How do the light sensors in a night light work that allow it to turn on in the absence of direct light?
one is to use a light sensitive chemical - cadmium sulphide - it reduces its resistance as increased light falls upon it. This is a classical LDR = light dependent resistor. Look up part ORP12. Another method is to use a diode junction. i.e. a doped PN junction in silicon or germanium. Many original transistors and diodes were photosensitive and had to be painted or encapsulated in a can. With the BC107/8/9 parts - you could snip off the back of the can to make a phototransistor. Later transistors are embedded in opaque materials. Another technique is to use the photoelectric effect - where UV light causes jumps in energy levels for electrons - the material begins to conduct if the wavelength is small enough (i.e. high enough energy.)
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