IR_RELAY_CIRCUIT

Published:2009/7/14 11:17:00 Author:May | From:SeekIC

All IR is received by the module via an IR photodiode. The diode is operated in its reverse-bias current it permits depends on the intensity of the received infrared light. The fluctuations in the reverse-bias current are then amplified by a high-gain stage. The output of the amplifier is then limited by the next stage. The limiter chops the extreme highs and lows off the amplified signal, and the result is a quasi-digital pulse train. The simplified wave then passes through a bandwidth filter that has its center frequency at 40 kHz. At that point, the circuit has effectively retrieved the 40-kHz remote carrier. The reproduced carrier is then integrated. The next stage is an inverting Schmitt trigger. It will not go low unless the filter's output signal surpasses a certain amplitude, and it will not go high again until the signal drops below a certain minimum. Thus, the Schmitt trigger responds only to large changes in the filter's output (caused by bursts and pauses) and ignores small changes (caused by the 40-kHz carrier, to which the filter can't respond quickly). The Schmitt trigger's output is thus low when a 40-kHz burst is received, and high during pauses between bursts. The resulting waveform is an inverted version of the pulses that were modulated and transmitted by the remote. The oscillator circuit is based on a 555 timer. With D3, R2 is bypassed (or shorted out) while C1 is charging, but is in the current path during discharge. Therefore, with R1 and R2 made equal, the charge time equals the discharge time, yielding an output with a 50-percent duty cycle. Components R1, R2, and C1 (all precision, drift-free units) have been chosen to provide a 40-kHz output in this configuration. That output strobes the IR LEDs via Q1. The oscillator functions only when pin 4 of the 555 is high. Because that pin is connected to the inverter circuit, the oscillator functions when the inverter's input is low. The inverter's input is connected to MOD1's output, which goes low with each remote burst received. So overall, the circuit produces a 40-kHz IR burst when it receives one.



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