how would one make a light that turns on when it is dark?

I am making my sister a doorbell for her room for her birthday and need help with the circuit

Lurch9 years ago
Was looking into this the other day as it happens for an IR project.

There is a simpler circuit without the op-amp, there's loads of those about, top of the list in Google. ;)
westfw9 years ago
Sandisk1duo9 years ago
UziMonkey9 years ago
That's pretty easy. You can use either a photoresistor or a phototransistor. When the light level drops, so will the resistance, which you can use to light the light. The problem is, if the light is strong enough, it will fool the phototransistor into thinking it's light out, and shutting the light off.