Friday, January 8, 2010

Chapter 1 - Exercise 4

So, Exercise 4 has quite a bit of reading... I'm actually going to go back through this once more and make sure I really understand all of this - there's a lot of math and equations, but fortunately it's fairly simple stuff - no Calculus or balancing of chemical equations.

I guess this is as good a time as any to remind my readers that I'm not going to always be going over the reading material or necessarily summarizing it - I'm more interested in documenting the experiment and hands-on stuff. That said, I'll probably still discuss the theory stuff here and there because there is some stuff that I either don't get (right away) or just need to write down to cement it in my head.

So, the first thing I'll share with you here is my taking apart of a potentiometer. Just grabbed a pair of needlenose and pulled up 4 metal tabs. Easy enough. And a closer look under the hood shows me how this thing kind of works... you can see the little "bump" in the metal wire under the rotating knob and how it rubs against the metal ring-thingy... (Sorry to get really complicated and throw a lot of techno-babble at you.)

So, after putting it back together, I rigged up the handful of connections described and starting on page 19. If you missed my earlier post, I was only able to grab a 5k potentiometer instead of the 2k mentioned in the book - this just means I can crank up way more resistance with mine.

After hooking it up to a yellow LED, I got a little bit of light... then rotating the potentiometer's knob clockwise, the LED got brighter. Very cool - less resistance means brighter LED.

I did test this with my multimeter at the two different areas mentioned on page 21 - and you can see in my pictures that my voltage is about 6.4v - add up the voltage measured at the LED and then at the potentiometer and I'm getting a fairly accurate reading. (The author tells you why there's some slight variations in readings... did you catch that?)

Next, I hooked in my multimeter to measure amperage. Even with the gator clips, it can be a bit tricky to connect a wire to a probe. And taking measurements at the two different spots, I got the same value. Not a surprise as that's exactly what I was expecting.

And finally, I took two 1k resistors and measured their resistance in series and parallel (page 25) - this isn't an actual experiment that's shown, but I highly encourage you to do it - see my photos for how I clipped the resistors together (in series and in parallel). As you can see from the two photos, resistance is exactly as expected (2k for series and 500ohms for parallel).

The rest of the pages for Exercise 4 focus on some math, but don't ignore it. We finally learn about wattage, how to read a data sheet (and understand why you can burn out one LED but maybe not another similar looking one), why our tongue didn't burn off back in Experiment 1, and, my favorite, how to do the math to figure out the proper resistor to put into an LED circuit.

I know I said I'd be covering Exercise 5 today, but you'll have to forgive me... a slight dusting of snow today caused schools to be canceled (crazy - we Southerners just don't know how to drive in snow/sleet/ice) and I wasn't able to get to the grocery store while watching my toddler son. Grocery store, you ask? Well, you either don't have the book or haven't peeked ahead to Exercise 5. No worries - I'll try to get Exercise 5 written up and posted by Sunday.


  1. This is fantastic. I ordered the book last week. Should be getting it pretty soon. I look forward to following along. It's going to be fun. Thanks for doing this.

  2. You're welcome - you're going to love the book - it's very well done.

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  4. Sorry to ask yet another question in your comments, but on this test, the multimeter i got from make is not working as it should at page 23 and 24. I'm getting all zeros. Does anyone have any idea why that might be? I'm doing it exactly how it says. The multimeter is working fine when testing voltage and resistance.

  5. one of the things I found is that I hate trying to clip things like the book said to do here so I broke out my soldering gun attached short leads to the potentiometer and with a breadboard from american science and surplus did the experiment. I hate doing things the hard way.

  6. Some notes from our run-through of this experiment:

    * The pot has three connection points on it - let's call them bottom/middle/top. If you connect to the bottom/middle, having the knob fully counterclockwise will provide minimum resistance, and fully clockwise will provide no resistance. If you connect to middle/top instead, you'll see the opposite result. Keep this in mind, because if you hook up the circuit wrong, your pot will be initially fully open instead of fully closed and you could burn out the LED instantly.

    * That being said, we never actually burned out the LED (as in 'go dark'), but I am pretty sure we ruined it. As we turned the pot up, we'd get a progressively brighter, pretty green (was a green LED) light, but after we passed a certain threshold, the point of light would change to this nasty flame-like color and the LED would get VERY hot (the blister on my fingertip speaks to this). Thereafter, that LED wouldn't respond smoothly to voltage changes like before. We tossed that LED.

    * I noticed that when I reversed the multimeter leads while measuring voltage (e.g., if I was measuring the LED with the red multimeter lead on the long LED leg, and then switched to the black lead on that leg), I'd get the same reading, only negative (e.g., 2.14 V vs. -2.14 V). Not sure the significance, but thought I'd point it out.

    * If anyone else is working through this book with little kids (my son is 9 -- bright with a mind for engineering/science, but still 9), this may be the part where the math gets a little over their heads. The key points I had my son take away were:
    1) The voltage used by the circuit's components adds up to the total voltage provided by the power source
    2) The amperage is constant anywhere in the circuit
    3) The volts/amps/ohms are mathematically related to each other, and thus we can use math (instead of trial-and-error) to figure out which 'strength' of components we should use in our circuits
    I told him I'd handle the math until he's a little older, but I do want him to absorb those simple principles at this point.

  7. I had some fun with this one. When I took the potentiometer apart I noticed that connecting to the third terminal would give you the opposite result (turn the pot one direction, and the resistance across one terminal increases and decreases across the other terminal).

    So I hooked up two LEDS at once, a red one and a green one. And I was able to turn the pot, smoothly dimming the red one as the green one got bright and vice versa. It was pretty neat, had a real "Divert power to the shields!" feel to it.

    In the process I noticed some differences between the red and green LEDS. The green one could get a lot brighter for the same voltage drop, and seems to come on more "smoothly" than the red. As in, the red stays really dim for awhile as you increase the current, but brightens really quick in a certain range. The green one had a much more consistent rate of brightening. I don't know how much this has to do with my eyes (People see green light easier than red, and I wasn't doing this in a dark room) and how much is physical differences in their properties, but it was pretty fascinating.

  8. As indicated in the book for experiment #4, I hooked up my meter in series to measure the amperage and found that I initially got a 1 mA reading (with the pot turn all the way down) but then the meter drifted down to .01 mA within a minute. This happened for both before the LED and after the pot. When I crank up the pot, the reading goes up briefly, but then drifts down again. Is this normal for amperage readings?

  9. Boy do I feel like a dummy. I'm using a multimeter that's new to me and I just saw the letters "AC" next to "mA". I toggled to "DC" and am back in business.