Posted on Mar 3, 2013

When long ranges must be worked with IR light sources, and when high system reliability is required, pulsed-mode operation of the IRis required. Additional reliability of operation is attained by synchronously detecting the photodetector current, as this circuit does. PC-1 is an IR and phototransistor pair which detect the presence of an object blocking the transmission of light from the IR to the phototransistor. Relatively long-distance transmission is obtained by pulsing theIR, with about 10-ftS pulses, at a 2-ms period, to 350 mA via the 2N6027 oscillator.

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The phototransistor current is amplified by the 2N5249 and 2N5356 amplifier to further increase distance and allows use of the HllA5, also pulsed by the 2N6027, as a synchronous detector, providing a fail-safe, noise immune signal to the 2N5249 pair forrning a Schmitt-trigger output. This design was built for battery operation, with long battery life a primary consideration. Note that another stage of amplification driving the IR can boost the range limited by theIR VF, by 5 to 10 times. A higher supply voltage for the IR can double this range. Today, optoelectronics are mostly used to transmit electronic information over light beams. These applications range from the use of optocouplers transmitting information between I C logic circuits and power circuits, between power lines and signal circuits, between telephone lines and control circuitry, to the pulse-modulated systems which transmit information through air or fiber optics over relatively great distances.

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