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Optical Communications Using Red LEDs
Steve M0SBY and John G4OTJ have carried out a successful two-way QSO using modulated light from an LED and picked up by a photo diode. Three tests were carried out on 10th August 2016. The first was just 320 metres to make sure the transmit and receive modules were aligned properly the next was over a path of 1.7 km and is shown in these two photos. Above is the view from Steve’s end and below from John’s. The red dot on the lower photo is not very clear but it was easily visible by eye. Steve’s transceiver uses a pair of 10″ fresnel lenses mounted in a large cardboard box and John’s a pair of 4″ convex lenses in plastic drain pipes. Both are using a 3 watt LED amplitude modulated directly with audio. Signals were 5 & 9 both ways.
Later on the same evening a more ambitious test was done over an 8km path. It was getting dark and the links were very difficult to align but two-way communications were achieved eventually. Signal strength and quality wasn’t as good although the transmitters were easily visible by eye both ways. These are early days in this project, others have done this over much longer paths but it proves we are heading in the right direction. We are both planning Mk2 equipment using new 12 watt LED assemblies originally from video projectors. John is going over to a fresnel lens too which should allow more light to be collected on receive.
