LDR Darkness Sensor Circuit


Joined Jun 4, 2014

Has a basic "dark sensor" design that will do what you want.

I'd imagine that you'd end up doing something like this...
View attachment 246339

... but as xox said, you are going to be much better off if you learn how to use a comparitor.
That circuit needs a resistor, perhaps 10k, in series with the pot otherwise the transistor will be destroyed when the pot is dadjusted to one end of the track.


Joined Sep 24, 2015
Using a 12v input and the LED's I originally linked to, would I be able to just add an LDR to the circuit and have it work?
It's unlikely to work as the LDR has a variable resistance. High resistance would mean very little current flow, but there will always bee current flow. Low resistance would mean more current can flow. And likely MUCH more. To eliminate the potential for over stressing your LED's you'd need an additional resistor to protect it. However, again, the circuit is constantly using power. If from a battery - count on it not lasting more than a day. If plugged in - it's going to be wasting a lot of power.

Why are you so hesitant to explore Op-Amps? But if you want to use the transistor - you have been given suitable schematics.