Triangle Wave LFO and Bipolar Supply

[leeranya]

hey everyone,

I know almost nothing about op-amps. I'm trying to build a triangle wave LFO to drive a LED, and kind of got lost there.
I'm using this schematic for the LFO:

and powering it with this:


I connected pin 4 of the LM358 dual op amp I used to vcc-, so, the -4.5V I got from the supply, and pin 8 to vcc+, so, the 4.5V I got from the supply.
I also connected pins 3 and 6 to ground (the meeting point between the two 10k resistors on the supply).

Is it ok for me to use LM358 instead of the TL072?

I'm using this to flash a LED, but for some reason the LED is always on... what seems to be the problem?

[Dan N]

Funny, looks too easy to be a working LFO. Is it oscillating?

You don't show how you hooked up your LED.

[leeranya]

right - I connected the LED to pin 1 of the op-amp, and to ground.
the led is simply always on...

[Dan N]

Unfortunately, I can't breadboard anything at the moment. If all you need is an led driven by a triangle wave, how about the "Fading Red Eyes" circuit down on this page?

http://www.bowdenshobbycircuits.info/page5.htm

Or, maybe borrow that transistor driver for your circuit?

One thing, both that and this circuit:

http://www.synthdiy.com/files/2005/LFO.circuit.font_fixedsystxt.txt

want a higher value resistor for your R2 so either bump up R2 or decrease your R1.

I don't know...

[leeranya]

thanks man, that actually helped me.

[mac]

I have to try it in my phaser, I want to compare sine vs. triangle.
Is there a way to make it quite independent of Vcc so as to keep the same light and sweep as the battery goes down?
Maybe a zener diode to keep Vref estable, like in the P90.

mac

[Cliff Schecht]

I have to try it in my phaser, I want to compare sine vs. triangle.
Is there a way to make it quite independent of Vcc so as to keep the same light and sweep as the battery goes down?
Maybe a zener diode to keep Vref estable, like in the P90.

mac

Modulating the VCC of an op amp won't change the gain, just where the upper and lower limits of the clipping thresholds lie. As long as the supply is above the peak of your signal by a volt or so then you're fine.

[George Giblet]

Is there a way to make it quite independent of Vcc so as to keep the same light and sweep as the battery goes down?
Maybe a zener diode to keep Vref estable, like in the P90.

The swing problem is
- triangle amplitude depends on the schmitt levels
- the schmitt levels depend on the opamp output swing
- the output swing depends on the supply rails

Regulating the Schmitt opamp supply will fix that, but another solution is to clip the output of Schmitt opamp before it feeds the Schmitt feedback resistor.

Even that doesn't solve the whole problem for the phaser because you not only have to regulate the LFO swing you also have to regulate the midpoint of the swing - you make the DC component of the LFO track the DC bias point the Phaser JFETs.

[mac]

My phaser is based on LEDs/LDRs. I just do not want the LED to go out of tune when the battery begins to drop, say a voltage or so.
I was thinking of a LM317 feeding the LFO -> Buffer -> LED

mac

[leeranya]

thanks for all of the good replies! you've all been helpful.

how would I adapt the R.G. Keen ramp up/down speed mod (http://www.geofex.com/FX_images/ramp-lfo.pdf), or any other mod to ramp the speed, to this circuit?

[R.G.]

You would cut off that schematic at the base of the 2N3904 and throw away everything to do with the opamps. Then substitute in my circuit and connect the opamp base to the opamp output marked with a triangle wave.