Mini DIY signal generator

[newperson]

That sounds a bit much and perhaps a cat killer.  I was hoping for some imaginative electric replacement.  My wife and I was looking at some low power stuff at Lowes to keep cats/dogs out of our raised beds and felt that the low powered stuff might even cause some 'issues' if a random kid came along.  I told here that it does not hurt too much and it should pulse so it cannot grab and hold onto you.  My brother and I used to dare each other to touch the fence at my aunt's house that kept the goats in.  Since it pulsed it was a 50/50 chance of getting zapped a little. 

[PRR]

We had our Corgi at a farm and she wanted to play with the piglets. Then a "YELP!!", and I realized it was an electric fence.

This is a student farm and they are all organic and humane, so I assume they don't think the shock will do real harm to the piglets. (On the other hand, they ARE just future bacon.) The dog was confused but showed no lasting harm.

However "random kids" have parents who know lawyers. Outside a farming district, I think an electric fence is probably asking for serious trouble. Some kids don't know what it is, others (you?) will dare each other to touch it. If ANYthing bad happens, it's all your fault.

Also many towns have laws. In the suburb I used to live, barb or electric had to be 8 feet off the ground and angled 45 degrees inside the main fence, as well as placarded. (And today you can not bet that ALL children read english.)

Wire-mesh fence is a pain when you have to cultivate, and won't stop deer, but is cheap and widely accepted as safe. (Except cutting the stuff-- I always bleed.)

[Kipper4]

I bit the bullet and built this simple little circuit (famous last words since my usual success with vero has been haphazard) tonight.
I'm looking forward to finishing it up and doing the off board wiring tomorrow after work.
I think it will be fun to use to test pedal reaction using my old scope.
I hope it's not going to be a problem but I used a 100k 200k ,10meg dark LDR. It's one of the Gl 55 series I forget which exactly right now.
Wish me luck fellas

[Kipper4]

I'm Impressed It worked hot off the iron. Thanks Merlin.

[merlinb]

I'm Impressed It worked hot off the iron. Thanks Merlin.

[PRR]

Merlin> probably early 90s.

There is an article about 1-pot RC oscillators in Wireless World, Dec 1980, page 82 of the magazine, page 44 in this scan. So the tricks were known then. Unfortunately there are no citations.

http://www.americanradiohistory.com/Wireless_World_Magazine.htm

[PRR]

Here's the same thing again.

80MB PDF, you only want page 34 (pg 35 of the PDF)
https://archive.org/details/PrecisionMonolithicsInc-PMI-LinearAndConversionApplicationsHandbook1986



To convince myself they *are* the same thing, I pasted them together:

http://i.imgur.com/fixHmKO.gif

MerlinB uses a more sophisticated AGC loop (omitted here), and takes AGC in the vary-gain inverter rather than the output stage (I don't know what difference this makes, if any). And of course the single-supply re-engineering.

Both together give a better idea of what that variable resistor really does.

[bluzeyonecat]

Thats really cool man! After I get some more of the basics down, definitely a project Id like to try. Thanks for sharing this.

[Jarno]

Ping, old thread bump

So, I did a layout with four of these sine oscs on a single board, and four VCA's on the same board. I am going to use these in my modular synth in an old school electronic music context (those guys used old HP measurement equipment to make music), to produce a set of partials.

I converted the circuit back to dual supply, since I have that available in my modular (dual 12v).

But, I am getting some weirdness.
For the low end, I am using 2x 470nF caps, and it takes some time for the amplitude to stabilise, sometimes it keeps on wobbling or it takes a bit of fiddling with the pot to get it started. I tried a vactrol, but that didn't work well.
Also, on the mid range (2x100nF), it rises to high amplitude (runs into the -12v rail) and then dies again.
The high range (2x 10nF) works fine (with a vactrol, NSL32-SR2).

So, is the speed of the vactrol important? (gut feel says it should be sloooowwwww)
Furthermore, should the caps be matched? (I am using 10% film caps now)

If the sine is oscillating, it looks pretty damn good, and without trimming

Edit:
I will also try to increase the resistor across the LDR, I'll try it tonight.

[merlinb]

I converted the circuit back to dual supply, since I have that available in my modular (dual 12v).
But, I am getting some weirdness.

Caps don't need to be matched, and vactrol speed shouldn't be an issue since the feedback servo is even slower (C10). I suspect the problem is that with +/-12V supplies, the servo opamp is trying to drive enormous current into the LED, and failing. Try a beefy opamp like NE5532. Perhaps try increasing R9 too.

[Jarno]

Hi Merlin, thanks for your reply.

That's a very good point, changing to NE5532 will be hard though


I am using SMT TL074, that indeed has a puny output stage. Judging by the short circuit current, they are comparable though 5532 is 38mA and TL07x is about 40mA. I could go with OPA4227 (expensive!) that has a slightly more beefy output. But I will also change that series resistor to the LED to 220r, those vactrols are pretty sensitive, so driving them to the limit is not necessary I suppose (max 25mA). Maybe I'll go for 390r, that keeps max current within max limits.
Some soldering to do, I'll let you know the proceedings.

