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# Schematic-reading problems ( Square wave oscillator )? Answered

Well, I try to follow an instructable for quiet a while now, but I still couldn't understand its schematic .. its not that I don't know what the symboles mean, but I dont understand the connections - Those arent physical-connections, but still ...

The instructable: www.instructables.com/id/Build-a-Square-Wave-Oscillator-Part-1-of-DIY-Mod/
The Schematic: https://www.instructables.com/files/orig/FG9/MG8C/GFRWMNFA/FG9MG8CGFRWMNFA.png

It's all over the place! There are 10k resistors appearently and 1 more resistor ( I can see 3x 1m resistors ), but he didnt mentioned those in the instructable.
I'd contact the instructable-author, but he's a student and has thus no time at all.

If somebody has time to explain this to me, or yet better, just mentioning the connections in english, I'd appreciate it very much.
This is driving me insane. ._.

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## Discussions

TBH, I don't see why he hasn't just used the good old 555 instead of an op-amp here.

frankly, I don't know what difference it would make. the 741 sounds pretty stable and solid to me. But anyway, despite that, i really have a big problem with the schematic - why did he mention 10k resistors when there are none in the list? and why 3x 1m resistors? urgh, help...

Its a LOT more complicated than a 555 circuit would be, and a square wave is a square wave.

What frequency range does it cover ? I reckon 600hz to 1khz ?

Forget the 1M and 500K output resistance - just put a nice 100K Log pot there

Find a 1uF cap, non-electrolytic instead of the "100uF", and use a 10K resistor instead of the final 1 Meg - I think the output impedance is stupidly high in the original, and is begging the creation of noise and pickup problems.

Does that help ?

Not really. It seems like people expect you to know everything that seems to be common-electronic-sense, but I don't have that. It's nice that you try to give me tips on how to make it smarter, but nothing can help when I don't know what's going on. :/ I don't have any theoretical-knowledge how the square-wave gets produced, and I think thats why its so hard for me to understand the schematic, and dont forget the problems I mentioned a few times now, heh.

Now, I really want to make a square-wave-oscillator where I can add some more options later on. But I don't have any knowledge whatsoever. And sadly, I don't know anyone around me that has some experience. I'm confused. Ehh...

Ah, if you want comprehension as well...that costs extra......

SO. The op-amp here ISN'T working as an amplifier, its working as a switch. Its comparing the voltage on the two pins on the left side, if the voltage on the - pin is higher than on the + pin, the output goes -ve, and vice versa.

Now, imagine that the output is hard +ve. That means that ~2/3 of the output voltage is now connected to the + in pin. The -in pin voltage is now rising at a rate determined by the R (1...1.5Meg) and C (1nF) cap.....After a period, the voltage on the -pin will be > the voltage on the +pin....so now the output will switch over, and become hard -ve (-4.5V).....

The voltage on the - pin is STILL > 2/3rd of the +ve output, but now the RC network will charge up -vely, over a period, until the - pin is > the +pin.....so the output swings back to +ve

and the cycle repeats.

Steve

Wow, well thats explaining like a boss.
Thank you very much for a lesson in electronics.
Now then, I have only one thing that I don't understand: the freakin' schematic.
There are several flaws that I can't explain, perhaps because I have 0 experience with schmematics: first of all, there are some 10k resistors on the schematic - how is that possible when I, if I would use his method with the same material-list, dont have these in the first place? Secondly, he also used 3x 1m resistors - his instructable mentions only 2x 1m resistors in the material-list. If only there would be a version of that schematic in text-format. "Solder the positive LED-lead to the 9V-jack" ( I would laugh if even that was wrong )

Well, it looks cleaner, definitely, but ... where did you get the 2k and 10k resistors from? Where are the other 3x 1k resistors? Where did the second poti go? :P This looks way different, and by far more complex. ( well that is probably because there are symbols that I don't know about now )

( I''m really sorry if everything above is stupidly easy-to-answer )

Its the same circuit, only I've used more sensible component values. This does exactly what the original did.

There ARE two pots - one for volume, one for frequency.

Ohh, now I see the second one. It's connected to the J-Box, which seems to be the power-jack. Well it makes sense, so the adjustment of how much voltage is allowed to flow controls the volume, eh? clever ... ( If its correct, hehe. )

Can you please tell me what the thing on the left is? as well as the thing that is apparently connected to it? If that is clear, then I think I might actually buy the god damn parts very soon.

So let me list up all the materials I can spot on the clean-version of the schematic:
-LED ( I use a big 20mm one, yellow )
-a resistor for it, the original author used a 4,7k one, but I think I need a different one, right?
-2x 1k resistors
-1m resistor
2x 10k resistors
-a 741 Op Amp
-1x 500k poti, 1x 2k poti
-1nF Capacitor
-that mysterious thing on the left
-a power jack
-an audio-jack ( ahh, so thats what the thing on the top with the leaking line is, correct? )

Hmm. The battery and a switch ?

There's no power jack on my circuit. There's an audio jack on the right though

Steve

Ohh, so the box on the right with the "J" on top is the audio-jack, alright

But tell me: What are those two things on the left? the one on the left-middle and that thing on the left-top.

... ohh, woops.
Ehem, anyway. Thanks man, you really helped me out.
Now I only have to actually build it. Oh well. Anyway, question very well answered, thanks a lot. :)