Dark Detector Circuit

Here is a dark detector circuit designed with well known parts to work smoothly when the surrounding light changes. When the LDR is in dark the relay will be ON. And when the LDR is in light then the relay will be OFF.

The circuit used an ordinary op amp configured as a voltage comparator. In this comparator mode when non inverting pins voltage below the inverting pin voltage then the output will be low. When the non inverting pins voltage goes above the inverting pins voltage then the output will be high. Change the R3 preset until you get a smooth ON-OFF function.

The circuit has hysteresis, it is done by the R4-100K resister. Feeding a small amount of current back to the input reference. This will prevent the relay toggling in ambient light.

The whole circuit works with a 5V power supply. If you wish to power this from a 12V supply then change the relay to a 12V one.