CIRCUIT IDEAS

ELECTRONICS FOR YOU���AUGUST '99

Using this low-cost project onecan reproduce audio from TVwithout disturbing others. It

does not use any wire connection be-tween TV and headphones. In place ofa pair of wires, it uses invisible infra-red light to transmit audio signals from

TV to headphones. Without using anylens, a range of up to 6 metres is

possible. Range can be extended byusing lenses and reflectors with IR sen-sors comprising transmitters and re-ceivers.

IR transmitter uses two-stage tran-sistor amplifier to drive two series-con-nected IR LEDs. An

audio out-put trans-former isused (in re-verse) tocouple audiooutput fromTV to the IRtransmitter.TransistorsT1 and T2amplify theaudio sig-nals re-ceived from

TV through the audio transformer. Low-impedance output windings (lower

gauge or thicker wires) are used for con-nection to TV side while high-imped-ance windings are connected to IR trans-mitter. This IR transmitter can be pow-ered from a 9-volt mains adapter or bat-tery. Red LED1 in transmitter circuitfunctions as a zener diode (0.65V) aswell as supply-on indicator.

IR receiver uses 3-stage transistoramplifier. The first two transistors (T4and T5) form audio signal amplifierwhile the third transistor T6 is used todrive a headphone. Adjust potmeter VR2for max. clarity.

Direct photo-transistor towards IRLEDs of transmitter for max. range. A

9-volt battery can be used with receiverfor portable operation.

Infrared CordlessHeadphonePRADEEP G.

G.S. SAGOO