Logic Probe Plans

Posted on Feb 7, 2014

This is a set of plans for an inexpensive, high performance digital logic probe that you can build in a few hours. It is small enough to use a plastic ball-point pen as a chassis, giving it a personalized touch and added style. The main advantage of this design is that the LED will only light when the probe tip is touching a voltage source (not fl

Logic Probe Plans
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oating). This is a necessary feature when working with tri-state logic, and is overlooked by most logic probe plans found in textbooks. Ball-point pen - This will be the housing for the logic probe. Chose a pen that unscrews at the middle, has a large barrel, and is made of plastic. (Metal will cause short-circuits, unless you are really careful. ) Dual op-amp - Chose an op-amp that is sensitive, has good rail-to-rail characteristics, a large input impedance, and an output current greater than 20 mA. Radio Shack part number 276-1715 should work well. Prepare the pen by removing the ink cartridge and by cleaning out any ink residue. Solder the probe tip to R2 and R3 as shown in the schematic and epoxy this assembly into the bottom of the pen. While the epoxy dries, prepare the top half of the pen for the LED by drilling a mounting hole if necessary. Drill another smaller hole in the side of the pen for the power leads to enter. Assemble the rest of the circuit as shown in the schematic above with wire wrap wire. Try to keep it as small as possible. Leave about 3 feet of loose wire for alligator clips one and two, but do not yet connect the alligator clips themselves. When the epoxy is dry, attach R2 and R3 to the rest of the circuit. Test the circuit by attaching the wire for alligator clip one to the positive terminal of a five volt power supply and the wire for alligator clip two to the negative terminal of the power...

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