This is a stand-alone probe for testing voltages in your projects. It only requires you to clip the ground wire to your circuit to be tested. Then you touch the probe tip to the point you want to measure and press the button. You get a 3 digit reading of the voltage (up to 30 volts). It's very convenient as the display is inline with the probe tip, so you never have to move your head to see the reading. Very simple circuit but does require some simple soldering. Runs off a 9 volt battery. (Note: it only measures positive voltages.)

Note: Below are the instructions that can be downloaded. This has a bill-of-materials, assembly instructions, more pictures, etc.