Here is a logic probe made out of some basic parts: A hex inverter, some resistors, transistors, and a paper clip that is acting as a probe. This instructable contains the PCB art so you can etch your own.
Step 1: Diagram & Schematic
Below is a PCB layout explanation and a schematic. A drop down resistor was included for the first input of the hex inverter to prevent it from hanging states whilst not in use.