Picture of 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

Picture of 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.

TommyT19 months ago

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

pretty please

billyboy6663 years ago
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!
manuka3 years ago
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
Mattosx (author)  manuka3 years ago
True. I just have a tendency to use 5V supplies.
ikožar3 years ago
The schematic is wrong. LEDs are connected wrongly.
Mattosx (author)  ikožar3 years ago
Fixed rightly now!
Should the ironing be done for 15 minutes or 15 SECONDS? I'm guessing it should be for the shorter time period.
Mattosx (author)  ckoehler19043 years ago
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,