping the handset in position or by touching the off plate first and then the on plate for issuing a warning after the lapse of 2.5 minutes.
The circuit draws 3.5 mA in off state. It remains off for incoming calls. It is automatic in the sense that it switches itself on and off within the normal procedure of making a phone call.
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1, R3 = 10 KΩ
R2 = 1.5 MΩ
R4 = 32 KΩ
C1, C3 – C5 = 0.01 µF
C2 = 100 µF/16V
C6 = 1 µF/16V
IC1 – IC3 = NE555
LS1 = 8Ω, 0.5W