Sunday, January 24, 2010

Chapter 2 - Exercise 11 Step 4

This post is for Step 4 (of 4) for Exercise 11.

The goal with this final step is to take what we've learned about the transistors and PUTs and use them to create an oscillating sound similar to a siren. We've already built 95% of the circuit, and the new circuit (on page 91, Figure 2-113) requires removing the LED and adding in a resistor (R10) and a capacitor.

I lacked another 2.2 microfarad capacitor (I used my only one for C1) so I found two possible replacements - a 0.1 microfarad and a 4.7 microfarad. One is less than the required 2.2 and one is more... so my guess was that if the 2.2 microfarad was going to cause a wah-wah-wah alarm sound, that one of the substitutes would likely cause the alarm to sound faster... but which one? Best guess was that it would be the 0.1. Why? Well, one of the tasks in Step 4 was to insert ONLY the R10 resistor... no capacitor. I did this (but forgot to shoot video) and the sound it made was super-fast wup-wup-wup sound... (sorry for getting super technical again in my descriptions - these things cannot be helped).

the 0.1 microfarad cannot hold as large a charge, so it's obviously going to discharge but faster... that was my thinking. The first video will let you see the sound I heard when I plugged it in.

Next, I pulled out the 0.1 and put in the 4.7 microfarad capacitor. The second video will let you see the results of that circuit.

All in all... a very fun exercise. Tomorrow I'm going to do a few of the tweaks on page 92... so I won't be taking this circuit apart just yet... I'll try and make some guesses for what's going to happen when I sub in various parts, but I hope I'm wrong... because it'll make me examine the circuit and figure out why I was wrong. I learn better that way...


  1. I'm catching up to you, now!

    I've got the Ex 11 done, up to the point where the book offers suggestions for further investigation, and I'll call it a night for now.

    But I'm puzzled about why the circuit continues to run after I disconnect the power. The LED flashes faster, but slowly gets dimmer. And when the speaker is connected, the sound gets higher-pitched and quieter for a good few seconds after the power's off.

    Where's that residual power coming from?

  2. In an earlier posts' comment, a reader suggested that the AC Adapter also has it's own capacitors inside for smoothing and that they are discharging, too. That's the best suggestion I heard about this issue.

    As for the higher pitch... my best guess is that as voltage/current drops, the speaker attempts to maintain the same volume and somehow the physical aspect of how the speaker works forces the chirp to be at a higher pitch. I could be 100% wrong...

  3. Yep. That was it. When I unplug the adapter from the wall, the circuit keeps running, but when I unplug the adapter from the breadboard, it stops immediately.

  4. I had an interesting accidental discovery in the course of this exercise. I accidentally shorted the positive leg of C3 to one of the legs of R3, and heard a radio station...! Touching some other components on the right side of the board (notably R8 and R6) seemed to amplify the signal. I started trying to figure out what was causing me to pick up the signal, removing components until I could have a smaller working set. I eventually stripped off everything upward (and including) R10. C2 seems to be a clear contributor to this, and I suspect that the capacity of that capacitor is what determines the frequency that I'm tuning into. (I haven't yet figured out the specific radio station frequency, though.)