A logic probe

The best tool is the one that you create yourself. Not because it has better quality than a bought tool, but because of what you learned in the process, and because, since you understand fully how it works, you understand the tool’s limitations.

This is a logic probe I made, using a design by Tony van Roon. I can’t suggest his circuits page enough.

Here’s the circuit I used. If you want to replicate the same circuit, I suggest you go to the circuit page, where you can find a lot of more information about it.

(the circuit at the left of C1 is the probe, and at the right of C1 is the pulse)

A logic probe is a tool that can be used to identify the logic level of a signal: 0 or 1. Of course, you can do that with a multimeter, but this is much slower, and the multimeter also makes no distiction between a 0 signal and a high impedance line.

Logic probes are specially useful for debugging logic circuits with wide buses (for example, a 16-bit bus).

This probe has two advantages over simpler designs out there:

There are two leds, one for 1 (red) and one for 0 (green). When the probe is not connected to anything (or is connected to a high impedance line), no leds light up.

There’s a yellow pulse led, that lights up for 200 ms whenever there’s a a pulse, regardless of its length. This way, a 1 ms pulse becomes visible.

My goal was to be able to put this whole circuit into a magic marker. For that to be possible, after prototyping the circuit in a breadboard, I decided to solder it using the dead bug method. In this method, the components are soldered directly one to another, in a format that ends up resembling a dead bug.

So, this was my final assembly. Doesn’t it look like a dead bug? It is terribly ugly, but it was just perfect to fit in my magic maker.

I envolved the whole thing in hotglue, so that the thing would remain firm after inserting into the magic marker. That was a mistake. A few wires broke, so I had to remove the hotglue (breaking even more wires), and resolder the whole thing again. What a mess!

So I finally mounted the circuit into the marker. I hotglued a nail on the tip of the magic marker, attached a little board with the leds and added a socket to the end of it, in order to attach the power wires. Here’s the final result:

All things considered, I was pretty happy with the result! Now I have a logic probe for years to come. However, if I were to do this project again, where’s what I would change: