Experiment 11 in Make: Electronics (2nd edition)

Question

Hello,

I am having difficulty with Experiment 11 in the second edition of Make: Electronics.

Specifically I am having trouble figuring out where to connect the components in Figure 2-118 to the breadboard. They are meant to replace the 4 color components at the lower right of Figure 2-116 / 2-117.

I am able to generate sound by guessing where to connect the speaker, 100uF capacitor, and 1K resistor, but the capacitor and resistor have no bearing on the sound..... it makes the buzz whether those components are attached or not. Also, replacing the 100uF capacitor with a 1uF capacitor as the upper portion of Figure 2-119 instructs does not change the character of the sound, so clearly I am not putting things in their proper places.

Obviously this is an issue of being able to translate schematics to functional design, but as a complete beginner on page 96 of an introductory electronics book, I find this very difficult.

Could someone please help me figure out Figure 2-118? I also foresee difficulty with the lower portion of Figure 2-119.

Thank you for your assistance!

jc2048 · Accepted Answer

Here are possible layouts for the first circuit with a speaker and then the third one with the bypass capacitor. I'm assuming you can manage the second one where you just substitute a 1uF cap for the 100uF one. You need to make sure you get the capacitors the right way round (normally the negative lead is marked on the body of the component, not the positive which you might expect) - I don't know if the book has explained, but if you get a large voltage the wrong way across an electrolytic capacitor it will rupture. That immediately then raises a question mark over the 33uF capacitor and what the author intended, since it will have a fair AC voltage across it. He seems to have cheated by showing it as non-polarised, but that doesn't help you. One possibility might be to put two capacitors back-to-back. Does the book give you a specification for the loudspeaker? Although most are 8 ohms, you can get small speakers that are 32 ohms or 40 ohms. It would make a great deal of difference to the filtering if you are using something different to the author. The resistance is normally marked on the back like this That's the DC resistance due to the resistance of the wire used to wind the coil inside and is what you'd measure with a test meter set to its resistance range (approximately - the one on the left actually measures 37.5 ohms).

beacon_dave · Answer

The book says "Speaker (small)" which is not very helpful, however the kit option would appear to include a "Small 8-ohm loudspeaker". http://www.plattkits.com/kits/ Does the book give you a specification for the loudspeaker? Although most are 8 ohms, you can get small speakers that are 32 ohms or 40 ohms. It would make a great deal of difference to the filtering if you are using something different to the author. The resistance is normally marked on the back like this

jc2048 · Answer

I don't think it is the same. The capacitor + needs to go to the emitter, not the ground end of the resistor.

Having said that, some additional resistance in series with the speaker would be a good idea - at the moment the dissipation is high, even with a 32 or 40 ohm speaker (if it's running from 9V or something of that order). With an 8 ohm speaker it would probably damage the output transistor.

jc2048 · Answer

Don't worry - I once made a mistake, too. [Now I'm trying to think who it was I stole that joke from...]

I'm partly wrong as well. I assumed that an 8 ohm speaker would be too much of a load for the emitter-follower but, if you look in detail at what the circuit does, it ends up with a fair bit of voltage being dropped by the coupling capacitor and a 2N2222A would probably just about manage because it's got quite a good collector current rating.

If you simulate it, you'll see it's a horrible way to drive a speaker. You can immediately understand why they quickly went to push-pull output stages on the early transistor radios.

Experiment 11 in Make: Electronics (2nd edition)

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