My washer and dryer have indicator led light labelled completed which turned
on after completing the washing or dryer process. I would like to use
photoresistor to act as a swith that I can connect to a digital input.
You might certainly do it with a photo resistor, but ambient light may
prove to be a problem. You'd have to hang the photo resistors over
the LEDs and make some effort to exclude ambient light (unless you're
in the habit of working in the dark).
Photo resistors are not photo switches - the output of a
CDS photo cell drops to low hundreds of ohms when light versus megohms
when dark. A silicon photo cell puts out a weak .5 volt signal in the
presence of light and may put out little or nothing in the presence of
red light. LED's can also be used to sense light - they output a
weak voltage depending on the color of light and color of the led (red
led emitter should be matched to a red LED for a detector) White LED
will detect blue light best. Photo transistors would have similar
drawbacks.
Better and easier to wire an opto coupler in series with the
indicators and bring those outputs to a connector. That would
eliminate ambient light problems and avoid kludging up the front
panel.
If, by chance, the LEDs are modulated with a high frequency, they
could be detected over some distance and may even work with high
ambient light. There's probably no reason they would be modulated.
With an unlimited budget you could come up with a way to detect the
LED from a distance if it is that important.
