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.
Step 2: Making Your PCB
The PCB itself is 0.625 inches by 3.5 inches. The PDF of the artwork is included below. I filled in alot of the empty white space with some shapes to lessen the amount of etchant needed.
Etching the board:
1) Lightly sand the copper clad board. This seems to make the pattern stick better.
2) Set your laser printer on max toner density
3) Print out the PCB art on magazine paper. It may jam a few times, but it gives great results. The more pricey magazines have slightly thicker paper and this jams way less often than the cheaper magazines.
4) Iron the PCB art onto the board at your iron's maximum temperature. Apply alot of pressure for 15 minutes. Iron with alot of pressure around the edges. The pattern had a tendency to be poor toward the edges without care enough to iron there.
5) Soak in water to dissolve paper. Remove any excess paper.
6) Fix any broken traces with a sharpee.
7) Etch in ferric chloride which you can get at Radio Shack.
Step 3: Drilled Holes With a Hand Drill
I prefer to drill my holes with a hand drill. It just gives so much better control.
Step 4: Populate Your Board and Solder!
Populate your board and solder and you're done!