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DON'T GET CAUGHT IN THE DARK

Building a Darkroom Timer
by Noel Rios

Start ý Circuit Operation ý Troubleshooting The Timer ý Using The Timer • Sources and PDF

My brother, who is a photographer, recently asked me if I could build a darkroom timer for his enlarger because he has to hurry and locate the switch every time he uses it. A light bulb lit inside my head, and I said to myself, "This would be a great idea for a project." Not only did the prototype work, but I also found other uses for the darkroom timer. One use is for my spin dryer. As we all have experienced in the past, mechanical timers usually break down, and usually break down often. I also discovered that this gadget is a good replacement for all devices using a mechanical timer.

Washing machines are a good candidate, but they have multiple contacts to modify the connection of the starting capacitor and the windings of the motor to reverse rotation. This applies to newer washing machines, those not using gears. Equally important, I use the darkroom timer to control my homemade UV exposure unit for making PCBs from positive pre-sensitized ones. If you replace the relay with one that has a greater current capacity, you can also use it for toaster ovens. The limit of its usefulness is only your imagination.

The circuit is also easy to construct and inexpensive compared to the $100 cost for most darkroom timers. You can use the flash memory version of 87C51, the 89C51, which costs approximately $1 each.

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