Photo transistor vs LDR vs Jfet in a modulation, tremolo, etc. context.

[Danich_ivanov]

Hi, everyone.

Currently i have a circuit that uses jfets to modulate things, but recently i discovered that phototransistors are much more widely available and cheaper compared to jfet's and especially LDR's. And i was wandering as to whether or not i should just go phototransistor route, and whether they will be a sort of in between jfet and LDR in terms of "feel".

I'm not necessarily looking for a technical info, as I worked with bjt's before in that conext, but rather personal experience and caveats.

Thanks.

[Kipper4]

It's a tough one to answer since we dont have much context from your post.

I suspect that bjt optos won't work that well in many circuits. As they tend to be more used for logic type switching and wont really have a "feel". It's either on or off.

I'm intrested to see what you have in mind and what phototransistors.

I could harp on about vactrols for hours. For me in many applications where a variable resistance is what creates the modulation vactrols are a no brainer.



Rich

[rankot]

I have never tried to use them, but as far as I know they have too steep on/off slope, so they are not a good choice for that.

[Mark Hammer]

The appropriateness of any sort of control element will depend on a couple of things:

1) What sort of signal amplitude it needs to handle.

2) How smooth the driving signal, whether LFO or envelope of some form, will be.

3) How fast one needs the control element to function.

So, for instance, LDRs do not distort, as FETs are susceptible to.  They also have a nice tendency to introduce a small amount of lag that helps to smooth out the last little bits of envelope ripple in many contexts.  But, unless you find one that has fast transition times, they can't handle fast modulation very well.  In phasers that use LDRs, often the fastest modulation speeds will be accompanied by a reduction in sweep width, because the LDR can't change from full dark to full bright as fast as the LFO wants it to.

Conversely, JFETs and other solid-state devices can handle fast transitions admirably.  BUt you need to be mindful of what sort of signal amplitude you expect them to handle.  There are things you can do to improve the signal-handling of FETs, but that involves more parts, and is not without its own issues.

[drummer4gc]

This article is specifically about envelope detectors but Table 1 has a good comparison of a lot of different factors related to different control elements:

http://www.geofex.com/Article_Folders/ECFtech/ecftech.htm

[Rob Strand]

i discovered that phototransistors are much more widely available and cheaper compared to jfet's and especially LDR's

LEDs and LDRS:
- controlled resistance     
- they don't distort the signal for reasonable level signal.
- the dependency of the control signal to the controlled resistance is fairly useable over a wide range.

Common Garden Phototransistors:
- largely intended to be used as a switch
- the resistance is fairly non linear, and
- only works over a small range of signals before distorting (unless fully on or off)
- the dependency of the control signal to the controlled "resistance" is barely useable

The take home message is they are quite different to JFETs/LDRs etc.

[Danich_ivanov]

Thanks guys, that's some helpfull info!  Although i'm still thinking of trying phototransistors just for educational purposes, i'm also looking forward to try mosfets. It's just that i like to buy all the stuff from one place, and Ldr's are quite costly there, and availability isn't inspiring either, so in the end i might just go with the jfets. Unless i forget, i'll try to report on wether or not phototransistors are worth considering in any way for this sort of stuff.

[Kipper4]

Photo transistor will likely be ok for a tremolo.

[Danich_ivanov]

Photo transistor will likely be ok for a tremolo.

I guess you're right. But there's only one way to find out!

[Kipper4]

I have some 4n25 on order.
To do some hacking with. Back in a bit.
Let us know how it goes.

[Rob Strand]

Photo transistor will likely be ok for a tremolo.

You can actually use a normal transistor and get the same result.
There's a few transistor temolos out there (as well as some envelope filters and fewer voice compressors).

[Danich_ivanov]

So i found an old toy which by accident has a phototransistor in it, i tried it (phototransistor, not a toy) in a magnavibe (sort of) context, and i have to say that it works really well and smooth. I can't say what model is it or anything really, rather than that it looks like a basic LED. Definitely gives a more interesting sound than jfet, and i'm looking forward to get a dosen of them. Not discouraging at all.

[Kipper4]

Let us know how it goes please.

[Danich_ivanov]

Will do!