Many a times one needs
an ex- tra telephone ringer in an ad- joining room to
know if there is an incoming call. For example, if the
telephone is installed in the drawing room you may need
an extra ringer in the bedroom. All that needs to be
done is to connect the given circuit in parallel with
the existing telephone lines using twin flexible wires.
This circuit does not require any external power source
for its operation. The section comprising resistor R1
and diodes D5 and LED1 provides a visual indication
of the ring. Remaining part of the circuit is the audio
ringer based on IC1 (BA8204 or ML8204). This integrated
circuit, specially designed for telec- om application
as bell sound generator, requires very few external
parts. It is readily available in 8-pin mini DIP pack.
Resistor R3 is used for bell sensitivity adjustment.
The bell frequency is controlled by resistor R5 and
capacitor C4, and the repeat frequency is controlled
by resistor R4 and capacitor C3. A little experimentation
with the various values of the resistors and capacitors
may be carried out to obtain desired pleasing tone.
Working of the circuit is quite simple. The bell signal,
approximately 75V AC, passes through capacitor C1 and
resistor R2 and appears across the diode bridge comprising
diodes D1 to D4. The rectified DC output is smoothed
by capacitor C2. The dual-tone ring signal is output
from pin 8 of IC1 and its volume is adjusted by volume
control VR1. Thereafter, it is impressed on the piezo-ceramic
sound generator |
