I chose this project because I do a lot of work in the school theatre and one of the problems there, along with the bureaucratic budgeting system and the damage caused by small pupils who think they know what theyíre doing, is the large number of cables that donít work. The project is aimed at helping theatre technicians to test audio equipment, and cables. The tone generator was included in the specifications in order to test the considerable number of audio amplifiers, mixers, DI boxes and speakers in the theatre.
A two-box device was needed because it would often be impracticable to bring both ends of a cable together or introduce additional leads. The circuit tests a 3-core lead using a square wave generator and pairs of LEDís to indicate discontinuities, shorts or swaps in the lead.
The only problem with the initial circuit was that a significant current was drawn even when a cable was not plugged in, so battery life would be unacceptably short without an ergonomically annoying power switch. Modifying the circuit to use a CMOS chip reduced the power consumption to the extent that the power switch would only need to be turned off when the tester was not to be in use for months at a time. The circuit was developed on breadboard and then soldered on a printed circuit board for the one-off prototype.
Various options for the case were considered before developing a simple design based on two tubes that fitted together. The prototype proved to be easy to use, with two green lights meaning a pass and anything else being a fail. It was found that the bi-colour LEDís could sometimes appear to be green, seen from some angles, when they should be yellow. Using tri-colour LEDís at slight additional cost should solve that problem.
One other modification would be the level of the tone produced to test audio equipment. The level produced by the prototype is too high for sensitive amplifiers and is actually loud enough to drive a speaker directly. An adjustable level would be ideal but a fixed, lower level would be simpler to use and cheaper as well.
Manufacture of the prototype raised some difficulties, notably those of accurately setting up and turning Perspex tube on a lathe to create a step on the inside (right). The problems are fairly easily solved when mass production methods are used, notably through the introduction of injection moulding.
The prototype provides a practical solution to the problem of conveniently testing cables and compares favourably with other products on the market. Comparison with other products highlighted some further features that may be easily added to a mass-produced model.
Before attempting to mass-produce this model, I would carry out the following: