Film SLR camera shutter trigger


I have an SLR film camera that can be triggered by shorting two pins of the camera’s remote control port. So I build a super simple switch based on the MOC3021 optocoupler (see the attached diagram). It works – well, sort of. It triggers the camera only once. To trigger the camera again, I have to physically disconnect and connect one of the pin wires. Interestingly, this issue doesn’t occur when I use this switch to trigger a flash. Any ideas what causes the problem and how it can be fixed?

By the way, I tried to use a simple transistor switch, but it didn’t work with this particular camera.



I do the same for a camera remote, but use a transistor rather than the optocoupler as you said you'd tried. I'm really not sure why it would be that wouldn't work but this does.

Is there a 3rd wire in the remote cable? Every model I've seen has one for prefocus and I normally pull both down.

Not having used an optocoupler I'm not sure if this setup might leave the output floating after you stop driving it too. That could be a dead-end but since I've not used them I'd need to do more reading to confirm.

Can you share your code just to make sure there are no wrinkles there?


Hi Geoff,

Thank you very much for your reply! No, there is no third wire (it's a manual focus camera). The code I use for testing is an adapted version of the blink sketch, and the setup works with my other cameras.


Have you checked the opto issolator operation by using a meter across the output?

Checking the first datasheet I could find for an opto issolator starting with "mc03" I find the forward voltage of the diode is 1.3v Subtracting this from 5v gives 3.7v. Dividing this by your 470 ohm resistor gives about 7ma. Yet the datasheet I'm looking at requires a trigger current of 15ma.

Sounds like you're under driving it. (although I'm guessing because I haven't got the full part no.)

Your MOC3021 is a TRIAC output, great for AC signals but will stick on for DC.
Get an opto which has a transistor output and you'll be in business.