You can use an LDR in a circuit that detects darkness and turns on an LED (Figure 2). The circuit uses a high-voltage threshold-detector IC that features a current output and operates as a comparator. The LDR and potentiometer R₃ form one side of a Wheatstone bridge. Fixed resistors R₁ and R₂ form the other side. You can operate the circuit from a 5 to 65V battery because the bridge excitation comes from an on-chip 2.4V series regulator that is referenced to the supply voltage. The chip keeps the 2.4V regulation voltage below the supply voltage. Resistors R₁ and R₂ form a fixed reference voltage at the noninverting input of internal comparator A₁. The LDR and R₃ form a variable voltage at the inverting input. When the light level falls, the voltage on the inverting input falls below the reference voltage until the comparator trips, activating the relay and the LED. The total voltage across the resistors is always 2.4V. Choose the values for these resistors based on your desired threshold voltage using the equation V_TH=−2.4×(R₁/(R₁+R₂))=−2.4×(LDR/ (R₃+LDR)), where V_TH is the threshold voltage.

You can reverse the position of the LDR and potentiometer R₃ to switch on the relay when the light exceeds a preset level. You can adjust the potentiometer to preset the switch to any light level, making it an ideal light sensor. The IC’s output current is less than 100 nA when the negative pin’s value is greater than that of the positive pin. The output current goes to 1 mA when the positive pin’s value is greater than that of the negative pin. This current drives a ground-referenced resistor to develop a logic-level signal at D_OUT. The logic signal is buffered with the NPN transistor that then drives relay switch S₁. You should use a latching relay, which uses permanent magnets to hold the armature in place after the drive current is removed.