I have a similar need, for a remote cue light, powered by 9V battery, activated by a photocell taped to a camcorder's onboard record/tally light.
A friend sketched me a wiring diagram (I don't read schematics). A very simple circuit; good, because I have very limited electronics knowledge. It's basically a battery, wired in series with a photoresistor and a red LED. I am pretty good with soldering, it's just the circuit details have me confused. Specifically, it appears that the photoresistor will basically substitute for the current-limiting resistor, true? I bought the RS grab-bag and in bright light the most sensitive one has a minimum resistance of 1K ohm.
This is much higher than the calculated R for the current-limiting resistor.
Given my 9v supply voltage and 1.7V forward voltage, and 20mA current rating on the LED, it's calling for 390R, 1/2watt
Questions:
do I also need a current-limiting resistor, since it will be in series with the photo-resistor?
am I wasting a bunch of current and possibly heating up the photoresistor unecessarily?
am i wasting significant potential brightness?
is there a better, but not too complex, way to use the photocell to trigger the LED? Is there a wiring diagram available?