Oscillators -> LED's

[Barracuda]

So I've been wanting to do some circuitry resembling the ROG Phozer:




However I want to change the LED/LFO section, and have the frequency triggered by an LDR. I'm looking to use a dual opamp. So the first amp will create a twin-t type wah looking circuit, and the second *hopefully* to produce an LFO and hook that up to an LED/LDR. I've been looking at this schematic alot because it looks alot like the one in the Phozer, and I place the LED with the cathode at ground and the anode at the LFO output.




All that comes out is just a bright light. This was on breadboard however so maybe my connections were just funny? would this work?

[samhay]

Two things.
1, The op-amp LFO you posted is a phase shift oscillator, but I'm guessing you worked that out. What you may not yet have worked out is that Rf has to be about 30x larger than R. In practice, it may have to be even bigger to get it to start in a reasonable time. This circuit isn't as popular as others, but I like it for a single op-amp design precisely because you can easily drive an LED from it.
2, If/when the op-amp output is wiggling, it will give a somewhat clipped sine wave that varies between ~2 and 7V for a typical op-amp on a 9V supply. However, to get this to work, you must reference the op-amp to Vb (or Vf, whatever you call it; i.e. the half supply point created with 2 resistors and a large cap to ground). To do so, connect the (+) input and the R connected to the (-) input to Vb.

If you do this, you should see the LED pulsing. You may not quite be there yet as depending on supply voltage and LED colour, it may not go out at the bottom of the LFO swing. To get it to go out, you can either put additional diode(s) in series with the LED or use another voltage divider as e.g. is used in the Tremulus lune LFO LED - see the 2 1k resistors:
http://img32.photobucket.com/albums/v95/mancubus/blinking_schem.bmp

[duck_arse]

However I want to change the LED/LFO section, and have the frequency triggered by an LDR.

what do you mean, you want [signal] envelope controlled LFO frequency, or you want the LFO to start and stop under LDR control, or something else?

[Barracuda]

Hi guys,
Thanks Sam for that useful info, I'm going to have another crack at that and let you know how it goes! Also for the resistors to ground will they all go to VB instead of ground? The Tremulus Lune looks like a different LFO, is there a name or link I can follow to read up on it?
also here are the values I've been using


At the Twin T bridge is a resistor to ground which will control the frequency, I will connect the LDR from ground to the center of that bridge. The LFO is to control the LED which will vary the resistance on the LDR.

Thanks!

EDIT:
I have got it working now with 1uf Caps instead, but I'm confused on what is the best method to change speed and depth?

[duck_arse]

in the transistor PSO we change the freq by varying one (or both) of the resistors to ground in the C//C//C string. same applies if you use an opamp (but leave the R at the input alone). for an opamp on a single supply with Vref, the R from the (-) input should go to Vref, but the other 2 R's can go to ground or Vref, as they are DC isolated by the caps.

[Barracuda]

I did try this out with success! but I'm not sure it was sine wave oscillation from watching the LED :/