LFO / LED Relations

[HamSandwich]

Or, how to drive an LED with an LFO.

I've been goofing around in LTSpice. Below are four schematics of a couple of ways to drive an LED from an LFO. LFO is 0-5V. The LED specs are If=25mA, Vf=3.6 (not a lotta choices in LTSpice). The opamp can supposedly swing down to ground.



On a DIYStompboxes thread, the fourth schematic was recommended as the LED in the opamp loop gets a constant current in exact proportion to the LFO. I've read that the issue with the first three schematics are that the brightness of the LED won't track with the LFO as it won't turn on until the LFO reaches a certain voltage/current.

So I simulated them and plotted the current.



Note that D1 doesn't appear because it is exactly the same output as D3.

The problem with D1 (and D3 as it's basically the same thing) is the LED won't turn on until it hits that magic voltage number, so you get the big bottoming out in the troughs.

D2 and D4 appear to be similar, but flipped along the x axis. Is it safe to assume the flatline of D2 is because the LFO isn't turning on the transistor at that point?

The clipping looked a lot like headroom issues. So I tried biasing the LFO in the transistor up a few volts and messed with the CLR and base resistor, and it looks like the current tracks pretty well with the voltage from 0mA up to the If 25mA. Besides trying to get 3.1V as a bias voltage, is there anything inherently wrong with this?



The headroom issue also seemed to fix the LED in the IC loop problem. I had to guess at the CLR in this case, as I'm not sure how to determine it in this setup. They both seem to be opperating similarly. Is there any reason that one is better than the other? Real world problems that don't show up in a simulation? Transistor circuit should take up less space than a big ol opamp.

[Scruffie]

IIRC some Morely phasers used a transistor set up, nothing wrong with it, just requires a trimmer to set up.

[HamSandwich]

Thanks Scruffie!