Continuity Checker

     While working on my Homebrew Gameboy Cartridge PCB, I needed a way to check the connections on the board. Unfortunately, my cheap multimeter doesn't have a continuity checker, so I had to make one myself. It's a very easy circuit based on a 555 timer. I made it on a scrap piece of PCB and mounted it into the remains of a Wireless Door and Window Alarm that I used parts from on another project. It was the perfect fit because it could be battery powered and already has a buzzer built it.

555 Timer Astable configuration
http://www.555-timer-circuits.com/astable-multivibrator.html

     I used the above schematic for my circuit. The astable configuration makes the 555 timer work as a frequency generator producing a continuous stream of square waves at a frequency based on the values of R1, R2, and C. I wanted an audible frequency so I used a 1kΩ resistor for R1, a 100kΩ resistor for R2, and a 0.01uF capacitor for C. These values should produce a frequency around 700Hz. Pin 3 is the output so the buzzer was connected between there and ground. The red and black leads coming from the box act like a switch -- when they are apart, there is no power going to the circuit, but when they are together, power is supplied and there is output on pin 3. Thus when the leads are placed on a circuit and the traces make a closed loop, the buzzer sounds, if there is no connection, there is no power and no sound. Simple!

Very portable

Powered by three button cell batteries at 1.5V each

Very compact circuit board layout

Everything that is needed