This is a very interesting and useful circuit. You can use it for many purposes like when you require continues beep. When you want to test an audio frequency system you can use an audio generator but how do you test your microphone? Saying "mic testing, hello test, hello check" gets
very tiring. Instead of doing this tiring work you can use this circuit to easily check you microphone.
Attach the circuit to the front of microphone and the circuit will produce continues beep in microphone. The circuit is very small containing only six components which are two resistors, one capacitor, one unijunction transistor, speaker and 9 volt battery. This circuit can be build very easily with fewer efforts and you can fit it in a small plastic box. You can use any size of speaker from 1 inch to 3 inch.