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Stairstep generator using 555 and op amps

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lordvader88:
I made this Stairstep generator using 555 and op amps, according to w2aew's version



A good schematic shot is at 1:12s

I used a LM358, and it didn't work any amount. Those never seem to work well in any type of op-amp oscillator I've tried, so time to order some modern op-amps.

So I used a TL072 and it basically works. However the step-level plateau's are not staying level, so the 100nF cap at pin5 of the op-amp is discharging when it's meant to be fixed.

Looking at the base of Q2 it only turns fully on (~0.7V) for under 10% of a cycle, to discharge the cap. And then it seems to gently slope from 0.6V to 0.45V or so iirc. And so the 1st 2-3 stair steps decay more like a stepped sawtooth, and the last 2 or 3 are still not very level, as Q2 turns off a bit more.

The same base signal drives Q1 for the 555-reset, it turns on/off just fine., making a nice square wave.

I made this on perf board, with 2n3904 and I haven't tried any other transistors.


I've started trying the DC analysis for the oscillator, I'm really new at that on the rigorous level. So IDK what to expect the base voltage to be yet, or how to tweak it.

Any idea's ? I want to use this to make the curve tracer Allan makes by adding more sections.

Also I only had about 6-7 steps I think, there's 14 in the video. I used the same value caps/resistors, and used a good plastic potted type 10nF cap for the 7555 (the modern CMOS 555). That's quite a difference . I should have made this on a breadboard for easy parts changes, but I love soldering stuff.

t1d:
I have played around with building a function generator... Mostly, I have learned how the circuit works and traditional approaches and ICs.

I do not know how those circuits compare, to what you are doing, with a 555. But, you mentioned considering other Op Amps, so I just wanted to say that a lot of slightly better, basic, Function Generators are built around the XR2206, or the AD8038. FGs built on LM324s seem to perform less well.

spec:
Hi lordvader88,

Looks like the opamp is the problem. You need an opamp capable of working with an input down to 0V, like the OPA191.

But, can I just advise that if you are making a curve tracer, better to go for a more accurate and flexible circuit.

I would recommend a 74HC4060 counter oscillator chip and an opamp. If you are interested in this approach just say, and we can hammer out your requirements and then point at/design a circuit for you.

lordvader88:
Yeah at 1st it worked a little bit w/LM358, but then i did something wrong with the 7555 part of the board, and it also wrecked the desired B-side of LM358.

Everything on the 555-circuit seemed to measure ~ right/expected with a DMM to me. But something was very wrong, some soldering mistake, or maybe it's the chip, I have fried 2-3 of them and I can't say that isn't 1 of them, I must check that chip.

I re-built the 7555 circuit on the proto-board, checked it worked, then fully soldered it.

And it at least works, but only as a saw-step generator, not a nice level stair step


IDK really know much DC or AC analysis of op-amps when caps and diodes are included, I need to refresh Thevenin Eq/node voltage/mesh current, because I still make silly mistakes with them.




I varied Vcc, (not over 11V yet tho), and that made little effect. I tried a cheap SMPS and a 60Hz linear LM317 regulator, and that didn't seem to change much on my oscilloscope.



Also when trying to 'figure out' these circuits on paper, without having a formal education on it, a lot of situations egg up like the 'chicken and the egg' type situations, and IDK if it's the Rooster. I can do most the calculus/diff.EQNs/linear algebra, but I don't have any EE textbooks. So I'm missing a lot

lordvader88:
And the TL072 seems to be 0.5V to Vcc-1.5V, and that was running off 5V to 10.8V , so that can't help. IDK my op-amps, yet.

w2aew mentioned about rail-to-rail op-amps, so he means op-amps that actually get very close to R2R.


I have to order a bunch of newer and more expensive op-amps

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