opto-transmitter for speech

elektor july/august 1979 52 Transmitting speech by modulating a beam of light isn`t new. Usually, infra-red is used -as in some `wire-less` headphone systems. However, it is also possible to use a normal torch bulb. The only point to note is that the filament must be run at a fairly high temperature, as otherwise the response will be too slow. For this reason, the bulb must be run at almost full brightness, with a very low modulation depth -about 1 %. On the other hand, this has the advantage that the high light output carries over greater distances -cer-tainly if a so-called halogen lamp is used, as in this design. Using amplitude modulation would have the advantage that it makes for a simple receiver design. It would also have a few major disadvantages, particularly the fact that a suitable output stage for the transmitter (capable of driving a 60 watt lamp) would be rather expensive. For this reason (and some others), pulse-width S1 Test This circuit is intended for use in conjunction with the `opto-transmit· ter`. The modulated light beam is detected by a photo-transistor, T 1; the output from this transistor is amplified by IC1. The overall gain of this stage can be adjusted by P1 and P2 (fine and coarse adjustment, respectively). The optimum adjust-ment depends on the ambient I ighting; it can be found (once the transmitter and receiver are correctly aligned) by `tuning in` the 1 kHz test tone from the transmitter. After the volume control, P3, the signal is amplified by IC2 and IC3. The out-put stage is derived from an earlier Elektor circuit.