Silicon Chip photographic beam break trigger

Recently I constructed the beam break trigger and photoflash trigger kits featured in Silicon Chip magazine. Both look good in general however I ran into an issue getting the beam break kit to work with the trigger because of the short duration of the output. I thought I’d share a letter I just wrote to Silicon Chip about the problem in case anyone else runs into the same problem before it’s published in print form:

After constructing the beam break trigger from the June 2009 issue I didn’t have any luck getting it to trigger a camera either directly or via the photoflash trigger kit. Unless I’m missing something doesn’t the 10nF coupling between Q1 and Q2 lead to a time constant of mere microseconds? I didn’t measure the timing before-hand but after doing a rough calculation I placed a 1uF cap over the 10nF which lead to a trigger time of somewhere in the order of 10mS and all was fine with the photoflash trigger kit. I left it at that because it’s my intention to use it with the trigger kit however readers should be aware that some cameras require a longer pulse on their external trigger to fire. For example my Canon EOS 450D seems to require a minimum duration of about 60mS in manual focus mode, presumably if the pulse is shorter than the normal shutter lag time it gets ignored. Other than that they are a pair of excellent projects and looking forward to exploring the possibilities they offer.

2 thoughts on “Silicon Chip photographic beam break trigger”

  1. Hi
    Did you ever get the beam breaker kit to work?
    I recently bought it, after assembling it, and trying for over a week now, i’m still having trouble getting the kit to work.
    It says in manual, that the light will only glow when a beam is found, but my light glows the moment i turn it on.

    Any suggestions?

    1. Hi Viv, mine worked fine apart from directly to the camera because of the short trigger time. Being a fairly simple design though without modulation it can end up being triggered by other sources of IR pretty easily, I noticed pointing at a window for example would trigger from IR light from the sun. If you haven’t thought to try it already maybe fire it up in a fairly dark room at night or cover the photodiode entirely to see if the LED goes out. Also try measuring pin 7 on IC1 to see if it rises as the beam is pointed at the photodiode. According to the article it should be around 1V when dark and > 1.7V with the beam, so if it’s higher than that maybe you could say drop the value of the 2.7K resistor that goes to ground to see if you can get a decent result.

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