Darkness detector using LDR and astable mode of 555 timer IC

This circuit based project demonstrates the principle and operation behind the darkness detector. For example lamps which switch on automatically in the night. Various decoration lights are needed to be switched on automatically in dark.

The circuit described here is more of a sensor application with a speaker attached to it as output. However, depending upon the application, user can attach a night lamp or series of small LEDs to the circuit too.

Constructed around a 555 timer IC and LDR, this darkness detector circuit uses a 9V battery as a power source along with a couple of passive circuit components: resistance and capacitance.

In this circuit 555 works in astable mode producing a frequency of about 56 hertz. Pin 4 which is reset pin is connected to ground through a LDR. Reset pin which is active high if gets directly connected to ground and circuit does not functions, but in this circuit when there is dark, the resistance of LDR becomes very high and conduction does not takes place so high voltage is retained at pin4 and circuit functions; there by making it a dark activate circuit. In presence of light the resistance of LDR becomes low and reset pin takes ground and speaker does not sounds while circuit works in dark and speaker sounds. This project can have varied applications like by using this circuit for high frequency we can make dark activated decoration lamps or night lamps (they could be closed by switch too). In security systems where we need continuous light we can place this circuit along with speaker and this could intimate us if light fails.