Aural Continuity Tester
This project is an aural continuity checker. It performs the same types of tests as the circuit in project Circuit Continuity Checker in Testers Section, but use a tone from the Speaker instead of the Lamp to indicate continuity. That makes this circuit more desirable when you aren't able to look at the Lamp each time you make a test.
For example, this circuit allows you to use your eyes entirely for locating the wires and terminals to be checked, while your ears detect the results of the test. The continuity of the circuit under test completes the supply connection to a standard pulse type oscillator which uses a sensitive Transistor (the 2SA).
Because of this, you can check the tester by checking almost any component on the board except the Meter (it is too sensitive and may be damaged).
When checking Diode and Transistor junctions, try switching the probe leads around to check operation with both polarities. You can check almost anything with this continuity checker because of the lower current (15mA or less).
You will want to try measuring the continuity of pencil lines on paper, water, metallic surfaces and many other things.