This infra-red receiver is the counterpart of the infra-red transmitter. The output from the infra-red diode D1 is amplified by IC1. The diode is a Type BPW41N which works well with the transmitter and is fairly fast 200 ns). The output pulses of IC1 are taken to comparator IC2, which separates them from the ambient light. To that end the d.c. component of the output of IC1is stored in C9 and used as a reference for 102. The received signal is decoded by a simple binary scaler, IC3a. Each pulse results in a change of level at the output of this stage, I so that the output resembles the original pulse-duration-modulated (PDM)
signal.The output is filtered by 117-C1 1, decoupled by C 12, resulting in an audio signal that is suitable for driving high-impedance headphones. The binary scaler stage contains a delay network, R6-Cio, which prevents the scaler being clocked by ambient noise. Owing to this network, the previous level is retained at the D input for a microsecond during which the state of the clock input is immaterial. The output signal may be enhanced by an active 3rd or higher order low-pass (10-12 kHz) filter, which may also serve as a buffer.
In combination with the infra-red transmitter, and ignoring the residual high-frequency components, the audio signal is virtually undistorted. With the receiver at a distance of 1 m (3.3 ft) from the transmitter and 30% modulation, the signal-to-noise ratio about -50 dB. The maximum distance the prototypes worked without interference was about 5 m (16 ft). The circuit draws a current of some 15 mA. which makes it advisable to use a rechargeable (NiCd) battery.