Over my workbench I have a LED strip powered from 12V @ ~1A. Currently I simply switch it on and off by a button. I would like to add a simple circuit that switch it on when there is another light source (there are no windows, so another light source must be from the ceiling light). It seems simple: turn off the light for a millisecond or so, measure light level with a phototransistor and if there is light turn the light on again.
But the LED strips are two long parallel wires carrying quite large current. I am afraid abrupt turn off and turn on will cause lot of noise. Maybe it would be better to slow down the switching a bit. But I am not sure how to do it. Adding a RC delay to the Gate of the switching MOSFET seems as an obvious choice but how do I size the resistor and capacitor? Active region of MOSFETs are not well defined in datasheets and usually only typical characteristics are shown (and it is said the real values may vary greatly).
And after all is it worth the effort? I have no idea how much noise the LED strip may cause even when switched quickly.
It seems simple: turn off the light for a millisecond or so, measure light level with a phototransistor and if there is light turn the light on again.
That will be annoying!
You don't need to turn the LEDs off, you just need to put the photo transistor at the end of a piece of black tube, such as 40mm drain pipe, and point the tube at the other light source.
But the LED strips are two long parallel wires carrying quite large current. I am afraid abrupt turn off and turn on will cause lot of noise.
Don't worry about it, it's not a problem.
You only need to size the gate resistor. The gate is a capacitor already. Use something like 10k for slow switching for occasional use, but 100 ohms or less for fast switching (PWM) to avoid heat build-up.