AN ELECTRONIC PHOTOCELL
By Gerry Shand
I have always been interested in building a circuit that can compute the
sunrise and sunset time for use in lighting control without having to rely on
the sun like conventional photocell technology does. So, I set out to design a
stand-alone circuit so external interfaces like a time signal, GPS input, or a
computer interface would be unnecessary. I was also looking for a small but
robust design with an LCD where setup is done with two buttons in less than
five minutes — as if you were setting a digital watch.
In other words, I wanted something high tech but simple nough to easily replace photocell lighting control
because — while cheap — photocells have the following
1. They can be affected by things like shadows, so
they have to be mounted carefully — typically,
high above the building or object being controlled.
This means poking another hole somewhere. This
can create a possible issue for security, and the
40 June 2013
■ FIGURE 1. Completed board.
ingress of moisture and pests.
2. They are susceptible to the effects of vibration,
animals trying to take them thinking they are food,
and dust/dirt build-up.
3. Wiring issues — while rare — can happen. Long
wiring lengths also mean higher installation costs.
4. Mounting them high creates maintenance issues
and expensive maintenance costs when they
eventually do fail.