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Circuits > Telephone related > Telephone Headgear
Telephone Headgear
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
Title: Telephone Headgear
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-09
Reads: 317
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