If you're building a circuit that requires an optoisolator, you have to choices (hopefully, that is) - either buy the device or "roll your own".
Sometimes 'rolling your own' can result in 'happy accidents' - you may find a combo of LDR and LED that you like better. For quickest results, buying the specified device is recommended.
However, that may not even be an option - so many devices have been discontinued. If you can find the datasheet of the device, you're one step closer to success.
I'll use the VTL5C4/2, a 'dual' optoisolator as an example.
You can build something close to a VTL5C4/2 using one LED and two light detecting resistors (LDR's). You would need to rig it so that the LED shines on both LDR's. The five legs of the improvised VTL5C4/2 would consist of:
(1) The anode of the LED
(2) The cathode of the LED
(3) One leg of LDR #1
(4) The other leg of LDR #1 connected to a leg of LDR #2
(5) The remaining 'free' leg of LDR #2
Note that this is a dual optoisolator. If it's a single opto, like the VTL5C4, then there is only one LED and one LDR. Therefore, it has only four legs:
(1) The anode of the LED
(2) The cathode of the LED
(3) One leg of the LDR
(4) The other leg of LDR
Normally, people use heatshrink or some kind of potting solution to fashion optoisolators.
The real trick, however, is finding the right combination of LDR's and LED to mimic a VTL5C4/2's characteristics.
To begin with, here is a link to the VTL5C4/2 datasheet:
http://optoelectronics.perkinelmer.com/ ... l5c4-2.pdf This may help you somewhat in your selection process.
The most important factors that determine the characteristics of an optoisolator are:
1. Light Resistance (the lowest resistance when the LED is on)
2. LED current needed to reach the light resistance.
3. Dark Resistance (the highest resistance when the LED is off)
4. The time it takes an optoisolator to go from dark resistance to light resistance
5. The time it takes an optoisolator to go from light resistance to dark resistance
Items 1, 2 and 3 should be easy to figure out with a DMM. Items four and five are not so easy, unless you have an oscilloscope, function generator and a test circuit set up on a breadboard or perfboard. Items 4 and 5, however are *very* important and will impact the performance of the circuit if the optoisolator is being used in things such as compressors, filters, VCA's, etc, where it has to respond to (or in some cases, *slew*)fast control transients. Many circuits depend on these characteristics.
However, without the mentioned test equipment, here is something you can do:
1. Buy a bunch of LDR's, and a bunch of LED's
2. Get a 'ballpark' figure on points 1 and 3 (just measure the resistance when the LDR is covered or exposed to fairly bright light) and select the one's that come close to the figures in the datasheet.
3. Get a film cannister (those little black containers that photographic film comes in) and punch 5 *tiny* holes in the lid of the cannister.
4. Mount the LED and two LDR's on the inside of the lid, with the LDR's pointed at the LED as closely as possible. Run the legs out through the five holes. Snap the lid back on the film cannister.
5. You now have a rather large and bulky optoisolator. Get a breadboard and mount this contraption on it.
6. From the points of your circuit that connect to the VTL5C4, run jumper wires to the appropriate legs of the contraption.
7. Listen to how it sounds/works. This way, you can mix and match LED's and LDR's until you get something you're happy with.
8. "Hand Roll" your custom VTL5C4/2 using the components that sounded best to your ears.
For more in depth information on actually putting an optoisolator together, along with a way to actually measure and select the response time of your device using a scope (as mentioned above), I highly recommend Motohiko Takeda's page here:
http://www.aleph.co.jp/~takeda/radio/phaser/indexE.htmlLots of pictures, graphs, and charts - Motohiko is very thorough!
Cheers,
Scott
