Opto-Coupler

[Papa_lazerous]

Hi all, I've just found out about optocouplers. As i needed something for switching on a valve project i'm working on.

Being the sort i am i want to make one as i figure a LED and a LDR are all thats needed and some heat shrink to keep them in the dark. Who here's tried this before? Just interested in knowing any pitfalls or tips before i waste my time. I don't know anything about LDR's i'm assuming u can get different power ratings like resistors. What size led's work well also would be good to know also colour of led does that matter?

Thanks for any advice

[fikri]

I think someone has done this LED-LDR switching thing with good result. As for me, it is very hard to tweak the 'on' value between 100Ohms. You got to have a very good LDR though. I ended up using Relays with driver for my tube preamp switching because it is cheap enough and still Quiet. If you have the money, i suggest you buy the 'real' Opto-Coupler.

[d95err]

Soldano and some other companies use optos for switching in tube amps. Typically the Vactrol brand is used, because some of their 'couplers have the appropriate power and voltage ratings for tube amps.  You can find power and voltage ratings for LDRs in the corresponding datasheet.

The most important data for switching is the on- and off resistance. Some LDRs will only go up to 100k or so, and some won't go below 5k. For proper switching, you need a very high off resistance (several Megs) and very low on resistance (a few hundred ohms at the most). The Vactrol VTL5C3 goes from about 100R to 10M.

[Papa_lazerous]

I think I will buy the opto's then.  No sense making a good project crap by trying to make one when they are available cheaply enough.

I am a little concerned about it having soe resistance when its meant to be on, surely this will suck some of the signal?

[fikri]

I am a little concerned about it having soe resistance when its meant to be on, surely this will suck some of the signal?

Yep

[Jay Doyle]

I think I will buy the opto's then.  No sense making a good project crap by trying to make one when they are available cheaply enough.

I am a little concerned about it having soe resistance when its meant to be on, surely this will suck some of the signal?

Yes, most likely, but if you buffer the following stage to a high enough input imedance, you should be able to get by with minimal, and unnoticeable, loss.

Unfortunately, nothing comes free in this hobby, everything is a tradeoff...