|
Many a times
one needs an extra 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 telecom
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.
|
