ON/OFF photoresistor switcher?
Depending on the current
required by the different LED loads,
the value of the Cds sensor is critical.
In addition, you should include hysteresis to snap the load ON or OFF,
rather than have it operate erratically
at the dawn or dusk threshold. If the
Cds sensor has a large value at the
critical threshold, there will not be
enough current to bias the transistor
ON to its saturation state.
To remedy this problem, I have
included two circuits which solve your
issues. Figure 1 uses two transistors
with positive feedback to create the
needed hysteresis and gain to drive an
output stage that provides 100 mA
current limiting in case your load
circuit shorts out.
Figure 2 uses a 4093 quad-nand
Schmitt trigger (that has one volt
hysteresis) and drives the same output
stage as the first circuit. The second
circuit is probably the easiest one to
implement, but requires the 14-pin
4093 IC. You will still need to select a
Cds cell that has around a megohm of
resistance when dark and 10K or
less resistance when light. Hope this
helps you.
Ron Hoffman
Solon, OH
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