Wednesday, July 23, 2014

Experiment 10 (Chapter 10) -- Let There Be Light

Experiment 10 takes the circuit built for Experiment 9 and just adds in an LED, a transistor, and a few resistors. It's still using the LM741 chip for amplification of the voltage, so the idea here is that the electret can be used to light up an LED with sound.

I had no problems getting my LED to light up, but it did require tapping on the electret. The electret I'm using just doesn't appear to be as sensitive as the one used in the book... or I may have a wiring issue. Capacitors and resistors are all unique, so it's quite possible I just have a combination of components that isn't helping increase the sensitivity of the electret. Tapping on the electret, however, does yield results as you'll see in the video.

What I'm taking from Experiments 9 and 10 are an understanding of the concepts of a split voltage supply, amplification, and the function of the LM741. The experiment may not behave exactly as desired, but I understand what I'm supposed to see when this circuit is powered up.

Up next is Chapter 11 and Experiment 11 -- the chapter is somewhat involved and lengthy, and I'm only one read into it... I'm going to read it again and try to tackle all the parts involved over the coming week. There's a LOT to learn and absorb, so if you're heading towards Chapter 11... prepare for some good stuff.

Video for Experiment 10 below...


  1. Hi Jim,
    Hope you're well! Thanks again for the posts - this blog is great, and its exciting to see the kind of projects I could be doing when I complete the original Make:Electronics book!
    If you have some time, I'd appreciate your thoughts on this...
    Ive literally just started the original book, and I'm using a really cheap meter that i bought several years ago. Its manual ranging, and I was using it for the first experiment, checking the resistance of my tongue. With the setting at 200K, the value on the meter was 85....which I assume to be 85 kilohms. When I did the same measurement with the meter on a setting of 2000K, the display read 350?! Does this indicate a faulty meter? Shouldn't they read the same value?
    Thanks so much for your help! Keep the blog posts up, I really enjoy reading them! Luke

  2. Hi, Ginger.

    Glad you like the blog. I've got a LOT to do in this coming week with Experiment 11... it's a big chapter.

    I'm no expert on meters, but I imagine your tongue's resistance is difficult to measure given that the human body made up of water. You're dealing with saliva... so one reading your tongue might not be as dry (or wet) as the next reading. You'd think the meter would give an identical reading, but I don't think you can expect that when measuring the resistance of your tongue. To test your meter, be sure to use the pincer/grabber ends so you're not touching a resistor with your fingers. Take a reading with the 200k and let's say you get 85k. Switch to 2000k and yeah, you should get the same 85k. If not, you might have a faulty meter, but I've got a cheap-o meter that I've used for years and it seems to get the same results as my more expensive one. Let me know what kinds of values you get when reading a resistor.

  3. Hi Jim,

    Thanks for your reply! I tried measuring a 100k resistor, and it seemed to measure it perfectly whichever setting it was on! Bizarre! Ive just completed the first experiment, and in measuring a glass of water, the results seemed to differ again! It seems to be struggling with liquids!! I think this is the only experiment where Ill be measuring the resistance of liquids, so i think ill stick with my current meter, but keep an eye on the results Im getting in the coming experiments....if they are fishy, and don't make sense, Ill invest in a new meter! What do you reckon?


    1. Hi, Luke.

      Sorry for delay -- took last week off. My only guess is that liquids just aren't consistent enough in their mixture to be easily measured. If it's measuring resistors properly, that's probably a good thing. You might want to test its reads on voltage (that you know) as well as known current... just to verify that your meter is working properly. i wouldn't worry about liquids right now.