Posted with permission of Frank Rivard
It is quite a simple circuit but rather more effective than the one using the 741. It uses a simple comparator LM311
as the master piece. This comparator is powered from a +12V DC supply and does not require negative supply to work
efficiently. The feedback given to the comparator provides some hysteresis and the potentiometer allows to adjust
the sensitivity of the detector for darkness. I am quite sure you could inverse R1 and R2 positions to inverse the
action of the detector but I haven't tried it!
The resistance of the light sensitive resistor is a trial and error game but I have been using values around 300 ohms
under visible light and 3k under darkness... but as I repeat, you must try several types of photocell before it works
correctly and you have enough span to adjust it correctly. It all depends what you have in your "junk" box!
The magic thing about this light sensor is that it does not trigger on and off at the same level of darkness
(hence the purpose of hysteresis) and it makes it good for everyday uses. It will trigger from one state to the
other when it is dark enough but will not trigger back and forth several times when you are on the edge of darkness.
It requires a higher level of light to trigger back to the previous state.
So, if you use it to control outdoor lighting for example, it will produce a single clean trigger from on to
off ... It will turn off one at dawn and will turn back on only once in the morning when light will be strong
enough to energize the comparator. This is quite good because it avoids the relay to trigger several times under
If you have questions or design improvement upon the above circuit, please contact
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Page Copyright © 1995 - Tony van Roon
Circuit Copyright © 2001 Frank Rivard