musical continuity

A continuity tester is handy for checking that there is an electrical 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 th
musical continuity - schematic

an looking at the tester itself. The circuit diagram shows that the continuity tester is made up of a sensitive one-transistor switch which turns on both 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 about 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 printed circuit boards (pcbs) and their piezo speakers are shown in the picture alongside. 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. This next photo shows the components soldered together. The contacts used here are connected to 4 mm "banana" sockets. This makes the circuit...

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