[sdiy] Vactrols subbed for dual-gang pot

[Tom Wiltshire]

Cheater,

> I understand how you get maximum and minimum resistance (the led being
> on and off), but how do you get anything inbetween?

There are two ways; one is to control the LEDs brightness with PWM, like you suggest. The second way is to use a current source to control the current through the LED.
I'm a fan of the first method over the second because it is simple to implement in digital systems and mitigates the effects of the non-linear relationship between current flow and brightness in LEDs.

> I understend you can use PWM to change the statistical average of the
> diode's brightness, but doesn't the LDR respond to those changes
> extremely quickly? Wouldn't it be then like a 'switched resistor'?

In practice, LDRs are slow to respond to changes (often 10s of mS). If the PWM runs at a high enough rate, the filtering effect of the LDR alone is sufficient to provide the averaging.

As an example, the Iron Ether Cygnet tremolo pedal:

http://ironether.com/pedals/cygnet/

This uses a vactrol to control the gain in an op-amp (a classic simple VCA design). The Vactrol's LED is controlled by my own Tap Tempo LFO chip, which outputs a 19.5KHz PWM pulse train. If you have a listen to the samples, you'll hear that there's no PWM breakthrough. There's no filtering anywhere in the circuit (I've seen the schematic).

On a different note, Dave's dual-vactrol BPF that started this thread would be cool as a pedal with one of these LFOs...

Regards,
Tom