Monday, March 22, 2010

Chapter 4 - Exercise 21 - Completed

One thing I'm certainly learning through this process is that no matter how many times you check and double-check your circuit, you've missed something... that is, until it works. The author does a good job of warning us about accidental damage to many components, and my first thought is always "must be a faulty component" - I need to change that to "must be a faulty human."

The good news, for me, however, is two fold - one, my regulator wasn't damaged. But two, the 7432 chip WAS damaged. But the question is whether that damage came about because of my faulty wiring - I'll likely never know, but when I switched out my 7432 (after finding a wiring error - more on that shortly), it started working.

If you look at my picture closely, you'll notice a few differences in the circuit I built and the one on page 210. First, I didn't have a .33 capacitor, so I substituted a 2.2. I tested this before building the circuit and was able to determine that it didn't have any effect on the 5V regulated voltage...

Next, I didn't have two of the 0.01 capacitors so I took a chance and substituted two .047 capacitors for each... I wired them in parallel (opposite of resistors - parallel you add their values) and crossed my fingers that this substitution was okay.

Now - for the faulty wiring. Figure 4-95 on page 210 is fairly easy to follow... but don't make the mistake I did. I had that 10K resistor at the bottom going into the negative column, not the positive. Could that have damaged the 7432 chip? Who knows, but after catching and fixing this error, the circuit didn't work. I went back... carefully checked off each wire in Figure 4-95 and verified I had no shorts and that all wires were done correctly. They were (hopefully).

So next I wondered if it might be a faulty chip - luckily I started with the 7432 instead of the 555 chips... as soon as I replaced the single 7432 with a new one, circuit worked. I pulled it out, put in the old one (same orientation) and the circuit didn't work. Put in the new one... circuit worked. Hmmm... how about that?

I'm including a video below showing the circuit working... I actually soldered wires to my S1 because I couldn't get good connectivity by just twisting the wires into the ends.

So, many lessons learned... again. Check my wiring. It's likely MY fault, not a component. I really am not rushing these builds, but I guess I need to slow down even more... the errors are easily corrected... but also easily avoided if I put a little more effort into slowing down and verifying all my wiring first... I hope my readers are learning from my mistakes and avoiding the frustrations I'm encountering.


  1. Hello James,

    I'm glad you figured it out. I don't see how applying negative voltage to the reset pin of the 555 timer could have damaged the logic chip; it would just put the timer perpetually in "reset" mode, so that its output would not go low, no matter what you did. So, it's a mystery. But, the logic chip is CMOS and vulnerable to static electricity, also easily damaged if you applied power to it with reverse polarity. I have never zapped a CMOS with static myself, but supposedly it happens.

    The whole issue of circuit errors is very interesting to me, because it's undoubtedly a common problem, and I don't know how to fix it. Or rather I do know what helps, but, I don't think the advice is useful.

    I just built a new circuit for a game (for a column that I have started writing for Make magazine) and initially, I had all kinds of weird problems. When it's your own circuit design, naturally you have to wonder if the design is the problem. But after I forced myself to stop and think instead of charging blindly ahead, I realized that the *type* of symptoms I was seeing would be typical of a "floating" pin on a CMOS chip, i.e. a pin which is not connected to positive or negative, and is free to pick up any ambient electromagnetic radiation.

    So, I knew what to look for, and sure enough, I had omitted one pulldown resistor.

    The lesson here is that it's so much easier to "debug" a circuit if you have an accumulation of past experience. But the intended audience for my book consists of readers who do not have past experience! So, I feel a bit helpless, as the writer. Maybe I should have devoted more space to the process of tracing and fixing errors. Maybe a whole book could be written on this subject alone.

    I also wonder if errors afflict some people more than others. I was always a lousy proof reader, for instance, because I couldn't "see" the errors. Probably there is a personality type that is very calm, super-methodical, and just follows steps A, B, C without missing anything. Alas I am not that type.

  2. Charles,

    99% of the errors I've encountered in my exercises were MY errors, not faults of the book... so rest easy. Those errors I have found were reported as errata but were not deal breakers AND I had enough knowledge from reading previous material in your book to figure out that it was an error!

    I think others have mentioned in previous comments that debugging is an art and something you get better at over time... some of the best advice I've received is to always check voltage and current readings throughout the circuit and that has given me at least a heads-up when I'm wondering if a component is functioning properly.

    I'm glad to hear you'll be having a regular column in Make - will it be a unique circuit every issue or sometimes just theory? Either way, it'll be something nice to keep the blog updated once I'm done with all the exercises.