Interesting Ideas: Two-way Optoisolator made with two LEDs
When I was but a wee lad around the age of 10, I wandered into a Radio Shack with my father. Of course, this is back when Radio Shack sold good electronics parts. I wanted to know what they did. My dad didn’t know. Somehow I ended up with a copy of Getting Started in Electronics by Forrest Mims III. The book was handwritten and the illustrations were hand drawn, and it was perfect for kids. Or adults. I still have a copy of this book, not the original one my dad bought me — I’ve since lent that out many years ago. But I was able to pick up a copy from the third printing, still with the original green cover, and I’ll hold on to that one forever. It cost something like three bucks at the time. The book is still in print — you can get it from Amazon, Jameco, or from the W5YI group.
I found this article at Jameco.com that Forrest wrote about using two LEDs as an optoisolator by mounting them facing each other and wrapping them in electrical tape. I would probably use heat shrink tubing if I tried this. This is an excellent application showing that an LED can also be used to detect light. The example circuit uses the homemade optoisolator feeding a 4011 CMOS NAND gate as a driver for an output LED.
For best results, use LEDs of the same type to make a two-way optoisolator. The LEDs should both be encapsulated in clear epoxy. I’ve had very good results with red AlGaAs super-bright LEDs and AlGaAs and GaAs:Si near-infrared LEDs. All that’s required is to point the LEDs at one another and secure them in place with black electrical tape or heat shrink tubing. When using tape, cut a 1.25-inch length of tape and place it on a work surface with its sticky side facing up. Then place the two LEDs across the tape so that they point directly at one another. For best results, rotate the LEDs so that all their leads are in the same plane. Then simply roll the free tape over the LEDs and over onto itself.
(published from DFW, Texas)