auto answering circuit
Here is a circuit of a simple telephone answering unit which may be used with any telephone. The circuit consists of three main sections:
1. Ring detector section.
2. A timer controlled electronic switch and telephone line interface.
3. A voice IC having a specific pre-recorded message.
This is not very critical and any other device (such as tape player with message recorded on an endess tape) which does not overload the circuit or the telephone line, may be used. The incoming line is protected by metal-oxide varistor (MOV) RDN 130/14 followed by polarity guard circuit comprising diodes D1 through D4 in bridge configuration. Transistor T1 (MPSA92), having a high breakdown voltage (Vce max.) is used as electronic switch/telephone line interface. It is controlled by transistor T2. Ring detector section comprises capacitor C1, resistor R10 and opto-coupler NEC 2505. (In case of its non-availability, one may substitute it with MC2TE or a similar opto-coupler with additional diode 1N4148 placed with its cathode connected to pin 1 and anode to pin 2 of the opto-coupler.)
Timer NE555 (IC1), configured as monostable flip-flop, is powered by external 230V AC via transformer X1, rectifier diodes D5, D6 and filter capacitor C5. When switch S1 is on, the incoming telephone line as well as transformer X1 get connected to the circuit and D8 LED lights up.
Normally the monoshot IC1 is off (output at pin 3 is low) and so also are the transistors T1, T2 and relay RL1. When ring is received, the opto-coupler operates and takes trigger pin 2 of timer from high-to-low potential, thereby triggering it. This causes the output at pin 3 of IC1 to go high, which causes relay RL1 to energise and create a bypass for the ringing voltage to the opto-coupler as transistor T2 also gets forward biased. Conduction of transistor T2 causes forward biasing of switching transistor T1. This, in turn, causes the positive votage from rectifier diodes D3, D4 cathode junction to be extended to voice IC chip after being limited to steady 6.2 volts by resistor R3, zener D10 and capacitor C2. As a consequence, the voice IC operates and feeds the voice message into the telephone lines via capacitor C6, base-emitter junction of transistor T1 and polarity guard bridge.
The pulse width of timer monoshot (decided by in-circuit value of potentiometer VR1 and capacitor C6) is so adjusted to cover the period of the voice message. At the end of the mono period relay RL1 de-energises and the circuit returns to its original condition. In this circuit about 100 mA is catered for the voice IC. The circuit can be easily modified (such as by use of DPDT relay etc) for operation with different answering devices. For testing the circuit you may use a musical COB IC.