infra-red headphones transmitter - also usable for other audio signals

also usable for other audio signals This circuit is used together with the receiver described elsewhere in this issue to form the simplest infra-red wireless headphone system im- aginable. It uses a pulse width modulation (PWM) system which, although unsuitable for critical hi-fi applications, gives a reasonable qual- ity and has an acceptable range. The transmitter is based on an LM 567 tone decoder IC. The layout used is somewhat unusual but the chip"s internal VCO and switching stage combine to give much better linearity than could be achieved with, for example, a simple circuit based on a 555 timer IC. C5 10 06 V Tant. =I= R7 7-90 50 mV BC 550C infra-red headphone transmitter modulate IC1. Pin 6 of the 567 is the trigger input so that the audio signal is superimposed on a HF (about 50 kHz) triangular signal. This causes the rectangular output signal to be pulse width modulated. The re- mainder of the IC is used as a buffer so that the 567 can drive infra-red LED D1 directly (a...