A full… explanation of circuit

[windmill john]

Hi All,
Probably worse than my isolation question, but here’s an example of why I struggle. Please see the circuit below. I know all the components.
I’m happy to find someone local who can show me, but can’t find them! If this is too basic to ask here, tell me and I’ll stick with searching.

Okay, please take me through this ‘electric motorcycle” circuit. When you hold the two wires, the harder you squeeze, the faster the clicking. Yes I understand that is capacitor discharge.
Right, let’s ignore fingers for the minute and just connect those two ends together maybe with a resistor.

I think I understand more or less what is going on, but what is the point of the 0.1 micro farad capacitor? If it were not there, to me it looks like it would still work? Can someone who is willing, explain?

Thanks.

[Thunderer]

Once read a comment on Stackexchange. Someone said that people do not jump to explain stuff to others when they ask for help online. So, that person trick was that he will try explaining it but put some very wrong or controversial explanation in the request. Human nature will drive someone to try to refute that explanation by giving the right explanation. And, lots of times it just works.

Back to your circuit:
1. Yes, it will work without the 0.1uF cap. But there will be some unwanted behaviour. Sometimes.
2. Can you please post the complete page of the magazine where this circuit comes from?

[themadhippy]

When you hold the two wires, the harder you squeeze, the faster the clicking. Yes I understand that is capacitor discharge.

is it?or is it the resistance of your body completing the circuit between the transistor base and a voltage rail?

[windmill john]

Ahh, I should have separated those two lines, the capacitor comment was independent of the holding the wires.

The circuit comes from experiment 6 of my Maxitronix 130 in 1 kit. Way beneath all you guys’ experience!

“Unwanted behaviour, sometimes”? Surely if you remove a component, you’ll get a predictable result, be it nothing, something etc?

[Thunderer]

Putting more pressure on the wires (decreasing the resistance between the wires) will make the simulated motorcycle "run" faster.

Playing with the caps will get you lower or higher frequency noises.

But, as you have the will to make it work, I'd go for a LM386 instead of the output transformer. Which you should source it from an old small portable radio.

[wasedadoc]

The capacitor is part of the AC feedback which makes the circuit oscillate. It will not work the same without it.

[windmill john]

Thanks wasedadoc, you’re heading towards the question I asked; maybe I ask too many questions!
I’ll build it again missing out the .1 micro farad capacitor, I am struggling to see what it does.

Staring at the circuit as a beginner, it looks like the .05 micro farad capacitor should achieve the same result. That’s where my understanding falls down.

[rstofer]

Hand waving this type of circuit is a lot more difficult than it seems.  That's why nobody is jumping in and neither am I.

I wonder what https://www.falstad.com/circuit/circuitjs.html would do with it.  You would have to make a guess at the winding inductance unless the kit manual gives some values.  Try 1 Henry for a first approximation.

As a guess, I think one capacitor deals with the on time and the other deals with the off time.

Resonating circuits are difficult.

Build it and use a scope...

[ledtester]

This looks like a circuit from one of the Science Fair NNN-in-1 kits where NNN is a number like 150, 200, 300, etc.

I bet the numbers indicate which spring terminals you have to make connections to. The pencil that you can see through the page is probably for an area for your notes.

I never figured out how their audio oscillator circuits work... many of them seem to use the speaker itself as part of the oscillator.

Update: I see now that the OP has posted the full page from the manual.

[windmill john]

Thanks both.

rstofer, your line “ As a guess, I think one capacitor deals with the on time and the other deals with the off time. “ sounds a fair guess. When my scope arrives, I will use it with my kit as it’s safe and will increase my learning. Plus, I totally forgot, I got the app EveryCircuit, so could virtually test.

ledtester, yes, this is a Maxitronix 130 kit model and they are springs.

Getting off the start line in this field is very hard. About five years ago when I thought I’d try this the first time (not letting go this time!) I bought an old dead radio and thought I’ll get that working…. How naive…

I did the right safety thing and made a current limiting socket. But fell at the first hurdle as I didn’t know how to test the circuit. Yes I have a DMM, yes I can stick it places, but had no idea what I was doing. Okay I didn’t have a schematic, but if I did, I wouldn’t have understood it. This time I want to get over that hurdle. I guess I need to watch more videos on actual fault tracing.

[windmill john]

Well, for those who are interested...... here is a short video of what happened when I removed the 0.1 micro farad capacitor. First half of the video is with the capacitor in place.

What I got was squeaking.. Hopefully my guess of, it is just a much higher frequency of triggering the remaining capacitor is correct?

https://youtu.be/LkvLZdzC-ks

[wasedadoc]

Put "blocking oscillator" into a search engine to find pages such as https://en.m.wikipedia.org/wiki/Blocking_oscillator

[windmill john]

Just to help me understand, or check if I understand!

From that page:

“Instead of a resistor, a potentiometer placed in parallel with the timing capacitor permits the frequency to be adjusted freely, but at low resistances the transistor can be overdriven, and possibly damaged. The output signal will jump in amplitude and be greatly distorted.”

