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A Simple Logic Probe

Gene Elliott

A few quick tips for the quick easy testing of your electronics / robot projects. We could all use a volt,ohm,amp meter, 'o' scope, signal generator, etc. for testing of our electronic boards. Now that is said, really how many of us can really afford or for that matter understand how to use all this equipment when first starting into this hobby? There is a tool, which is easily made to assist in this venture, a simple logic probe. A trip to the local Radio Shack, Active Electronics, Fryes store will grab the parts needed to get started.

probe.gif (22453 bytes)


Alternate for above is #10 or #12 ‘Romex’ copper house wiring scrap or ground wire (can be purchased by the foot at local home improvement store)


Disassemble the pen, toss the ink cartridge, and strip out the push button assembly. Drill out the push button hole to fit the LED snugly. Use a file to sharpen the brass rod or romex wire to a pointed shape (Like sharpening a pencil lead), Drill out the cartridge opening for a snug fit. Insert the brass rod into opening with about 1/4" poking out. Measure the taper inside pen housing to opening and remove brass rod solder on a blob of solder to the measured area so rod won't slip out. Solder a heavy wire to brass rod ( or use the ‘Romex’ wire) and to one leg of LED, measuring the wire so you get a snug fit between brass rod and LED when pen reassembled. I.e.: rod poking out, LED poking out. Drill a small hole for the jumper wire in the lower half of the pen tube. Insert the wire and solder the jumper wire to other LED leg, insulate LED and wiring from shorting together (heat shrink tubing or tape?) Test on 3 to 9 volt battery by clipping the alligator on a terminal and touching brass point to other. Now reverse - lights red one way and green the other. Test on 12-volt supply and LED turns orange.

You have just successfully made a very useful tool to test electronic circuits.


Clip the alligator to a known ground on your circuit a carefully touch various points to see if the LED lights, if you get a steady light you have found your + or - voltage, if blinking you have found a pulsed or switched signal. You can see a PWM signal as a ramped low to high pulse.


You can buy this device ranging from a few dollars for a cheaper version to lots for one with all the bells and whistles. The experience gained from 'Home Brewing' your own will assist in your future electronics ventures. Always take care not to short out anything and beware of shocks if more than 12 volts are in circuit or high voltage in TV, Radio, Microwave or A/C circuits. (Even low voltage can give you a nasty shock if enough amperage is there!)

I have not given exact measurements as your pen and electronics might differ from mine. This could be built in many ways and is intended to give you a basic idea on which to build some knowledge. Experiment until you find what works for you!

Good Luck and Enjoy!


I do not claim to be an electronics expert and have no formal education on the subject. Please be careful and do not blame the messenger for any failure of your circuits, or harm to your person from the use of this information.