Having a Continuity Tester / Signal Injector is nice to have to pinout a circuit. Most Multimeters will have a Continuity tester built in but if you only have one meter and it’s being used for something else a standalone tester is great.
How the tester works is it just tosses a low voltage around 1.5 volts into a circuit and makes a beep/buzz when the two probe ends touching together creating a short.
The unit I built is just a old stereo PC speaker amp and a PICAXE 08m2 micro controller.
The right channel on the amp is only used and Pin3 on the PICAXE sends out a square wave so when the output of the PICAXE finds a connection to the positive input of the amp it closes the circuit and a beep is played. Both the micro and amp uses the same power supply. Since the PICAXE can only handle 5volts I tossed in a LM7805 to power it but use the same common ground. The speaker was also replaced with a small three inch 16ohm 25watt speaker I had in the parts box.
The amp also has a AUX input for a external device and a headphone jack. So I can always toss on some remote speakers and also inject another audio source and use it as a signal tracer for Radios and such.
As I said it is a very simple circuit to add to a audio amp. Just add a LM7805 voltage regulator to power the PICAXE add a 0.1 cap for protection on the output of Pin 3 of the PICAXE. The Positive probe would also be Pin 3 and the Negative probe side would be the input of the amp. Both the PICAXE and amp share the same ground. If you put it on a scope you’ll see a 2.5V Square wave.
The code for the PICAXE is very straight forward.
main: sound C.1,(100,100) ; play the sound pause 10 ; wait goto main ; repeat
Here is the Schematic
I might use the AUX input for a signal generator.