In Tutorial 6: Rising Pitch Siren [ http://startingelectronics.org/beginners/start-electronics-now/tut6-rising-pitch-siren/ ], we have instructions on how to build it but nothing about how it works. It is probably that I do not understand oscillators yet that I am having problems with this circuit. Apparently the Resistor-Capacitor Combinations are having some interplay here. The Transistors are up to something. By the way I am writing, you can tell I am just guessing, using meaningless abstract terms to refer to things I have yet to understand.

Can I have a Blow by Blow description from Power Source to Speaker as to what is going on here???

 

R1 and C1 have a time constant of 33 seconds, which is very slow. Q1 and Q2 are a Sziklai pair, which is a form of Darlington, lots of current gain plus voltage gain. As the base current through R2 slowly rises, positive feedback through C2 slaps the base high and the transistors saturate. When the transistors saturate, there is no change in voltage which can pass through C2 so the oscillation stops and the current into base of Q2 falls toward the voltage on C1 divided by R2. Eventually, Q2 turns on and the cycle is repeated. As the voltage on C1 rises, the time between cycles decreases because the base of Q2 can not fall towards zero current from C1. How this gets to the place where it repeats escapes me because there is no circuit that will dump the voltage on C1 and cause a reset. It won't do, "whoop whoop whoop" it will do, "tweeeet" and quit.

 

I'm going to put this circuit together tonight and play with it. I've been looking at the diagram for about ten minutes and I still don't know what it's doing. Neat circuit, but no description.Typical. I recognize a few things. C1 is fairly large and acts like a current dump (short) when first turned on. R1 is fairly large too, and is choking the current to C1 so it takes several seconds to charge up. As C1 charges, the voltage drop across C1 rises until C1 starts looking like an open circuit. Mean while, the transistors and C2 are oscillating. Why, I don't know. They

 

I'm going to put this circuit together tonight and play with it. I've been looking at the diagram for about ten minutes and I still don't know what it's doing. Neat circuit, but no description.Typical. I recognize a few things. C1 is fairly large and acts like a current dump (short) when first turned on. R1 is fairly large too, and is choking the current to C1 so it takes several seconds to charge up. As C1 charges, the voltage drop across C1 rises until C1 starts looking like an open circuit. Mean while, the transistors and C2 are oscillating. Why, I don't know. They

mtonge:

1) You are right that startingelectronics.org does not go into too much detail about their circuits. They expect you to be a full-fledged Electrical Engineer while they have you construct a beginner circuit.
2) You left me hanging with your incomplete sentence at the end that starts with "They" but goes no further. You were clearly going to say something but did not say it. What were you saying?
3) Your explanation about the capacitors and resistors are a good starting point for my own analysis. I'll get back to you later when I figure out what's going on.

 

R1 and C1 have a time constant of 33 seconds, which is very slow. Q1 and Q2 are a Sziklai pair, which is a form of Darlington, lots of current gain plus voltage gain. As the base current through R2 slowly rises, positive feedback through C2 slaps the base high and the transistors saturate. When the transistors saturate, there is no change in voltage which can pass through C2 so the oscillation stops and the current into base of Q2 falls toward the voltage on C1 divided by R2. Eventually, Q2 turns on and the cycle is repeated. As the voltage on C1 rises, the time between cycles decreases because the base of Q2 can not fall towards zero current from C1. How this gets to the place where it repeats escapes me because there is no circuit that will dump the voltage on C1 and cause a reset. It won't do, "whoop whoop whoop" it will do, "tweeeet" and quit.

#12, that's quite a bit to digest. I am going to have to look up "Sziklai pair" and Darlington . I did gather that R1*C1 = 33kOhms * 1000 mFarads = 33 seconds from your reply. You are gracious enough to admit that you are giving a partial answer, but you have given me something to work on tonight.

I will probably be back to this thread for some fine tuning.

Regards

 

I try to adjust my presentation to the questioner, but I guess I didn't go far enough.
A Darlington pair is two of the same polarity transistors in the same configuration so they just act like a single transistor with a current gain of 10,000
A Sziklai pair is two transistors, a PNP and an NPN in a configuration that still resembles a normal transistor, but with a lower saturation voltage on both the base emitter direction and the collector-emitter direction. They also have a current gain of about 10,000 plus they have a voltage gain.

