Date of completion: 2015
When teaching either the topic of Total Internal Reflection (of light) or Communications to GCSE students I always felt that it would be much better to see optical fibres in use than simply to talk about them. The device here measures the time for an optical signal to travel along an optical fibre, and from this a fairly accurate value for the speed of light in a glass fibre can easily be calculated. This is clearly useful when teaching about refractive index. I bought a 100 metre reel of double optical fibre from Ebay (ensuring it had the correct optical connectors) and then another much shorter length of optical fibre (blue in the picture) to return the output signal back along the input fibre (making a total length of just over 200 metres.) Given that the refractive index of glass is about 3/2 and so the speed of light in it is about 200,000,000 m/s, this gives a time delay of roughly 200 / 2 x 10^8 seconds or 1 microsecond. This is just measurable using a 50 MHz oscillator and its accuracy is greatly improved by averaging over multiple transits.
Inevitably there a delay due to electronics response times (about 2 microseconds, dependent temperature), but a reset button was added to subtract this from the times displayed once the unit had warmed up (about 10 minutes) to keep the demonstration simple for GCSE students!
Inevitably there a delay due to electronics response times (about 2 microseconds, dependent temperature), but a reset button was added to subtract this from the times displayed once the unit had warmed up (about 10 minutes) to keep the demonstration simple for GCSE students!
Photos and School Instructions
Design
The hardest part of the project was finding a cheap optical transceiver on Ebay, but it wasn't long before I spotted one buried in an old non-functioning electronics unit. It turned out to be an HFBR 5803 and a search on the internet revealed its characteristic were just fast enough to make a reliable timer. Some level shifting of input and output voltages was needed, but it became apparent that with the help of an op amp it could be connected to a resettable gate made from fast TTL NAND gates in a 74HC132. This is controlled by a PIC16F628A (my favorite!) and the output is displayed on a 16 character LCD (also my favorite!) The 5 volt-powered 50 MHz oscillator came, as ever, from Ebay, and these signals are fed through the gate to the PIC which counts them repeatedly (via its TIMER0, set up as a fast counter.)
The coding is performed in assembly language so that timings could be calculated easily and reliably (with the PIC driven by a 20 MHz quartz crystal.) Most of the coding is concerned with calculating the time correctly and displaying it on the LCD display.
The coding is performed in assembly language so that timings could be calculated easily and reliably (with the PIC driven by a 20 MHz quartz crystal.) Most of the coding is concerned with calculating the time correctly and displaying it on the LCD display.
Circuit Diagrams
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