Ode to Bodacious Breadboards, Part 5

As we are all coming to discover, there’s a lot more to breadboards than first meets the eye. For example, I just ran into a problem with one of my own breadboard-based projects that had me scratching my head and saying, “Well, bless me” (or words to that effect).

We will return to my conundrum a little later, but first let’s remind ourselves that, in Part 4 of this ever-growing series, we considered one way to power a breadboard as shown in the image below.

As part of this, we discussed the reason for deploying the links in the middle of the power and ground rails. We also discussed the rationale behind having the wires linking the lower and upper rails on the far side of the board to where the power and ground wires come into the board, while the two light-emitting diodes (LEDs) and their current-limiting resistors are located on the same side of the board as the power and ground wires coming into the board.

In a crunchy nutshell, if both the LEDs light up when we apply power to the board, this gives us a high level of confidence that all our power and ground rails are intact before we plunge headfirst into the fray with gusto and abandon (and aplomb, of course).

One way to power a breadboard. CLIVE "MAX" MAXFIELD

Our original discussions on this topic were prefaced by my saying, “Assuming we don’t want to do any soldering, one way to power a breadboard is…” Suppose we aren’t afraid of engaging in a little soldering? Even if you are new to all this, as I said in Part 3: “… if you feel you’d like to dip your toes into the soldering waters, then may I be so bold as to suggest a great book called The Basic Soldering Guide Handbook that was written my good friend Alan Winstanley.”

I’ve also mentioned my friend Joe Farr on multiple occasions. One of the things Joe does is to take a pair of 0.1-in.-pitch long-tailed header pins to which he attaches a LED and its current-limiting resistor. Joe has a drawer containing a bunch of these pre-built, so he can pull them out and attach them to his breadboards without wasting any time.

Joe’s projects typically involve multiple voltages (–12 V and +12 V for the analog portions of his circuit and +3.3 V and +5 V for the digital portions of his circuit). Thus, he uses different LED colors and resistors for the different voltages. By comparison, since most of my own projects employ only 5 V, I’ve created a bunch of green and blue versions with the same resistors. (Why two colors? Why not?) One trick is to always associate the current-limiting resistor with the LED’s anode because this is easy to remember and it speeds inserting the header pins the right way.

Pre-building some breadboard power rail indicators. CLIVE "MAX" MAXFIELD

Both the green and blue LEDs in my box of bits have forward voltage drops of 3 V and maximum forward currents of 20 mA. Assuming a 5 V supply (because that’s what I’m working with), using Ohm’s law of V = I × R, and rearranging things to give R = V/I, the minimum value we could use for our current-limiting resistor would be R = (5 V – 3 V) / 0.02 A = 100 Ω (with brown-black-brown color bands). But this would have the LED running at full brightness, which would hurt my eyes. Instead, I’m using 680 Ω resistors (with blue-gray-brown color bands). Rearranging Ohm’s law once again to give I = V/R will result in I = (5 V – 3 V) / 680 Ω = ~3 mA, which will drive our LEDs more than brightly enough for what we are doing here.

Based on our handy-dandy power rail indicators, a new possibility springs to mind as shown below. As before, we bring our power and ground into the bottom right-hand corner of the board. And, as before, we locate the wires used to link the lower and upper pairs of power and ground rails on the left-hand side of the board. Now, however, we can simply stick one of our new power rail testing LED assemblies in the upper-right-hand corner of the board.

An alternative breadboard power scenario. CLIVE "MAX" MAXFIELD

Once again, the LED’s lighting gives us a high level of confidence in the integrity of both sets of power and ground rails. Also, doing things this way helps to preserve valuable real estate in the middle (the “working area”) of the board. Of course, when you come to think about it, we could have used a single LED in our original “solder-free” scenario. In that case, however, it just seems more intuitive to link the LED between the upper and lower rails. And, in turn, this requires us to deploy two LEDs to verify both sets of rails. 

Can you think of any way in which we could further improve our power presentation? Well one of the things I tend to do is to overengineer everything. Bearing this in mind, I would opt for the beefed-up scheme shown below.

Another alternative breadboard power scenario. CLIVE "MAX" MAXFIELD

In this case, we are using two sets of wires to link the lower and upper pairs of power and ground rails—one pair on the left-hand side of the board and another pair on the right-hand side. There are several advantages to this scheme. First, by doubling the wires we’ve halved their resistance and their associated voltage drop (this will be miniscule, but still…). Second, we’ve increased redundancy because our system will keep on going even if one of these wires fails or becomes disconnected. Third, it provides us with a tenuous excuse to use two LEDs (as if we needed an excuse) and, since we are using two LEDs, we might as well use different colors.

At the start of this column, I mentioned that I ran across a problem with one of my own breadboard projects. In fact, this uses two breadboards connected as shown below.

My current breadboard-based project. CLIVE "MAX" MAXFIELD

This is a DIY clock project. In the lower-left-hand corner we see a DS3131 real-time clock (RTC) breakout board (BOB). In the upper-left-hand corner we see four 7-segment displays. We are using the same eight digital pins (2 through 9) from the Arduino to drive all four displays. We are multiplexing the displays using four BC377 NPN transistors.

Originally, the four control signals feeding the bases of the transistors were being driven by individual digital pins (10 through 13) from the Arduino. Everything was working perfectly. But I decided to free two of these pins up by implementing a 2:4 decoder using a couple of jelly-bean SN7400-series integrated circuits (ICs). The 7404 contains six NOT gates (we’re using only two of them) and the 7408 contains four 2-input AND gates.

“This is going to be easy peasy,” I thought. “There are only two 14-pin chips,” I thought. “I’m using only six primitive logic gates,” I thought. “What could possibly go wrong?” I thought. 

You’d think that, as old as I am, I would know better than to tempt fate in this way. I will explain more in my next column. Until then, as always, I welcome your insightful comments, penetrating questions, and sagacious suggestions. Also, if you have any breadboarding tips and tricks you’d care to share, please feel free to email me at [email protected].