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. |
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