Monday, July 7, 2014

Experiment 6 Part 2 (Chapter 6) -- Can I Get Some Feedback?

I've upgraded my breadboard from the previous post to include the second 500k trimmer. This new circuit is all about helping you understand positive feedback and how it can be used to eliminate "hunting." You'll also get a really good understanding of hysteresis (I also broke down and went on the hunt for the proper pronunciation -- Hiss-Ter-eesis ), that sticky type behavior that we see all the time in our daily activities but never think twice about -- why doesn't the air conditioner or heater constantly turn on and off with minor fluctuations in the surrounding temperature, for example?

It took me a couple reads of Chapter 6 to really REALLY understand all the concepts in this chapter, but I get it. I'm also quite happy with the explanation of how the LM339 comparator works -- of course there would have to be some sort of transistor tucked inside! (Actually, more than one!) It's a lot to absorb, so I'll probably have to come back and reference this section again one day. The good news is that I understand how and why this inexpensive chip can be so useful to circuit builders...

Don't skip over Charles' explanation for how these inexpensive circuits can be used in place of a microcontroller... like an Arduino. I love microcontrollers, and I've got a lot of experience with the Arduino, but at $20+ each, any kind of permanent project you are building may have a lot of wasted money inside if you go the microcontroller route. I really like how Charles is explaining why the old school methods of finding the right components (and the lowest priced version to suit your need) is so important. Not only do you avoid the programming aspect of dealing with microcontrollers, but with simple trimmers you get some manual control that may be all that's needed to fine-tune a small DIY device.

Here's my video showing the new 500k trimmer installed. As expected, the positive feedback is preventing me from intentionally triggering the "hunting" performed by the comparator and the flicker of the LED. I played with this circuit for some time, trying to get a flicker... and I couldn't do it. No amount of tuning either or both of the 500k trimmers would let me get a flicker. The LED was either On or Off. Cool!

I've read ahead a bit into Chapter 7... I'm going to have to go on the hunt for a cheap, inexpensive digital clock that runs on only 3V (two AA batteries) and not AC. Chapter 7 looks like it's going to be fun... a lot of circuits to build and some hands-on with a cheap clock, but still... fun. Once again I'll have to break Experiment 7 up into parts, so go ahead and read Chapter 7 so you'll be familiar with the next two or three parts that I will be writing up.

Note: By the way... in the "try to learn something new everyday" category, put me down as just learning that there is an upgrade to the 7805 voltage regulator that puts out a standard 5V to your breadboard. It's the 7806... and guess what voltage it's designed to provide? 6V. And here I thought so many of these numbers were just made up and not really useful... are there 7807 or 7808 versions? Yep and yep. and 7809, 7810, 7812, 7815... the list goes on.

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