LFO help

[idlefaction]

hi all

i have a dual opamp LFO, like out of the Zombie Chorus, driving an LED/LDR assembly, but the turn on voltage of the LED is annoying me.  i have two LEDs in series, one hooked up to the LDR and one external as a visual cue.  so i have this 9V triangle, but the bottom 3V doesn't create any light  

i was wondering if anyone had any ideas as to how to solve this?  i tried the way used in the zombie by biasing the LFO midpoint higher, but it stopped pulsing.  i used a 10k/20k divider, since i figured that would make the lfo oscillate about 6V.

any ideas?

[J. Luja]

well, you could use a transistor or fet to drive the LED. or you could put some diodes between the opamp (-) power and ground so that it could only sweep as low as the turn on voltage of the LED (I wouldn't put them in series though)

[idlefaction]

that's a pretty good idea, i'll try a 3V zener and 10k/10k string and run the opamp power to the top of the zener, the lfo ground to the 10k/10k junction, and run the LEDs between the LFO output and the bottom of the zener.  thanks!  

[gez]

that's a pretty good idea, i'll try a 3V zener and 10k/10k string and run the opamp power to the top of the zener, the lfo ground to the 10k/10k junction, and run the LEDs between the LFO output and the bottom of the zener.  thanks!  

Low power zeners only do what it says on the tin when a certain amount of current flows through them.  You might need to lower the value of the divider resistors to get a 3V drop across it.

The FET/trannie option, as mentioned by J, is a pretty good way to go.  You could use a MOSFET with some DC bias on the gate, set by a trim pot.  This is the method I use - consumes less power (trim-pot across the rails can be really high value) and allows a lot more control over the LED than the method you're proposing.

[Nasse]

I have very little experience of this subject but noticed once when driving some different leds (I socketed them) from opamp triankle wave generator that there is difference in leds, some "looked more linear" than others, some even "gated" the light intensity output. If I remember right some green superbright ones were nice. But did not drive LDRs with these...just looked with my eyes...