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Light-Activated Camera Shutter Timer


A number of years ago I took a course in mechanical camera repair. I needed a robust way to adjust the timing of the focal-plane shutters used in SLR cameras.

This isn't exactly robust, but it was an interesting way to learn a little bit about phototransistors and binary counting.

Fortunately, breadboard prototypes
aren't expected to be beautiful.
This one sure isn't.

A phototransistor is an ordinary transistor that turns on and allows current to flow when exposed to light. The DC resistance of the one I used in this project varies from about 11 MegOhms in the dark down to less than 1 MegOhm when it's exposed to bright light.

The plan was to insert a holder - the clear plastic object on the left - with the phototransistor into the camera's film gate and shine a bright light into the camera lens. When the camera shutter is triggered, the phototransistor is exposed to a minute flash of light. The resulting signal is amplified by a second, ordinary transistor and used to gate the output of a 32768 KHz clock source.

The clock is just a simple feedback oscillator using a CMOS Schmitt trigger inverter and the crystal out of a cheap wall clock. I used a Schmitt trigger in hopes of getting a cleaner clock signal.

The clock signal is then sent to a binary counter which in effect counts the number of clock pulses that occurred whilst the camera shutter was open. This is displayed in binary - this was an experimental circuit - on the 16 LEDs you see at the lower right. Dividing this by 32768 - which this little board doesn't do - should in theory give us the camera's actual shutter speed.

The plan here was to test the concept rather than to develop a full prototype and as it happened there were a few fundamental problems with the concept that relegated this project to an interesting learning exercise and little more.

The optical design of the light sensor turned out to be much more critical than I expected. To make the design work would require a custom optical system and a very intense light source.

This seemed to me to be exactly the sort of problem where you should really be looking for a simpler alternate solution, so this is as far as I went with this approach.

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