Optical Fibre ADC and DAC

Date completed: 2014

Here are a couple of circuits I designed and built for teaching Physics. At the time of the "Big Switchover" from analogue to digital radio and TV, we were required to teach the differences between, and uses of, analogue and digital signals at GCSE. These little boxes encode an input analogue voltage into a digital signal of 8 bits, sends it along an optical fibre, and then decodes it back to analogue (which can be connected to a voltmeter). The whole process is demonstrated slowly with a flashing LED (in a straightforward binary code), or it can be done rapidly, so that a low frequency signal (say 200 Hz) can be sent and listened to via an amplifier and speaker.

Photo and School instructions

Design

These circuits are based around two PICs (different ones, just because those are the ones I had at the time!) Each pic has a quartz crystal to control the timing, and they are powered via 7805 voltage regulators from PP3 9V batteries. The optical fibre connectors were bought cheaply, the aim being to keep the whole setup as visible as possible. TS912N op amps are used to amplify the signals (again because I had some of these available - 741s would have done fine.) Diodes are used for over-voltage or reverse-voltage protection. The programming is in assembly language.

Circuit Diagrams

Tiny CAD (.dsn) file: ADC-DAC

Source code and other files

ADC: ADC.asm  All files (.zip)
DAC: DAC.asm  All files (.zip)