Friday, September 12, 2014

Experiment 14 (Chapter 14) -- The Joy of Wiring!

First, I'd like to say for anyone following along that I'm sorry for the delay in getting a new experiment posted. I'm teaching an after-school science project club at my sons' school and have spent the last few weeks getting equipment, supplies, and instructions together along with fine-tuning the projects. The club has kicked off and is going fine, and things have calmed down a bit... now I can get back to Make: More Electronics.

Experiment 14. Wow. A good project to really dive into and think about what's happening in the circuit, but also... a lot of wiring! You're going to really want to pay attention and double and triple-check all your work. I thought for certain I'd done everything right after a double-check, but after applying power it still wouldn't work properly... no amplification and no LED lighting up. Another check was done and sure enough... I'd made a bad connection on one of the 555 chips. Finally, success... but more on that in a minute.

I wanted to share a bunch of photo with this experiment that detailed my building of this circuit. The reason I did this was not only to point out a few key differences between the layout of the components on my breadboard and Charles' breadboard, but also to talk about how I go about assembling a circuit. I don't always follow this method... but most of the time it works for me.

First, I inserted all the chips, LEDs, and the electret. Based on the schematic, I guessed at the best spacing of the components. Charles managed to get the entire circuit on half of his breadboard, but I spread it out over the entire breadboard. If I wanted to transition this circuit to a perfboard for a more permanent boxed project, I'd probably continue to close up my circuit and reduce the spacing and such. I did cut almost all of the leads of the resistors, but not the capacitors because I have a limited quantity and I never know where I wish those leads to stretch.

After placing the first wave of components, I took an inventory of the resistors in the circuit (including the 1M variable resistor). I keep all my resistors in little baggies and I really only want to pull them out one time. Therefore, when I pulled out the 1K back, for example, I pulled out three of them and clipped and placed them as necessary. A few times I realized I would need to move or relocate a resistor, but it was rare.

Speaking of resistors, take a look at Figure 14-1 that holds the schematics for Experiment 14. Look carefully where the 1M variable resistor (pot) is located and how it wires into the LM741. Now look at Figure 14-3 that shows Charles' wiring. Notice anything different? Yep -- he's flipped his pot upside down... the tip-off is that brown wire running from the top lead to Pin 2 on the LM741.  Notice the green wire running from the bottom lead (on the pot) to Pin 6 on the chip? Not a big deal, but if you try to wire up your pot to match the schematic, don't try to compare your final result to Figure 14-3. I was trying to figure out what was so strange about the pot, and then I realized it the middle pin was jumpered to the top pin (in Figure 14-3) with a teeny-tiny brown wire. Look close... it's there. Again... not a big deal. I wired it up my way and got the circuit working. But just be careful. Charles wires up things in a very efficient manner that also lends itself to photographing the final circuit. Because of this, you'll occasionally find workarounds that he uses that don't match exactly to his schematic.

Sometimes, however, you've got to come up with your own workarounds. Example? In the schematic, each of the 555 chips has a single 150k resistor coming on Pin 8. I didn't have any 150k resistors, but I did have 100k and 47k resistors. If you look at a close-up photo I took, I just used a 47k as a jumper to connect Pin 7 to Pin 6. The 100k jumps Pin 8 to 7... and 47k jumps Pin 7 to Pin 6. And then a small capacitor makes the final connection to Ground. If you look at Charles' circuit in Figure 14-3, you'll see his solution which is much more elegant and clean. I didn't have 150ks, so I had to improvise. Pin 7 to Pin 6 has to be jumpered anyway, and that 47k will be felt whether it's part of a 150k single resistor or a two resistors in series as I've done.

Testing continued to be a slight problem for me as I've never been able to get good results from these electrets. I probably should have purchased some new ones, but I got enough valid results from the circuit to understand what was happening. (Of course, I got a finger burned good by using the wrong speaker... won't make that mistake again.)

Below is the video of my test. As you'll see, the electret picked up the scraping of the pin and the input was amplified with the speaker. But once I stopped the input, the circuit began to cycle for some reason. I've played around with it a bit more to see if I can figure out how to break that loop, but no solution yet.

These sound circuits have been fun, but I'm really ready to keep moving forward and see what comes next. I've already read the next few chapters, and I've always enjoyed working with logic chips... so I'm looking forward to the next handful of builds.

Video below, along with some additional photos:


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  3. Just finished working through this experiment. I wasn't getting the pause duration timer to kick in as it should. I'm lucky enough to have an oscilloscope so I could see that the electret microphone and the 741 OpAmp was providing a good voltage output signal. Also the 555 noise duration timer was working fine and the LED was lighting up consistently with loud noises. But the pause duration timer just wasn't responding to the signal from the noise duration timer as it should be. If I manually grounded the trigger pin (pin 2) on the 555 pause duration timer then the LED would light up for a second, so I knew this part of the circuit was wired correctly. I remembered from previous 555 circuits that the trigger input usually requires a pull-up resister to keep things running smoothly. I added a 10K pull-up resistor into the circuit. It runs from pin 2 of the pause duration 555 to the positive rail. So guess what, everything works perfectly now. The noise duration timer LED now lights up consistently when a loud noise is detected and then is followed by the pause duration timer LED lighting up for a second. Just as expected. All is right in the world. Time to move on.

  4. Charles Platt also mentions the 10K pull-up later in the chapter and he shows it in place on the breadboarded circuit figure 14-3. He just neglected to add it into the schematic figure 14-1. The circuit definitely won't work well without it. I looks like the pull-up resistor is missing in your photograph as well James. If you were having problems with the circuit that's probably why.