The local music school had a need to test a bunch of audio cables (1/4" and XLR) in a hurry in preparation for the annual students' concert. Yes, it can be done with a multimeter but if you have lots of them to do this becomes very tedious very quickly. Sounds like a job for an Arduino
![DSC00120](https://bogpeople.com/blogger-content/uploads/2012/08/DSC00120-300x209.jpg)
![DSC00123](https://bogpeople.com/blogger-content/uploads/2012/08/DSC00123-300x247.jpg)
It does XLRs also...
![DSC00128](https://bogpeople.com/blogger-content/uploads/2012/08/DSC00128-254x300.jpg)
The work is all done by an Atmel Atmega mircocontroller
![DSC00134](https://bogpeople.com/blogger-content/uploads/2012/08/DSC00134-300x194.jpg)
All of the development work is done in the Arduino environment. Once it was all working I just transferred the microcontroller onto a piece of perfboard (so that I could reuse the Arduino board for my next project). The connectors on the far-left of the board (connected to pins 1,2,3,7 and 8 ) allow the Arduino board to be used as a programmer (see http://arduino.cc/en/Tutorial/ArduinoToBreadboard). That link explains how to configure the Arduino to use the built-in 8MHz RC clock in the Atmega so no external crystal is required (neither accuracy nor flat-out CPU speed are remotely important in this application).
I also stuck a normal ICSP connector on it so that it can be programmed using a normal programmer (bottom-left corner). This is handy: once the code was complete I was able to blow the Arduino boot-loader away so that the sketch starts immediately rather than after a pause. The downside is that if I want to use an Arduino to program it again I need to reinstall the boot-loader (for which that ICSP header will be very useful !)
The source-code is posted here. The comments in the code list the required connections. I haven't bothered to draw a circuit-diagram but if anyone wants one just leave a note in the comments below and I will post it here.