Simple LED Bar Logic Tester
Sometimes it just works, but for most of us that's the exception rather than the rule. If your circuit isn't working as expected, how do you track down the problem?
I've been using this LED Bar Logic tester to debug some of my Arduino™ add-ons. It's very simple - a row of sockets connects to a ULN2803A, which drives 8 of the LEDs in a 10-LED bar display. The ninth LED is unused; I use the tenth to indicate if the 5v supply is on.
Most of the boards I'm working on at the moment are driven by an I2C™ port expander. I use the LED bar tester to check that the state of the port expander outputs is what I expect. Of course you can also use it to check the status of the Arduino pins.
Here's the schematic. SCL and SDA are usused; they are part of the I2C bus which runs along the bottom of all my test boards.