Response on SC photographic beam break trigger

I received a prompt response from Jim Rowe at Silicon Chip Magazine on the beam break trigger. Interestlingly the pulse was designed to be short to work with the time delay trigger kit but I didn’t have any success using the two together, maybe some of the components supplied in the kit were out of spec:

Greetings Mr Johnson,

You are correct in that the effective trigger pulse width from the June 2009 Beam Break Trigger will be only a little over 110us, as determined by the 10uF capacitor and the total resistance of 11.1k# in the charging circuit.
The pulse was actually made this short to prevent multiple triggering of the Time Delay Trigger published in the February 2009 issue. However if you want to use the Beam Break Trigger to trigger a camera directly, I imagine that the pulse width will be too narrow – as you have pointed out.

Your remedy of increasing the capacitor value from 10nF to 1uF is fine, but if this does not give sufficient lengthening of the trigger pulse for some cameras, the resistor from the gate of Q2 to ground can also be increased in value from its present value of 10k#. It could be increased to 22k#, 47k# or even 100k# if a much longer pulse is needed.

Thank you for your feedback, and I hope you find the projects useful.

Jim Rowe

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