adding voltages ?

jonfoote · Post by **jonfoote** » 12 Jul 2009, 21:27

hi,

ive built this lfo http://www.experimentalistsanonymous.co ... %20LFO.gif

i get between 0 - 5 volts at the output which i use to turn an led on and off.

any ideas how could i change this so i get 2 - 5 volts on the output?

thanks

earthtonesaudio · Post by **earthtonesaudio** » 13 Jul 2009, 01:01

Are you sure that's what you want to do? If you're driving an LED the LED will limit the voltage to it's forward voltage drop, for example 1.5V for an "average" red LED.

jonfoote · Post by **jonfoote** » 13 Jul 2009, 09:05

thanks for the reply, probably could have explained myself a bit better.

basically i want to stop the led turning completely off. so a square wave output from the lfo would go from full brightness to dim light.

thanks

earthtonesaudio · Post by **earthtonesaudio** » 13 Jul 2009, 12:05

Okay, that makes sense.

One thing you could do would be to add a voltage divider bias to the LED (much like voltage divider bias for a transistor base) to have it always biased on at the voltage of your choosing.

If your LFO was connected straight to this bias point, it would still drive the LED to 0 and +5V, unless you added something between the op-amp and the divider.

If you put a resistor between, the op-amp's output will add to and subtract from the bias voltage, which makes things a little more complicated.
However, if you instead used a diode between them (anode to op-amp, cathode to divider), your divider would determine the low voltage, but the op-amp would then determine the high voltage, minus one diode drop.

jonfoote · Post by **jonfoote** » 13 Jul 2009, 13:11

thanks for that, took me about 10 read-throughs but i get you now.