Acompact, inexpensive and low
component count telecom head- set can be constructed
using two readily available transistors and a few other
electronic components. This circuit is very useful for
hands-free operation of EPABX and pager communication.
Since the circuit draws very little current, it is ideal
for parallel operation with electronic telephone set.
Working of the circuit is simple and straightforward.
Resistor R1 and an ordinary neon glow- lamp forms a
complete visual ringer circuit. This simple arrangement
does not require a DC blocking capacitor because, under
idle conditions, the telephone line voltage is insufficient
to ionise the neon gas and thus the lamp does not light.
Only when the ring signal is being received, it flashes
at the ringing rate to indicate an incoming call. The
bridge rectifier using diodes D1 through D4 acts as
a polarity guard which protects the electronic circuit
from any changes in the telephone line polarity. Zener
diode D5 at the output of this bridge rectifier is used
for additional circuit protection. Section comprising
transistor T1, resistors R2, R3 and zener diode D6 forms
a constant voltage regulator that provides a low voltage
output of about 5 volts. Dial tone and speech signals
from exchange are coupled to the receiving sound amplifier
stage built around transistors T2 and related parts,
i.e. resistors R7, R6 and capacitor C5. Amplified signals
from collector of transistor T2 are connected to dynamic
receiver RT-200 (used as earpiece) via capacitor C7.
A condenser microphone, connected as shown in the circuit,
is used as transmitter. Audio signals developed across
the microphone are coupled to the base of transistor
T1 via capacitor C3. Resistor R4 determines the DC bias
required for the microphone. After amplification by
transistor T1, the audio signals are coupled to the
telephone lines via the diode bridge. The whole circuit
can be wired on a very small PCB and housed in a medium
size headphone, as shown in the illustration. For better
results at low line currents, value of resistor R2 may
be reduced after testing |
