Sunday, January 24, 2010

Chapter 2 - Exercise 11 Step 3 revisited

Something just kept bothering me with the circuit I built for Step 3. I read and re-read the section and although I still have some questions (more on that shortly), I knew that the amplification should work and I should be hearing more than the very faint buzzing noise coming from my speaker.

So, I went back to the circuit and then re-examined my breadboard... and there was the problem. Something very simple and easy to overlook. The circuit calls for a 100 ohm resistor in series with the loudspeaker. I made the mistake of putting in a 100k resistor. A factor of 1000! No wonder my loudspeaker was barely audible.

So, below you'll find a video with the newly substituted 100 ohm resistor. That annoying sound coming from my loudspeaker isn't so annoying anymore... I love that sound. It represents another lesson learned - always check your resistors with the multimeter to verify your color-coded selection is correct. As a matter of fact, I should ALWAYS be verifying my resistors with the multimeter against the circuit. On page 86, I went down the list and my brain just saw everything in k-units... R4 470K... R5 33K... R6 27K... R7 100K... oops... wait a minute. That's 100, not 100K.

I'm currently working on Step 4, but Step 3 still has me slightly confused. Back in Exercise 10 on page 80, we saw how a small current entering the Base amplifies the current exiting the Emitter. Okay, I get that. When the transistor is triggered at B, the current is amplified at E.

So, jump to page 91 and Figure 2-109. My first thought before building this circuit was that I'd simply rewire the loudspeaker and have one of its wires connected to the Q4 Emitter... more amps coming out would mean a louder volume, right? But that's not how it's wired up as you can see in Figure 2-109. That speaker wire is connected at the Q4's Collector. Huh? Still, I wired it up as shown in the schematic and it works. I think what I'm missing here is an understanding of how the speaker is oscillating... back in Figure 2-104 on page 86, I see the oscillation from the on-off of the 2N6027... the speaker is getting its voltage/current from the Cathode which turns on and off at a high frequency. But back in Figure 2-109, the speaker is no longer connected to the 2N6027 but instead is getting power directly from the 6V DC positive column (with R7)... so what's causing the speaker to oscillate now? L1 is going into the Collector of Q4 which is oscillating on-off... so is that on-off oscillation what now causes the speaker to buzz? I'm guessing yes, but I'm just not 100% understanding why... feel free to chime in if you have a good explanation.

1. I'm following this in-actively... I don't have the book yet and don't want to get ahead of myself.... :D But thanks for doing this. Can you somewhere, add a total spent? Add up all components needed, maybe also if you need to buy tools?

2. Hi, Dean.

I'm doing that, little by little, in the Shopping List posts. Look at the labels and select the Shopping List tag to read over those. I've done my best to list part numbers, quantities, and prices... some things I haven't had to purchase because I already had them, but I'll do my best to keep inventorying what I've bought.

3. "I made the mistake of putting in a 100k resistor."

Very relieved to read that! I was wondering and wondering why you weren't hearing anything.

Personally, yes, I always check resistors with a meter before putting them in a circuit. This is especially important with physically small, quarter-watt resistors where the color bands are so narrow, apparently they don't stimulate a wide enough area of sensors in the retina, so it's hard to tell the difference between orange and red (for instance). You can of course use a magnifying glass--but if you're going to do that, you might as well use a meter. It's just as quick.

Of course if you have an auto-ranging meter, you can STILL mistake a 100K resistor for a 100 ohm resistor! That's why I stubbornly stick to my manual ranging meter, the dinosaur that I am.

4. Charles,

I'll try not to make that mistake again!

BTW, do you have any comments about the wiring of the speaker to the Collector instead of the Emitter? I'm still stuck a little bit on the "Why?" of oscillation of the speaker... is it the 2n222 opening and closing that causes a loss of voltage across the speaker and thus the sound it makes?

5. In answer to your question re the placement of the speaker in the circuit in Figure 2-110:

I experienced a similar feeling of puzzlement when I first looked up the typical wiring of a "Darlington Pair" of transistors, and saw it done like this. (Yes, I confess, I did not invent this part of the circuit. I think it was invented about 40 years ago!)

Bear in mind that the current going into the transistor is pretty much the same as the current coming out. The base is mostly just controlling the flow (see Figure 2-89). But if you swap the loudspeaker and its series resistor so that it's "below" the transistor instead of "above" the transistor, the sound volume is less. I encourage you to try this, as I did. The question is, why?

Since I lack a formal education in electronics, I get a bit sketchy when considering issues such as the saturation state of transistors. Saturation is mentioned briefly on page 80, and that's about it. But I'm thinking, if you wire the collector of the transistor directly to the 6V power supply, surely the transistor must become saturated.

If you have time to experiment (I didn't!) you could try various configurations based on Figure 2-110. For instance, instead of one 100-ohm resistor, try a 50-ohm resistor above Q4, and a 50-ohm resistor below, and then insert the loudspeaker above, and insert it below. And check the current and voltage in each case.

Sorry I don't have a better explanation. It really boils down to something like, "Other people generally seem to do it this way, and I didn't have time to research this particular issue in depth."

I regard myself as a writer who happens to know some stuff about electronics, rather than being trained in electronics and able to do a bit of writing. This is why I had a technical advisor who reviewed everything. His contributions were important.

6. In answer to the question that you wrote while I was writing my previous response:

Q2 is oscillating at a frequency which becomes audible if it is translated into pressure waves (by the loudspeaker). But Q2 isn't powerful enough to connect directly to a loudspeaker.

The output from Q2 switches the current flowing through Q3. The 2N2222 is able to pass a larger current than the 2N6027, and it only has a 2K2 resistor above it, so, the 2N2222 amplifies the fluctuations. The output from Q3 then goes to the base of Q4, to amplify the flow some more.

You can imagine a whole chain of transistors, the output from each controlling the flow through the next. But you reach diminishing returns because of saturation, where a transistor such as 2N2222 simply cannot pass any more current. You then get distortion, sometimes known as "clipping," which I discuss later in the book. You could of course substitute a more robust transistor for the 2N2222, designed for audio applications, but that would just complicate matters. Or you could use an audio amplifier chip, and forget about transistors. I get into that later in the book.