With analogue circuits, we need to measure voltages, currents, waveforms, and the like. With digital circuits we often only care whether a signal is logic high or  low.

A logic probe allows you to peek in at logic signals inside an operating circuit on an as-desired basis; no need to wire up LEDs to monitor signals (although that's sometimes exactly what you want, and we'll cover that later in this guide.

You connect the probe's Vcc and ground to their counterparts in the circuit being tested and you use the probe connection to touch points in the circuit where you wish to know the logic level. Be careful not to short connections (e.g. adjacent pins of a chip) with the probe. With digital circuits nothing really bad should happen (usually), but you won't get an accurate measurement.

You can buy a logic probe for $25-$50 from Amazon, Mouser, or the like. But we're makers and there's not a lot to it. So let's make one.

After looking around the internet for a simple, cheap, and easy to build probe I found the article below. It describes (including schematic and build suggestions) a simple transistor based logic probe.

I replicated the schematic in Eagle so that I could make a PCB. While you should have a look at the original article, it assumes some knowledge of discrete transistor circuits. A simplified description of the circuit's operation is below.

Operation

You start by connecting the 5v and ground connections to the 5v and ground of the circuit being tested. If you are powering the probe with another supply, the will need to have a common ground as usual. Then you simply touch the probe connection to the signal you want to check. If it is a logic low (< ~1.2v) the L LED will light (I use red) and if it is a logic high (> ~2.4v) the H LED will light (for which I use green).  If the probe is unconnected, neither LED should be lit.*

If the signal is switching between high and low the LEDs will flicker between the two, clearly toggling back and forth if the frequency is low, and both glowing at something less than full brightness if the frequency is higher. In this case the relative brightness can give you an idea of the duty cycle even though you can't observe it directly.

How it works

When the probe is unconnected, R1 and R2 set the voltage at the base of T1 such that it turns on just a bit which causes enough voltage drop across R5 to turn T3 turn on which keeps T4 off, meaning LED2 is dark. At the same time current running through T1 (and R3 and R5) is small and results in a small enough voltage across R3 to keep T2 from turning on, keeping LED1 dark.

When the probe is at a logic high (>~2.4v) T1 is held strongly on, keeping T3 on which keeps T4 off and LED2 dark as before.  In this situation the probe voltage will be higher than when unconnected so the current through T1, and hence R3 will be higher. This means a bigger voltage drop across R3, which lowers the voltage at the base of T2 making it turn on (notice it is a PNP transistor, not an NPN like the others so it sort of works backwards), lighting LED1.

The final case is when the probe is at logic low (<~1.2v). This will cause T1 to turn off, turning off T3. This, in turn, lets the voltage at the base of T4 rise (though R7 and R9) enough to turn it on causing current to flow through LED2, lighting it. There's no current flowing through R3, thus no voltage drop and T2 stays off, keeping LED1 dark.

Note that the BC177 (or BC557) can not be replaced with a 2N3906. I tried since I had 3906s in stock.

One unanticipated feature of this probe is, since it is transistor based and not TTL based, it works fine with 3.3v circuits. You do have to power it with 5v, though. And make sure you have connected the grounds together.

_{* The high LED might light dimly when the probe is unconnected. That will depend on the LEDs you use. I have some very bright LEDs which are especially nice for this project and the H led glows quite brightly (about half it's "on" brightness) when the probe is unconnected. Changing R3 from the 15K that is specified in the original circuit to 1K (meaning the voltage drop is lower for a given current) makes it work as expected. So depending on your LED, you might need to play around with R3.}

Building one

The circuit is quite simple to build on a piece of perfboard. That said, I've made a PCB shaped to work well as a probe (i.e. could be placed in a thin, transparent plastic tube or 3D printed case) that will be available to order on OshPark. You have to order 3 at a time there, so I plan to sell it on Tindie as a bare PCB, a kit, or assembled.

The power connection is easy: just solder on a couple wires (colour coding is a good idea) and terminate them as desired. I put a pair of female header pins on one using the make-your-own ribbon cable wires. One end gets soldered to the board, and the other can plug into my breadboard power supplies. I can easily use a couple long male header pins to convert it to plug into the power rails of a breadboard. You can use longer wires with alligator clips on one end if that's more useful. You could use a little heavier connector and make replaceable ends. It really depends on how you want to connect it.

The probe is a bit more problematic. This probe is designed to hold in your hand and poke at signals points in your circuit. That means you want something rigid, that extends from the end of the PCB far enough to reach into the circuit. For the same reason, it needs to be fairly thin; you want to touch the signal you are interested in, but nothing nearby. My solution was to use the same kind of wire as above, one with a male end in this case. To stiffen it I used pieces of ziptie like a splint you'd use for a broken leg.

I cut the wire to the length I wanted and soldered the cut end to the probe connection on the board. I cut one of the zipties into pieces of a length that would leave the probe tip exposed and extend over the board slightly.

I then used a short piece of shrink-wrap to secure the  pieces of ziptie to the probe wire.

Aanother piece of shrink-wrap was used to cover the entire thing (leaving the tip exposed) which secured the bits of ziptie to the wire. 

As a final touch, I used another ziptie to secure the splint to the board.