Musical Continuity Tester
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 than 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 connected the 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.
circuit diagram for musical continuity tester

Circuit Diagram

The base resistor value is not critical. I have specified 10 kΩ 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 kΩ seems to work just as well. Again the collector resistor (100 Ω) 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

Constructing the Tester
The following photos show steps in modifying the musical greeting card pcb and constructing the continuity tester.
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.

layout of the musical continuity tester

This next photo 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 can be attached to the sockets.

detail of the modified greeting card pcb

The photo alongside shows a detail of the modified music pcb.

A link has been soldered from the central negative terminal to a larger piece of copper track.

This makes it easier to solder the 100 Ω resistor and LED to the pcb.

Below are two views of the finished musical continuity tester in its box. The piezo speaker leads were unsoldered from the pcb and fed through two small holes drilled into the box before being soldered back onto the pcb. The red LED is glued to the box and attached by flying leads to the pcb. The banana sockets are attached on either side of the box.

the finished musical continuity tester in its box - front view the finished musical continuity tester in its box - side view showing one banana socket

Using the Tester
Here are three other ways I have used this tester apart from testing for continuity:
  • As a moisture tester. This circuit is sensitive enough to operate when the two contacts are placed in damp soil or on a damp wall. Your skin is moist enough to trigger the circuit - see the next use.
  • To demonstrate to students that components need to be connected in a closed loop for a circuit to work. Get a class of students to form a circle and hold hands. Get two adjacent volunteers - one to hold the tester in such a way that their index finger touches one socket/contact. The other volunteer touches just the other socket/contact (making sure not to touch the first volunteer). Once all are holding hands the music plays and I point to various partners to momentarily break from holding hands - the music stops and then resumes when they hold hands again. I also make sure the students realise they are "connected" together in a series circuit.
  • As the base circuit for a game of skill like "operation". A wiggly length of rigid wire (florist's wire or a bent coat hanger) is connected to one socket. A small wire loop is attached to the other socket using a flexible wire lead. The aim of the game is to move the loop along the wiggly wire without touching it and setting off the music.