Do you think that me removing the 0.1 micro farad capacitor has done the same as above? That is, the squeaking you hear at the end of the video is the output signal with a big jump in amplitude creating distortion?

[rstofer]

I just measured my body resistance with wetted fingers and came up with 1M Ohm.  The 10k is insignificant when compared to 1M and is in the circuit just to prevent a short circuit on the probes from overdriving the transistor.

You can replace the 10K and CdS sensor with a 1M resistor and get consistent results independent of skin contact and lighting level.

When you get to scoping the circuit, a consistent circuit will be a great help.

A DMM is pretty much useless on this kind of circuit. You really need to see the circuit on a scope to understand what is happening.

This type of oscillator is hardly the place to begin with understanding electronics.  It will make a useful demonstration.

[wasedadoc]

A capacitor in the location of the 0.1 uF in that circuit is an essential part of that configuration of a blocking oscillator.  If the circuit with capacitor is working OK, removing the capacitor will not cause any damage.

[Terry Bites]

It looks like a Hartley Oscillator. You'll find descriptions on the interwebbo.

[windmill john]

This type of oscillator is hardly the place to begin with understanding electronics.  It will make a useful demonstration.

I am discovering that finding the right path to electronics from understanding a circuit with a couple of resistors and a battery etc to something more substantial, is quite hard; give me time…..

I am looking forward to trying the oscilloscope on these little circuits.

[wasedadoc]

It looks like a Hartley Oscillator. You'll find descriptions on the interwebbo.

It is not a Hartley oscillator. In the classic Hartley (https://en.m.wikipedia.org/wiki/Hartley_oscillator) the amplifier is non-inverting and the feedback is non-inverting. In the oscillator under discussion the amplifying stage is inverting and the feedback is inverted. Hartleys operate the amplifier in linear mode and produce closer to a sinusiodal output. Blocking oscillators like this one turn the active device on and off, producing a pulse train.

[rstofer]

This type of oscillator is hardly the place to begin with understanding electronics.  It will make a useful demonstration.

I am discovering that finding the right path to electronics from understanding a circuit with a couple of resistors and a battery etc to something more substantial, is quite hard; give me time…..

I am looking forward to trying the oscilloscope on these little circuits.

Once you can sling AC and DC circuits around, you should start looking at active circuits - something with a transistor that amplifies.  w2aew has some videos on the topic and they are very good.

Try this one on for size:  https://youtu.be/zXh5gMc6kyU

You can search for the others in this list:
https://www.qsl.net/w2aew/youtube/W2AEW_video_index.pdf

AC circuits has the difficulty of imaginary numbers and gruesome impedance calculations.  Fortunately, we have Octave and MATLAB and these programs know quite well how to deal with i.  i is just a 90 degree phase shift and comes up in every AC circuit with reactive components.

Attached is one of my favorite examples using an Analog Discovery 2 with a 10k resistor and 0.1 ufd capacitor as a low pass filter.  Given floating inputs, I can get current by measuring the voltage drop across the resistor and voltage by measuring the voltage across the capacitor.  And, of course, the AD2 provides the 1 kHz sine wave.

You will probably run across the phrase "ELI the ICE man" where E leads I in an L (inductive) circuit and I leads E in a C (capacitive) circuit.  We expect I to lead E in a low pass filter and indeed it does.

Simple circuits are fun to play with.  I can take the same components, wired the same way (low pass filter) and hit the circuit with a square wave and verify the capacitor charge/discharge function.  See attachment 2

Or, I can look at the Bode' plot of the low pass filter.  The cursor is off just a touch, it should be at the -3dB break point and it's at -2.9...  See attachment 3

I can spend hours playing with simple circuits!  There's a reason I am a strong supporter of the AD2 but I certainly don't like the new and improved price.

[Terry Bites]

You are right to correct me. I'm too hasty sometimes- well, a lot actually!

[EPAIII]

Interesting comment.

I once saw a published circuit for a logic probe and I decided to build it. But it just did not work. I examined it for some time and finally thought that it would work if one of the electrolytic capacitors was leaky; if it had a very high ESR. I put a resistor in parallel with that capacitor and bingo, it worked. But it obviously wasn't a very good design. I suspect the designer had just tried things until something worked. But even he had no idea of how.

I designed my own circuit with some logic chips. I sent the improved circuit to the same magazine and they published it. It still works today, after 30 or more years.

One tends to think that something in print is good because it is published. Surely it has been completely checked and re-checked. But that is not always so.

Once read a comment on Stackexchange. Someone said that people do not jump to explain stuff to others when they ask for help online. So, that person trick was that he will try explaining it but put some very wrong or controversial explanation in the request. Human nature will drive someone to try to refute that explanation by giving the right explanation. And, lots of times it just works.

Back to your circuit:
1. Yes, it will work without the 0.1uF cap. But there will be some unwanted behaviour. Sometimes.
2. Can you please post the complete page of the magazine where this circuit comes from?