continuity tester circuit

Posted on Feb 6, 2014

A continuity tester is handy for checking that there is a conducting path between two points. The following circuit has the advantage that it is very sensitive and it gives both a visual and audible indication of continuity. An audible tester is handy since you are normally looking at where you are placing the tester contacts rather than looking a

continuity tester circuit
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t the tester itself. The circuit diagram shows that the continuity tester is made up of a sensitive one-transistor switch which turns on a LED and a circuit taken from a musical greeting card. Most greeting card circuits are powered by a single 1. 5 volt "hearing aid" cell so I have connectedthe music printed circuit board (pcb) across the red LED which maintains a fairly constant voltage of 1. 5 volts across it. Some musical circuits use two cells so you could change the LED to a green, blue or white one which would increase the voltage across the pcb. The base resistor value is not critical. I have specified 10 k © here but its purpose really is only to limit the maximum current that can pass through the base-emitter part of the transistor. A value of 100 k © seems to work just as well. Again the collector resistor (100 ©) is not too critical. Higher values will limit the current through the music pcb and LED making the music quieter and slower. The music`s pitch may may also be affected a little. Two musical greeting card pcbs and their piezo speakers are shown in the picture below. The one on the left is unmodified and has its cell, cell holder and off/on switch still attached. These have been removed from the pcb on the right. The photo below shows the components soldered together. The contacts used here are connected to 4 mm "banana" sockets. This makes the circuit more versatile since a range of probes or connectors...

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