 

Expert#12:

I did look up Sziklai pair. I am interested in "How it Works" as that will, as you imply, tell me how that circuit works. I see a lot of Math in the explanations, which is of no use to me at present. I am interested in tracing electron flow. For example, "The collector has a voltage of 9 volts which can't get through because the base has yet to acquire the 0.7 volt threshold to allow current flow through the emitter . . ." etc.

You are right in your unspoken thought that I plan to be a pest. As a former computer programmer, I have been burned by a quack electrical engineer (who was later found out and fired despite his smooth talking) and sustained some career damage. In my current endeavor in using the Arduino for Farming Applications (see my bio), I need to get up to speed.

With this in mind, what literature do I read or what video do I watch, or am I looking for a good one paragraph explanation?

Regards

 

Here is an old thread on this Compound Darlington circuit...


https://forum.allaboutcircuits.com/threads/simple-tone-oscillator-22mf-capacitor.95028/

 

Here is an old thread on this Compound Darlington circuit...


https://forum.allaboutcircuits.com/threads/simple-tone-oscillator-22mf-capacitor.95028/

Thanks! Will look into it.

 

I am interested in tracing electron flow.

Turn on the power. As the voltage on C1 becomes more positive than a few tenths of a volt, electrons are sucked up through the emitter of Q2 and out the base because R2 is allowing the lack of electrons on the top of C1 to do that. When electrons flow in the emitter-base circuit, a lot more electrons flow in the emitter collector circuit because transistors have current gain.

As the electrons come out of the collector of Q2 they are sucked up through the base-emitter junction of Q1. At that point, +9V is pretty much connected to R3 and C2 because transistors have current gain.

OK. So now Q1 has a fairly good connection between +9V and 100 ohms to a speaker and C2 which is 100 nf to the base of Q2. For a hot instant, the electrons coming up through the emitter to the base of Q2 get hijacked through C2 and that slams Q2 on as hard as it can go. This is called positive feedback and positive feedback is the basic principle for a lot of oscillators.

So, Q2 is slammed hard and that slams Q1 on and that condition holds until current stops flowing in C2. At that time, the electrons being sucked up through e-b of Q2 start to diminish in number toward the point that started this, electrons being sucked up through R2 into C1. Immediately the positive feedback resumes being effective. As Q2 and Q1 begin to diminish their electron flow, the junction of C2 and R3 stop being so much connected to +9V. They just got their electrons sucked out to nearly 9 volts, but that path through Q1 is diminishing so electrons flow up through 108 ohms toward C2. Those electrons push electrons out the other side of C2 and they are supplying the R2 to C1 path which further shuts down Q2 and Q1. Bam. A cycle is complete.

When the C2, R3 junction approaches zero current, the voltage on C1 resumes sucking electrons through the e-b junction of Q2. As this cycle repeats and repeats, the voltage on C1 is increasing slowly. The time required for the voltage at the base of Q2 to fall back down to the voltage on C1 diminishes and the cycle goes faster and faster until it reaches a balance. C1 stops getting a higher voltage and the oscillator stabilizes at a higher frequency that the first cycle it went through.

That's where I can't see how it starts over.

 

@ConstantineOfTexas
Error corrections: Q2 in the third paragraph became C2.

 

Expert #12:

My apologies for not getting back to you sooner. Farm Work is hard and time consuming.

I am making some progress! I am finding out that the Darlington Pairs are all but self-evident in their operation. Let me give a test explanation for you to critique as according to this diagram: http://68.media.tumblr.com/0d60f125f1d66083aeadfa48ba506503/tumblr_inline_n608jnkZEk1sx620x.jpg .

Conventional Current (Holes flows from Vcc to the Collectors of both Transistors. It stops because neither Base has been activated. When Holes flow into the Base of the Transistor on the Left (i.e. the Input Signal), TR1 turns on, amplifying the Signal which gets fed to the Base of TR2 as a Signal. This turns on TR2, which amplifies the signal again.

How did I do?

PS: I will still be working on the Sziklai Pair, which suddenly does not look so hard anymore. though I haven't understood it yet.

Regards

 

I remember when I wanted to know where every electron went. I decided, If you can have minus-a trons, you can have plus-atrons. (Most people call them, "holes".) It's just the difference between "conventional" current and electron current. Call it what you want, just be consistent! The quickest way for your model to fall off the horse in mid-stream is to try to change your thinking in the middle of a circuit.

 

I think I finally figured it out. C2 being nonpolarized is what makes this thing work.

Thanks!!!

 

All feedback, positive and negative, works for both polarities...unless there is a diode in the way.