February 2018

Lab 2: RC Circuits (Part 1 – low voltage experiments)

Exercise 2L.1.1 calls for figuring out the RC constant, and then R and C of an RC low-pass filter. In their lab, the authors hand the students a “wrapped” R and C pair. Since I didn’t have that option, I had to improvise. I had a pair of Elenco Resistor and Capacitor Substitution Boxes. I pulled off the knobs, spun them in my hands, and then reinstalled them and spun the boxes to random settings. Instant mystery filter. (Of course I then had to use a DMM and an LCR meter to reinstall the knobs correctly…)

Despite the book’s careful explanation, I initially set the oscilloscope time base too short, and struggled to get correct values. Eventually, I figured it out….

Exercises 2L.2.3 and 2L.2.4 gave me real pause and stopped me for quite a while. They called for using a transformer to examine noise on the 115V electric mains. While the transformer was one of the items on the parts list, it came as a bare transformer with some wires. I wasn’t about to stick the bare leads into the socket. Just soldering on an extension cord also seemed unsafe. So a more elaborate safety enclosure seemed in order. Details in Part 2.

Lab 1: DC Circuits

Lab 1 was very basic – using the breadboard, Ohm’s law, mostly things I already new. However, this lab helped me internalize one critical thing – output impedance. I’d read about it, but I didn’t really understand, on an instinctive level, why you want low output impedance and high input impedance in circuits. This was something that needed to be played with.

Exercise 1L4 was very satisfying – it involved measuring I and V through a diode and plotting the results. The resulting plot, done in Excel, matched exactly the expected diode curve. The science worked! See linear and log graphs – perfect straight line on the log graph, as predicted:

Exercise 1L3 required a little bit of work. The lab called for using a bare meter movement to do some of the experiments. Unfortunately, no movement was listed in the parts list. I ended up buying an uxcell 0-100uA ammeter from Amazon:

Worked well.

The lab guide called for soldering a pair of 1N4004 across the movement, one in each direction, to protect it from excess voltage. I did just that, as can be seen in the pictures:

The lab was otherwise uneventful.

Learning Electronics

I recently decided to learn more about electronics, and to do so in a more systematic manner. I’ve tinkered with electronics for decades, but my actual knowledge base is very catch-as-catch-can – bits learned for specific projects, online tutorials, half-remembered college physics, etc. So, thanks to recommendations on the eevblog forum, I picked up two books.

The first is “The Art of Electronics” (AoE) by Horowitz and Hill. It’s a textbook that purports to teach electronics “without all the math”. I find it does so to a degree. It’s certainly lower math than most EE classes on the topic would use, but there’s still tons of it. The math doesn’t really bother me, but it does assume at least some knowledge of integral and differential calculus. Regardless, the book is very good even if I’m not retaining 100% of all of the material on the first pass. I’m about 80% through the book at present.

I’ve also supplemented it with other sources, when I had trouble following something. Some of the videos from Khan Academy‘s Electrical Engineering program have been great. I watched a lot of their Amplifier and Semiconductor Devices videos. The other site I used a lot is the online textbook from All About Circuits. Frequently, by combining these sources, I’ve been able to figure out something I couldn’t from any one of them.

But all of the above is theory. I also wanted to build things. In comes “Learning the Art of Electronics: A Hands-On Lab Course” (LtAoE) by Hayes and Horowitz. This is the lab “manual” companion to AoE. It breaks the theory down even more, and presents real lab problems to do. I’m slowly working my way through the projects.

The biggest “flaw”, if one were to call it that, of LtAoE is that it’s meant for students taking the authors’ class, live, at Harvard where they teach. Which means it assumes access to parts and equipment that not everyone has. The book’s website helpfully provides a parts list, but it’s not quite complete. A few items are missing, but, more importantly, some of the items can’t be used as-bought. They need assembly or modification that are not necessarily obvious. In some cases these modifications are described in the book, but not always.

My plan is to write about my adventures with LtAoE in this blog. Specifically, I’m going to write about the things I had to do to make things work, as they might help the next student. No specific schedule is planned – this is a hobby, and my available time is very inconsistent. My current plan is to work through the Analog part of the book. Once that’s complete, I’ll decide if I want to do the Digital labs.