Infra-red headphone receiver: This receiver is meant to complement the transmitter describe in the previous article. Its design is based on just one FET. This has the advantage that construction is simplicity ‘itself, and the disadvantage that for a sufficiently low output impedance the value of drain resistor R3 has to be fairly low. That results with correct operation of T1 to a fairly large (certainly for a battery) current. The value of R3 Was chosen, as a compromise, at 560 Ω. which makes driving 600 Ω headphones possible. The load seen by Ti is then 300 Ω.
Both usable types of receive diode shown in the diagram have a daylight filter and are tuned to the wavelength of the LEDs in the transmitter (950 nm at 25 ∞C). At a couple of meters (5-8 ft) distance, a (no-load) output voltage of 200-300 mV is obtained, which is quite sufficient for most headphones. The circuit then draws a current of 9-10 mA.
The setting of P1 is fairly critical, but its control range may be reduced by adding a small resistor at either side of the present. The preset should be adjusted for minimum distortion. This is best done by applying a 1 kHz audio signal at a level of 150 my to the transmitter and adjusting both circuits for minimum (audible) distortion. This should be done without electric light because the transmitter does not modulate the audio signal on to a carrier, so that light bulbs, and particularly neon tubes (which emit an appreciable amount of IR light at 950 nm, modulated with 100 Hz) can cause quite a hum. Even normal ambient light causes a deterioration of the signal-to-noise ratio. However, with a little ambient light and a distance between transmitter and receiver of 3-4 m (10-13 ft), the distortion was 1-2%, which is not bad for such a sparse design.