[merlinb]

But I will also change that series resistor to the LED to 220r, those vactrols are pretty sensitive, so driving them to the limit is not necessary I suppose

Ah, quad opamps. In that case change the resistor before you change the opamp -it may be all that is required. This will increase the output ampliftude of the oscillator too, which is what you'd expect with so much more headroom.

[Jarno]

thanks!

Yes, common signal level for audio in eurorack modular synths is 10V pp. No worries if it is less though there's always places to adjust in a patch.

[Jarno]

Well, it appears I was asleep at the wheel when putting in components. Forgot 100k in one of the oscs, and mistakenly put 10k into the 1k position of another osc.

They are all humming along now

I get the following ranges:

10nF --> 1.61kHz - 14.44kHz
68nF --> 252.45Hz - 2.36kHz
100nF --> 153.00Hz - 1.47kHz
470nF --> 33.82Hz - 327Hz

I will be using 50k multiturn potmeters in the final build, and have put a 12k tapering resistor over it to get a 10k reverse log response (-ish). It seems to work well on the single turn pots I have on it now.
I used 390r for the resistor in series with the LED (rather than 100r), and 30k across the LDR (rather than 12k).

Right, onto the VCA's!

[PRR]

>> Cite? Hint?
> Newnes Circuit ideas Pocket Book.., probably early 90s.

Exploring an otherwise useless "3,630 Electronics Circuits" book, I found this from 1974.



No cap-switch, uses MUCH lower value freq-resistors, takes gain trim in A1 instead of A2, using diode-clip instead of opto. But same base idea of making amp-gain compensate for Bridge loss with tuning.

Now I am wondering how much further it may go. "Most" basic opamp schemes were known in the 1950s, but practice was limited by high op-amp cost. I had thought that with modules, chips, and quad-chips, the marketers dragged the old books for ideas which now made sense (and sold more chips). Certainly all the bi-quad and dual-integrator and gyrator ploys got space in application notes. Yet I had never seen this one until Merlin presented his rendition.

[merlinb]

I found the book. Very-very interesting. All these technical projects and then "Guard the vegetables without frying the cat".
Newnes Circuit Ideas Pocket Book
ISBN: 0750623365 / 0-7506-2336-5
Publication Date: September 1995
(FWIW, I can't find any "Part Three".)

W A Cambridge's 'Circuit Idea' does not include any references.

FYI by chance I discovered Cambridge's circuit was published in Electronics World, February 1994, p149, before being collected into the Circuit Idea Pocket Book.
PDF available here: https://www.americanradiohistory.com/Wireless_World_Magazine.htm

[PRR]

Dang bit-rot! Here again is my now-missing image in Reply #74:


> Cambridge's circuit was published in Electronics World, February 1994, p149

THANKS!! (A much spiffier reference than the pocket book.) To extract the key bit for convenience:

[amz-fx]

If you need an opamp with more drive than a TL072, then try the NJM4556D, which can drive 70ma into a 150 ohm load. Has an 8MHz GPB and voltage noise lower than the TL072. Slew is 3 V/us which is less than the 072 but more than enough for FX projects. Could be a good fit for a sig gen.

It is a dual opamp and can drive most headphones. Available at Mouser for less than $1 each.

regards, Jack

[tubegeek]

Just finished building one of these, using the vero layout from Sabrotone. https://www.sabrotone.com/?p=2951 I'm pretty sure I'll box this up nicely - useful little gizmo indeed! Might add one more opamp to provide a balanced XLR output option first.

Merlin's been keeping me busy!

[tubegeek]

So here's what I've been able to ascertain about my build so far - I tested the frequency ranges in each of my 4 switchable ranges. Range 1 covers 15.5 Hz to 160. Range 2: 150 - 1650. Range 3: 1500 - 14.4K. Range 4: 14K to about 15K.

Range 4 gets a little touchy - tuning upwards past a certain frequency, the amplitude drops out. Tuning back down until the amplitude picks back up to about where it was, I get to 15K. I rolled my own vactrol with an LED and a photoresistor, but I don't think this is the limiting factor for upper ƒ based on what I understand, the vactrol just affects level.

What *does* determine the maximum ƒ?

Anyway, Merlin's specs appear to be spot-on excepyt for the top range, which is said to reach 100KHz. I'm curious to know what might be limiting the top ƒ. I didn't measure level BTW, just sent it into my laptop so I could count peaks per second and determine ƒ's. I was using a 96K sampling rate in my audio interface so I should have seen something above 15K if it was there, I guess.

In all the ranges, at all frequencies (except as mentioned in Range 4) level stays VERY steady and the wave shape looks pretty damn sine-ish to me. If you bump up the frequency within a range, there is a VERY brief period of settling while the level overshoots a tiny bit and comes back down to where it had been.

My only difficulty getting it working at the outset was a brain fart: I wired the two switch poles incorrectly: the caps on one side were going up, while the ones on the other were stepping down. Once I noticed that it was all sweet and dandy. (I only had a 5-position switch, too, so I actually have one range that is just for DC sine waves.)

I'd say this thing is VERY much as advertised. What a useful simple little gadget. (I haven't tried adding a balanced/XLR output just yet BTW.) I'm particularly impressed by the accuracy of the overlap between ranges and their almost-exactly-decade spacing. Merlin, your slide rule appears to be properly lubricated.

I'm really glad I built this.

Thanks again for the wizardry, Merlin!