Andy Neil wrote:
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What about a 7414 (etc) Schmitt trigger?


Not a good idea; the positive threshold voltage is something between 2.5 and 3.5V.

Chris, I understood what your problem is the first time round. If you want to do this with any amount of reliability, you will have to use a comparator. The method with a transistor will probably work (provided the camera output can supply enough current to switch the transistor on), but is extremely dependant on the characteristics of the individual transistor. It's also very temperature dependant.

Regards,
Rob.