Experiment 11 in Make: Electronics (2nd edition)

Hi,

You are doing OK. The layout in 2-120 is exactly what is being called for in figure 2-119. I drew it out on a piece of paper but what I drew is the same as figure 2-120.

Keep in mind that the 2 vertical lines of holes associated with the red and blue lines are all connected together and the horizontal groups of five holes on either side of the middle grove are also hooked together. They serve as wires with multiple connection points. Compare my drawing to the figure 2-120 and see if you can see how they are really equivalent. Getting comfortable with the new protocols of a new course of study is always a challenge but also worth it. The book you are using to learn is an excellent one. Keep up the good work.

John

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.

Hi Jon,

I think you are absolutely correct. (Senior Moment). Please disregard my previous post s it was wrong.

John

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.

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).

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

Hi John,

Thank you for your response!  It is definitely helpful to remember that the breadboard is connected in horizontal rows.  Moving things left and right (up to the middle divide) seems to be ok (at least in these simple circuits), but moving a component up or down breaks the connection.   This actually came in useful on this circuit, as I will mention below.  Thanks so much for your encouragement!

Hi Jon,

Thanks so much!  As you can see, I followed your diagram and was able to produce the tone.  I did have to move the transistor from H42 to I42 to make room for the 100uF capacitor, but as John pointed out above, this shouldn't make a difference.  It's hard to see in the first photo, but the positive lead of the capacitor goes into I42 in line with the transistor's emitter.

Mr. Platt does explain the importance of polarity, so I was confused by lack of specifics in diagrams 2-118 and 2-119.

My question now is why the character of the sound doesn't seem to change when I swap the 100uF capacitor for the 1uF capacitor.  It sounds identical.  Adding in the 33uF capacitor does muffle the sound considerably, but I'm wondering why interchanging the 100uF and 1uF capacitor produces the same sound.  Could it be a fidelity issue with such a little speaker?  I am using an 8 ohm speaker as the experiment calls for.

I will now forge ahead and build the circuit in 2-120.

Thanks again for your help!

Hi Dave,

Yes, I am in fact using the Platt kit sold by Chaney Electronics.  Thanks for pointing that out.