Dead-Bug Logic Probe in a Magic Marker

Logic probes are simple but handy tools that can be had for a couple of bucks. They may not be the sexiest pieces of test gear, nor the most versatile, but they have their place, and building your own logic probe is a great way to understand the tool’s strength and weaknesses.

[Jxnblk]’s take on the logic probe is based on a circuit by [Tony van Roon]. The design hearkens back to a simpler time and is based on components that would have been easy to pick up at any Radio Shack once upon a time. The logic section is centered on the venerable 7400 quad 2-input NAND gate in the classic 14-pin DIP format. The gates light separate LEDs for high and low logic levels, and a 555 timer chip in a one-shot configuration acts as a pulse stretcher to catch transients. The DIP packages lend themselves to quick and dirty “dead bug” construction, and the whole thing fits nicely into a discarded marking pen.


It’s a simple build and a nice form factor for a useful tool, but for an even slimmer package like an old syringe you’ll probably have to go with SMD components. And when you graduate from the simple logic probe, you might want to check out the capabilities of this smart probe.

Adding a Digital Back to a Sweet Old Camera

[Eugene] wanted to use his vintage Leica M4 as a digital camera, and he had a Canon EOS 350D digital camera sitting around unused. So he Frankensteined them together and added a digital back to the Leica’s optical frontend.

It sounds simple, right? All you’d need to do is chop off the back from the EOS 350D, grind the digital sensor unit down to fit into exactly the right spot on the film plane, glue it onto an extra Leica M4 back door, and you’re set. Just a little bit of extremely precise hackery. But it’s not even that simple.

Along the way [Eugene] reverse-engineered the EOS 350D’s shutter and mirror box signals (using a Salae Logic probe), and then replicated these signals when the Leica shutter was tripped by wedging an Arduino MiniPro into an old Leica motor-winder case. The Arduino listens for the Leica’s bulb-flash signal to tell when the camera fires, and then sends along the right codes to the EOS back. Sweet.

There are still a few outstanding details. The shutter speed is limited by the latency in getting the signal from the Leica to the 350D back, so he’s stuck at shutter speeds longer than 1/8th of a second. Additionally, the Canon’s anti-IR filter didn’t fit, but he has a new one ordered. These quibbles aside, it’s a beautiful hack so far.

What makes a beautiful piece of work even more beautiful? Sharing the source code and schematics. They’re both available at his Github.

Of course, if you don’t mind completely gutting the camera, you could always convert your old Leica into a point and shoot.

TIQ Probe is More Logical than Most


We had [Mark] on our “dance card” for people to find at Maker Faire. But before we could track him down he bumped into us holding the TIQ Probe in one hand and a testing box in the other. TIQ is conceived in the form factor of a traditional logic probe but thanks to the Cypress PSoC 5LP inside it’s much, much smarter than the decades-old bench tools. Sure, it can tell you if that uC pin is a 1 or a 0, but it can also detect what type of signal it’s probing and has built-in protection for over-voltage.

The point of the tool is to bridge the gap between things which would be measured with a DMM and those measured with a proper Oscilloscope. We think he did a pretty good job of including the things that someone just starting out without expensive bench equipment might want. For instance, you can set it to trigger on common data protocols like i2c, and use the probe itself as a rudimentary pulse generator.

The bulk of the details on the probe can be found on its Kickstarter page (which has just a few days left). You may also be interested in his company page. We’re curious about the insides of the test rig he was hauling around. [Mark] is a regular reader so hopefully he’ll leave a comment below with the details of that black box.

Forget treating your diabetes – use your insulin syringe as a logic probe


Hackaday reader [JumperOne] was in need of a logic probe that he could use to reliably test some tiny .5mm pitch IC pins. The probe that came with his oscilloscope was a bit too big and not near sharp enough to do the job, but he figured that a syringe might do the trick nicely.

He drilled a small hole near the business end of the syringe, through which he fed a piece of stripped twisted pair cabling. [JumperOne] then soldered a pair of pins to a small piece of coaxial cable, attaching the opposite end to the twisted pair already in the syringe. After carefully coiling the thin cable around the needle, he secured the coaxial cable and its pins in place with a bit of hot glue.

[JumperOne] says that his makeshift logic probe works very well and the sharp needle would easily pierce through any oxidation or solder mask that stands in its way. One extra benefit of using a syringe as a probe is that they come complete with caps which help protect both ends of the delicate tool.

HEX Out reveals the secrets your data bus holds

[Quinn Dunki] is looking to augment the tools she has available at her electronics bench and built the HEX Out as a mock-logic sniffer. The device reads 8 or 16-bit inputs, showing the current state of those connections on a 7-segment display. This requires that you’re comfortable reading Hex codes, but if you’re not it’s almost like studying flash cards; before long you’ll be able to read them without thinking about it.

She’s blogging about the design and build process in three parts. The link above is the first installment where she shares the development process for the top layer which hosts the display hardware. The other two parts should be up for your enjoyment in the next couple of weeks.

You’ll notice her design on this portion of the project still requires a lot of point-to-point soldering, even though she etched her own circuit boards. We didn’t look too closely, but it seems this would be worth going to the trouble of etching a double-sided board if you can.

UPDATE: Part Two is now available

DC Entries: LVP/Logic probe and a wireless antenna

I’m sick today, but finally getting some entries up. [Albert] designed this board which can be either a low voltage PIC programmer or, if you lack a programmer to get the initial flash data on the PIC, the through hole components make a logic probe. He rounded out the board with a silk screened logo and a prototyping area. He was also kind enough to provide an eagle library with the Hack-A-Day logo.

Our second featured entry of the day: [Pablo] put together a simple Hack-A-Day 2.4ghz antenna. He built and tested it – It functions nicely as a directional 2db antenna. His proposed use: limit your neighbors access to your AP by aiming the unetched backside at them.