Is there such a thing as a simple triangle-wave LFO?

[GeorgeSunset]

Hallo
Is there such a thing as a simple LFO circuit that produces a triangle-wave voltage to drive a Vactrol?
Oscillisation frequency control is a must
Lower and upper limit control would be cool! (depth control?!?)

It's for a wah circuit that is modded to be able to act as a volume pedal, and I like the way you can get a tremolo effect by fast sweeping (but It's hard on the pot)

Any idea's? Any schematics?

I have been looking quite some time now, have giving up, tried again to search the web... now I'm here.

[nexekho]

There's a number of methods.  Using a certain filter on the square wave output by a 555, you can get an approximate triangle wave.

(note the voltage drop)

You can use op-amps.
http://www.falstad.com/circuit/e-triangle.html
I think replacing the 10k between the two op-amps should change the frequency.

[GeorgeSunset]

cool animation!

Thanks for the help!

what kind of opamp is usable in that amplication? is "any" opamp good enough?

EDIT: would 2 TL071 do the job?


Hm, heard the 555 gives a lot of high clock-noise into the powersupply - is that true?
And would 0.5 V not be to low to drive a Vactrol/LED?

[nexekho]

I don't know sorry, never worked with one.  I'd imagine almost "any" would work but I can't say for sure.  It also outputs AC, so you'd need to do something to make it DC - I think you can bias it but not studied enough electronics to know.

You're right that .5v is nowhere near enough for a standard LED.

Also, it's not an animation, it's a full simulation.  It's Java based and you can build circuits in it.  (Falstad)

[GeorgeSunset]

AC.. now it make's a lot more sense!

Actually, I have been looking at the LFO section of ROG's phozer. Thanks to the simulation I actually understand it now!

I got a couple of TL071 in a bag, and they use a single TL072 (double op-amp of the TL071).

It got already a Freq knob. 

[asatbluesboy]

There's a number of methods.  Using a certain filter on the square wave output by a 555, you can get an approximate triangle wave.

(note the voltage drop)

You can use op-amps.
http://www.falstad.com/circuit/e-triangle.html
I think replacing the 10k between the two op-amps should change the frequency.

Awesomeness.

As for my own ¢2, have you thought about adapting the Tremulus Lune LFO? You can have depth, spacing and smoothness as resistors (not pots) to get whatever shape you like. And it's made to work with a Vactrol.

[Hides-His-Eyes]

Yeah, I don't really see the benefit of using a 555 over an op-amp LFO like that in the lune; it looks simpler on a schematic but by the time you consider that a twin op-amp is also a DIP8 I don't think there's much in it.

[R.G.]

Is there such a thing as a simple LFO circuit that produces a triangle-wave voltage

Yes.

to drive a Vactrol?

Maybe. That last phrase puts a lot of restrictions on it.

The first thing to realize is that Vactrols are not controlled by voltage. They're controlled by current. The inside of a Vactrol is an LED which spits light onto an LDR. The LED's light is close to linear with current. The LED responds to voltage by hardly conducting at all until it gets up to some threshold voltage, then starting to conduct massively as voltage increases. That is - it's still a diode, even if it's light emitting.

So all common led/vactrol circuits come up with some way to at least fake driving the LED with a triangle *current* not voltage. The classical way is to use a big triangle voltage and a biggish resistor to keep the max current down to what the LED needs, and hope that the voltage lost to the turn on of the LED is not too significant; a close second is to offset the triangle voltage by a little bit so that it only goes down to the voltage where the LED is close to going off.

It's those considerations that make "simple" and "to drive a vactrol" conflict.

Oscillisation frequency control is a must
Lower and upper limit control would be cool! (depth control?!?)
It's for a wah circuit that is modded to be able to act as a volume pedal, and I like the way you can get a tremolo effect by fast sweeping (but It's hard on the pot)
Any idea's? Any schematics? I have been looking quite some time now, have giving up, tried again to search the web... now I'm here.

I'd go with a classical two-opamp integrator/Schmitt trigger type. That gives you really good speed control, good triangle, and really constant amplitude.  Then I'd use a pot to vary how much of the triangle versus the middle-DC level of the triangle got into a transistor rigged as a current source driver. This still has the offset problem, but a transistor's offset is about 0.5V, not 1.2 to 1.5V.

[GeorgeSunset]

Thank's R.G.

As always 

[boogietone]

This still has the offset problem, but a transistor's offset is about 0.5V, not 1.2 to 1.5V.

Maybe out of my depth, but would a darlington help here?

[R.G.]

Maybe out of my depth, but would a darlington help here?

No, you're exactly right - and that's the kind of thinking you always have to do with LFOs - figure out what offsets and scaling is needed, then figure out a way to go about that which is simple and cost effective. The trick is that to use a darlington with it's typical 1.2 - 1.4V base-emitter offset cancel out some of the offset voltage of the LED, you need a way to use it so the base-emitter voltage opposes the LED voltage, not adds to it. So you get into tinkering with followers, differential circuits, etc.

One really good way to make a part act like it's being driven with a current source (not least because it is!) is to use it in the feedback loop of an opamp. Putting the LED in as a feedback element makes the opamp run it with a current, and offset voltages don't matter because the offsets are divided by the open loop gain of the opamp. Then you have to realize that this is unidirectional, so you have to bias the input LFO voltage going into the opamp so the LED is at "half" or "middle" all the time, and the LFO goes up and down from there, if up and down is what you want.

It's always the range, scale, offset, and linearity (or lack of it) that makes LFOs complex. Getting a triangle wave is simple. Getting what you want out of them is hard.

[smallbearelec]

Try the LFO in my trem:

http://www.smallbearelec.com/Projects/TremBear/TremBear.html

You get considerable control of the duty cycle and frequency, and the bias pot sets a quiescent current level for the LED...controls the "hardness" of the effect.