Photocoupler Questions

[ParlorCitySound]

I'm in the beginning stages of building a tremulus lune. I see in a few layouts it uses a VTL5C2 photocoupler. If I'm correct, the Resistance of the photocell changes as the LED changes in intensity. Has anyone tried this in other places (minus trems/compressors)? Say, to control the "time" on a delay?

Just a thought.

-Kevin

[R.G.]

Yep.

[ParlorCitySound]

ha, how did it work out?

[liquids]

While humorous, it's too huge an answer to detail comprehensively...since the answer is yes, it's done' frequently. In brief, opto-based stuff of one kind or another has been used widely as a basis for trems, compressor, phaser-type stuff, filters, and yes, delays:

http://www.diystompboxes.com/smfforum/index.php?topic=74294.0

How did it work out? It seems to work out well enough, considering the relatively popularity of use...FYI, I find the datasheets of those photocouplers are worth looking at (and you'll end up wanting one of each type!) information, but the application notes are well worth looking at given your inquiry: 

http://optoelectronics.perkinelmer.com/content/ApplicationNotes/APP_AnalogOpticalIsolatorsAudioApps.pdf?bcsi_scan_4915F3EBBE8861A9=3yf16pLOwIZ2vyvILXKKBpcAAADxlg1V&bcsi_scan_filename=APP_AnalogOpticalIsolatorsAudioApps.pdf

[brett]

Hi
if you'd like to do some really nice research for all of us, you could look into optocouplers like the 4N25, where the resistance between the base and emitter of the phototransistor varies the current transfer and the delay.  There's a buck-load of untapped potential in those characteristics.

The other nice thing about optocouplers is that they can save you from grounding issues because they use 100% optical transmission (sorry if that was obvious).
cheers

[ParlorCitySound]

Awesome stuff!

I should have been a little clearer, I was thinking one day (total noob thoughts with little understanding of how they work) how cool it would be to control delay time with a photocoupler. The idea being using a an LED (in like a clipping light scenario) to alter the resistance of where the "time" pot would be. Harder you play, the more delay, softer->shorter (or vice versa). Looks like it's time to get wild with some research.

on the same lines, how cool is the Clari(not)!

Thanks!

[JKowalski]

Photocells aren't the ideal voltage controlled resistor (which is basically what you are trying to create with this). Photocells introduce a ton of distortion to your modulation waveform, because they are very slow to react to changes in the LED.

You can also make voltage controlled resistors from FET's, they tend to be cheaper/easier to find and work better in some cases. Of course, you can make your own photocoupler with an LED and LDR yourself, but there are a couple problems with that. One, is that if you need multiple of them in the same circuit, each one will behave very differently, as typical LDR's vary widely in their range. The other, is that you need to make a way to encase the two components so that they couple, but so any outside light does not enter in.

LED/LDRs are easy to install, but they introduce distortion. FET's are more reliable, but they require precise biasing to behave like a voltage controlled resistor.


LED/LDRs are typically what we use in stompboxes - I suppose because nobody wants to go through the trouble of biasing FET's. Alot of people have added them to designs in the same way you are talking about. I've seen them in echos, compressors, phasers, tremolos, vibratos, auto wahs - basically any time based effect. People also like to put a switchable one on the top of their stompboxes, so they can control a pot by using their hand to block or let in light to the open-air LDR.

[ParlorCitySound]

JKowalski, Thanks for the info! I wasn't too worried about the added distortion, I'm kind of into the qwerks of stuff like that; I didn't know they were slow to react to the LED intensity. That is handy information. Does anyone have an instance where someone has used a FET as a voltage controlled resistor? I'm pretty knew to the design aspect of pedals, so please pardon my ignorance.