Logic Probe With a Paperclip




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!



    • Trash to Treasure

      Trash to Treasure
    • Paper Contest

      Paper Contest
    • Pocket Sized Contest

      Pocket Sized Contest

    8 Discussions


    4 years ago on Introduction

    logic probe was awesome, what id love is a detailed explanation to your etching process

    pretty please


    6 years ago on Introduction

    The PCB shows the unused inputs 'floating'. They should really be tied to one of the supply rails to prevent stray oscillations. I would also add a decoupling cap (100nF) to get rid of noise on the supply leads. Great gadget, just built one!


    6 years ago on Introduction

    The supply ( which of course is usually from the circuit under test) doesn't need to be an exact 5 volts! Unlike TTL ( which needs a firm 5V supply), the "HC" logic family are not picky on this, & usually are quite happy with supplies between 2 and 6 Volts

    1 reply

    6 years ago on Introduction

    Should the ironing be done for 15 minutes or 15 SECONDS? I'm guessing it should be for the shorter time period.

    1 reply

    Reply 6 years ago on Introduction

    Minutes, I always go longer so I don't have to sand off the pattern and re-iron. Five minutes would probably do it though,