Here is a project of a simple dark detector LED circuit. In the previous topic we have discussed a light activated LED circuit. By simply changing the positions of the resistors and LDR we have converted the circuit in a dark detector LED or dark activated LED circuit. The circuit will light up an LED
when it will detect no light. An LDR (Photoresistor) is used in this circuit for sensing the light and dark conditions, and transistor 2N2222 is working as a switch. When the LDR will detect no light it will open the transistor 2N2222 due to which the LED will light up. The 50K variable resistor (VR) is used to adjust the desired light condition on which the LED will light up and 1K resistor is used to protect the LDR from direct connecting to the supply when the 50K variable resistor is on zero. The 330 ohms resistor is used to provide the required current to the LED. The circuit can be operated with four 1.5V batteries of any size.