Inverting an LFO

[seten]

I have an LFO out from a JRC4558D (copied the circuit block from this https://www.pedalpcb.com/docs/PitchWitch.pdf) that I'm using to wiggle an LED and I want to get an inverted LFO to wiggle a second LED. I'm using a standard inverting buffer configuration (http://www.muzique.com/images/buff10.gif except no 5pf cap and 100uf caps for input and output) but theres no signal at all on the output. If I put the LED before the output cap theres just a constant voltage.

What am I missing here? I can get voltages but the inverting buffer works and passes guitar signal so I'm guessing this is just not the right way to go about inverting the LFO.

[EATyourGuitar]

You didn't post your circuit. You didn't say where the LED is grounded. You can draw the circuit on piece of paper or ltspice or kicad or falstad. You also didn't say what that constant voltage is. Also you should mention if your black probe is on 0v or Vref 4.5v. maybe you have the LED backwards. Do you have an oscilloscope? The input resistor to the second opamp should be connected from the output of the opamp driving the LED.

[PRR]

> get an inverted LFO to wiggle a second LED

LED can float. One is wired from LFO to ground, right? Wire another from LFO to V+.

[Rob Strand]

For single supply LFOs the output never goes to zero.   The LFO output swings above and below some bias voltage.   Even at the lowest point the voltage is still positive.

To get the LED to go on/off you have to introduce some voltage drop so when the output is low there's not enough voltage to make the LED go on.

Here's an example which uses the VGS voltage drop of a MOSFET (and the LED voltage drop),
https://www.diystompboxes.com/smfforum/index.php?topic=122777.0

If the LFO output swing is very small there won't be enough output change even for that circuit.

To make the LED_on match-up with the desired sound of the effect, you can (optionally) invert the output with an additional opamp inverter and feed it to the above circuit, or, you can uses a PNP/P-channel version.

Sometimea you need to tweak the voltage drops with LED color or adding diodes in series with the LED.

If the LFO zero level (ie. the mid point) isn't symmetrical with the supply then you have to forcefully add in some DC offset.

[iainpunk]

either this:

> get an inverted LFO to wiggle a second LED

LED can float. One is wired from LFO to ground, right? Wire another from LFO to V+.

or this

which i used for a stereo tremolo experiment.

cheers

[amz-fx]

Edit: Oops, already covered above by Rob. 

-Jack

[seten]

For single supply LFOs the output never goes to zero.

AHH yes that was my mistake.

> get an inverted LFO to wiggle a second LED

LED can float. One is wired from LFO to ground, right? Wire another from LFO to V+.

and that is my solution. Thanks so much! I had a feeling I was missing something stupidly simple.