This light activated relay circuit presented here uses the 555 timer IC and a light dependent resistor or LDR to form a light sensitive relay in an intruder alarm system or for switching on a lamp at ...

As you probably know the LDR is a light dependent resistor. Normally the resistance of an LDR is very high, sometimes as high as 1MΩ, but when they are illuminated with light resistance drops ...

Light-dependent resistor (LDR) The resistance of a LDR depends on light intensity. ... So, let's dive into what they look like in a circuit diagram. Okay, ...

A photo-resistor, 9V battery and a PC speaker in series will pick up the audio if the light is focused on to the photo-resistor. I did this back in the late 80’s with my home stereo.