Well, I must admit to being flabbergasted. I think I can say without fear of contradiction that my “flabber” has rarely been so “gasted.” The cause of my current state-of-gast was the rapturously raucous response to my previous post on DIY Logic Probes for Breadboard-Based Prototypes & Projects. 

It never fails to amaze me how the simplest things—like a flashing light-emitting diode (LED)—can keep me amused for hours. “Show me a LED flashing, and I’ll show you a man drooling,” as I’ve been heard to say, and I’m not alone. I used to work for a company that made state-of-the-art workstations. One day I brought a small breadboard-based gizmo into my office. Essentially it involved only an 8-segment red LED bar graph display presented in a dual in-line (DIL) package (DIP) along with a simple circuit that cycled around lighting random combinations of segments.

You wouldn’t believe how many grizzled, hardened engineers passing the open door to my office caught a glimpse of the flashing LEDs out of the corners of their eyes, backtracked, and entered my office to drool alongside me. (It wasn’t a pretty sight.)

The reason I mention this here is that much the same thing occurred when my previous column posted. I received numerous emails from members of the Design News community relating their own creations and experiences with respect to creating DIY logic probes and using them to debug circuits.

Related:DIY Logic Probes for Breadboard-Based Prototypes & Projects

At the end of that column, I made mention of the fact that I knew a technique for creating an 8-bit logic probe using just two components. “Tell more,” you said, so that’s what I’ll do (I live to serve).

I love DIL bar graph displays. I commonly play with 8-segment and 10-segment devices containing only red LEDs, but there’s a lot of fun to be had with multicolored 10-segment displays, some boasting three different colors and others flaunting four.

Just for giggles and grins, the following image reflects two 8-segment DIL red LED bar graph displays coupled with two different resistor treatments. One interesting point is that the packaging of most of the bar graph displays I’ve run across makes it rather difficult to determine which pin is number 1. If you look closely at this image, you’ll see that the corner of the package associated with pin 1 has a slight chamfer. To be honest, I’ve exaggerated this for this image; you really need to look hard in the case of real devices.

8-segment DIL red LED bar graphs and resistors.  CLIVE “MAX” MAXFIELD

I should also note that, in my experience, pins 1 through 8 of the bar graph display are connected to the anode (positive) terminals of the LEDs, but I have no idea if this is a hard-and-fast rule. If you don’t have a datasheet to hand, then it’s always worth testing things beforehand. Also, you won’t damage anything if you happen to plug the display in the wrong way. (It just won’t work.)

Related:Ode to Bodacious Breadboards, Part 1

In the real world, we’d use some other components—or a microcontroller—to drive the displays. I’ve shown certain segments connected directly to the power rail only for the sake of simplicity.

As a somewhat related aside, have you ever wondered why the horizontal power and ground strips on breadboards are presented in 5-pin groups? This often makes it difficult to present your components (and create your diagrams) as neatly as you would prefer. I was chatting about this with my chum Joe Farr earlier today. One thought is that this allows the manufacturers to use the same 5-pin metal strips they use to connect the vertical columns in the main body of the board, thereby enabling them to reduce their build of materials (BOM). I’d be interested to hear if you have any thoughts on this.

Returning to the image above. This is loosely based on a project I’m currently playing with. My power supply is 5 V, and I’m assuming a forward voltage drop of 2 V for my red LEDs, so I’m using 560-Ω resistors to limit the current to ~5 mA. This is only 1/4 of the rated value for the LEDs, but it will be more than bright enough for what I want to do.

Related:Controlling WS2812 Tricolor LEDs (aka NeoPixels), Part 1

We could use eight discrete resistors as shown on the left-hand side of our breadboard. Alternatively, we could use a bunch of resistors presented in a single in-line (SIL) package as shown on the right-hand side of our breadboard.

You need to be careful here. The type we’re using is a 9-pin SIL containing 8 resistors sharing a common pin. This is called a “commoned resistor network,” but there are other types, such as an 8-pin SIL containing 4 individual resistors each using two pins. Also, you can get 5-pin SILs containing 4 resistors, 10-pin SILs containing 9 resistors, 10-pin SILs containing two 4-resistor commoned resistor networks, and others. There are also weird and wonderful SILs for specialist applications, bespoke configurations for special applications, and then things start to get complicated.

The reason I’m waffling on about all this here is that, just a couple of months ago, Joe taught me a simple but effective trick. Since he does a lot of work with 16-pin DIL logic devices, Joe has created a bunch of hand-crafted logic probes. Each is built using a DIL bar graph LED display and a SIL resistor pack as shown below. 

8-pin DIY logic probe. JOE FARR

As we see in the right-hand portion of this image, the pins on one side of the bar graph display are soldered to a row of header pins that will be used to plug into the breadboard. In the left-hand side of the image, we see that the pins on the other side of the bar graph display are soldered to a SIL resistor. A piece of single-core wire is soldered onto the SIL’s common pin.  

If you look carefully at the right-hand image, you can see that the left-hand corner has a slight chamfer. This means the header pins are connected to the anodes of the LEDs and the resistors are connected to their cathodes. The header pins will be plugged into the breadboard next to a logic IC, the common cathode wire from the SIL will be connected to a ground rail, so +ve on any of the header pins will light the corresponding LED.

Finally, on the off-chance you were wondering, the reason Joe used a 10-segment display and then covered two of the segments is that he purchased a job-lot of several hundreds of these components for peanuts a few years ago.

Joe tells me he has a drawer full of these assemblies, and that he invariably finds use for them when developing a new digital logic project.

How about you? Do you have any logic probe-related tips and tricks you’d care to share? As always, I look forward to casting my orbs over your captivating comments, insightful questions, and sagacious suggestions. (You can email me at [email protected].)