[sdiy] LDR optocoupler blowout

[Tom Wiltshire]

On 19 Sep 2012, at 21:07, David G Dixon wrote:

>> With the slow response of LDRs and the regular means of 
>> controlling LED brightness being PWM, I wonder if these could 
>> be used as a digitally controlled resistor, assuming that one 
>> clocked the PWM comfortably above the response time of the 
>> LDR. Just a thought.
> 
> Seems like a lotta trouble.  Just use an OTA or 2164.

Not at all. It's much simpler. Example (by Taylor Livingstone and myself):

http://musicpcb.com/pcbs/tap-tempo-tremolo/

This does a tremolo using Dan Green's classic Tremulus Lune audio path (basically just an inverting op-amp with an LDR setting the gain) and uses a uP generating a PWM signal to control the amplitude. Using an OTA would make the audio path much more complex - and we only need one, so we'd probably have to use a 3080 (ugh!!) or waste lots of stuff on the IC (LM13700 - one OTA and two buffers, or SSM2164 - three VCAs). Plus if we used an OTA or a VCA, we'd either need to filter the PWM, or we'd need a DAC too - add another chip.

The downside of the PWM/LDR combination is the same as it's benefit - the response time. Don't expect audio-rate modulation. An LDR just won't do it. But if you like your modulations smooth, it's lovely! (Ahem...Buchla anyone?)

T.