The circuit described below produces a musical sound whenever you put your hand on the sensor or a shadow of the person falls on sensor. It can be named as shadow alarm. This circuit can be used at our homes near door bell. Whenever somebody is present on the door it alerts us about the presence even before the door bell is pressed. A very useful technique in restricted area where only authorized person are allowed but in that case sensors need to correctly chosen. This circuit can also used to switch ON the light when darkness is detected with little modification. For example you can connect relay at output(in place of UM66 IC) to on the bulb or you can use LED’s in series. So when there is no light this can be our emergency torch. It can serve the decoration purpose, by switching the light ON when the darknessis detected at night.This can be done by connecting LED’s at output.This circuit can operated with pencil cell as it require less only 3V for its operation.Can easily be mounted because of less components used in circuit.
This circuit based project demonstrates the principle and operation behind the darkness detector. For example lamps which switch on automatically in the night. [[wysiwyg_imageupload::]]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 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. Keep on reading to find out how the circuit is assembled and how it works.