Darkness Detector Alarm

This dark detector 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. It can be used to alert that somebody is present on door even before the person presses the door bell. It is a perfect technique in a restricted area where only authorized person is allowded. It can be used by installing the sensor accordingly. The circuit also can to be used in the darkness by slight modifications in the sensors. The circuit can be iplemented by connecting relay at output (in place of UM66 IC) to on the bulb or you can also use LED's in series. So when there is no light it is switched ON serving as you as a emergency torch. It is used for decoratipn purpose. For example light will be switched ON while detecting darkness at night. This circuit can operated with pencil cell as it require less only 3V for its operation. It can easily be mounted because of less components used in circuit.

This darkness detector circuit explains the principle of operation of LDR (light dependence resistor). LDR is a device whose sensitivity depends upon the intensity of light falling on it. The resistance of LDR decreases when intensity of light falling on it increases and vice versa (resistance increases when intensity of light falling on it decreases). In dark or in absence of light, LDR exhibits a resistance in the range of mega ohms, which decreases to few hundred ohms in presence of bright light.

This phenomenon can be measured with the help of multi-meter by keeping the multi-meter in ohm or resistor measurement region. When you cover the LDR, its resistance will be very high and when you place it in light it starts decreasing. We are utilizing this property of LDR to act as a sensor, since a varying voltage drop can be obtained with varying light. In this circuit amount of light will vary as the shadow of the visitor falls on LDR. LED’s should be mounted carefully.