Make a Simple Optocoupler.

Working late one evening trying to build a time-lapse trigger for my camera, I realised i had no optocouplers in my parts bin. My electronic store was closed for the night, so what to do?

Being the ingenious ex roadie that I am (in my opinion, you can fix just about anything with a ratchet strap and gaffer tape), I thought about how an optocoupler works. Basically it is an infrared led that shines onto an infrared sensitive transistor.

Hmmm I think. How about a light dependent resistor with an led? Sounds like a plan!!!

(Technically, this isn't an optocoupler, but it 'optically separates one circuit from the other', so it's good enough for me... at least 'till the shop opens.)

Step 1: Parts You'll Need

I used an ultrabright white led that i had bought as a job lot for another project that didn't get off the ground yet (30 quid for 1000. Bargain!).

I used one that i had in the parts bin (less than a quid)

I've got a box of the stuff that i picked up from the army surplus store.

Step 2: First Some Safety Precautions.

To stop any shorts, cover the anode and cathode of the led with heat shrink, and also the legs of your l.d.r.

Step 3: Time for a Face Off!

Place the led and the l.d.r. face to face and cover the two with some more heat shrink.

Step 4: All Done!

You now have a rudimentary optocoupler. The resistance in the l.d.r. is high when there is no current to the led. when the led is lit, the resistance of the l.d.r. drops and allows current to flow.

This is my first instructable, but hopefully I'll get a few more posted in the near future.



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    13 Discussions


    5 years ago on Step 4

    thank you very much :-)
    time to make my lightning trigger XD


    8 years ago on Introduction

    I assume from your usage of the word 'quid' you, too, are English (or at least British?) as I don't think quid means anything other than pounds sterling. So, may I ask, where do you purchase these LEDs for £30 for 1000?

    Also, could anyone please explain to me what an LDR does? I know it senses light, but what does it do with the information? Would this 'optocoupler' allow one circuit (the one with the LDR) to flow when the LED is on?
    Thanks in advance.

    Finally, great, clear 'able.

    H4ZZ :-)

    5 replies

    Reply 7 years ago on Introduction

    As I did not see any reply to yr LDR question: An LDR is a resistor whose resistance depends on the amount of light that falls on it.
    More light, less resistance, although there are a few that do the reverse


    Reply 8 years ago on Introduction

    Oh, and as far as what an optocoupler does, it isolates 2 separate circuits.

    I know you could use a transistor, but this stops a surge from an unprotected side of the circuit messing with a delicate side of the circuit. Ii can also control one circuit of a certain voltage with a circuit of a different voltage. (Think controlling your expensive camera with a hodge podge intervalometer that you think MIGHT just work, but your not willing to risk it...)

    (I'm at work and this is being written as quick as possible before the boss gets back!!! If you need clarification, I'll take my time to explain fully at a later date.)


    Reply 8 years ago on Introduction

    Of course ebay, what else did I expect...

    Right I think I've got it. I'm pretty new to electronics at the moment...

    Thanks flip :-)


    Reply 8 years ago on Introduction

    I got these from ebay a while ago. Just keep your eyes out for '1000 ultra bright white led' and you should pick up a pretty good bargain. They are now going for about £25 at the moment with free shipment from Hong Kong.

    Get 'em while they're hot! (or as hot as leds get).

    Hope this helps.


    If you're using a lighter on the recommended teflon heat shrink tubing, I'd say that it'd be a good idea to not breathe the fumes. I don't remember at what temperature PTFE depolymerizes, I believe it's somewhere around 465c (800f)-( I may have the two confused) - Whatever it is, fluorine compounds have never been known to play nice with humans.

    Steve Potman

    8 years ago on Introduction

    The heat shrinking idea might work. Just like roadieflip said, if you are quick with your lighter everything should be good. I like the steam idea a lot as well.

    If anything goes wrong I buy all of my tubing, heat exchangers, and other components from Fluorotherm. Good luck. I hope everything goes well.

    If you're quick with your lighter, there shouldn't be a problem. Mine both work fine after getting the Flaming Gas treatment. Also, with them being used in a time-lapse trigger, they will only be on for one second every minute or so.

    I was concerned other people might not know to be careful with heat, dont forget while you might be experienced, there will be learners here, while not doing your project, they will picking up information on heat shrink tubing.