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A “SIMPLE LDR CONTROL CIRCUIT” is designed to facilitate the detection of changing light levels using a light-dependent resistor, commonly referred to as an LDR. This component is frequently utilized in applications such as automatic security lighting. Its resistance is inversely proportional to light intensity, meaning that in low-light situations or darkness, the LDR’s resistance is high, thus restricting current flow. With the integration of an LDR and a minimal assortment of components, this circuit offers a straightforward solution for activating a light source when confronted with low-light or dark conditions. In this configuration, the LDR functions as the light sensor, while a transistor acts as the electronic switch.
Simple LDR Circuit


The LDR undergoes a change in resistance when exposed to light. Specifically, its resistance decreases as the amount of incident light increases. By connecting the LDR in series with a 100K resistor (R1), a potential divider circuit is established. This configuration results in the division of voltage, which is influenced by the light’s intensity. Notably, due to the LDR’s connection to Ground, as light intensity increases, the voltage drop becomes more pronounced. This setup exhibits sensitivity across a broad spectrum of light conditions.

The center point of the potential divider provides power to an NPN transistor, such as the BC147 or BC548. Given that the base voltage of this general-purpose NPN silicon transistor surpasses approximately 0.5V in relation to the emitter voltage, the circuit forms a complete connection, leading to the illumination of the LED in low-light or dark environments.

Parts Used:

  • LDR
  • Resistance of 100K
  • BC548 is a transistor.
  • LED

This Magic Eye circuit is both inexpensive and simple to construct. It will only cost you about 20 cents (Rs 15).

Automatic street lights, Electronic Letter Boxes, Electronic Eyes, Automatic Vehicle Headlights, and Automatic Garden Lights may all be made with this LDR circuit.

According to the spectral characteristics of LDRs, there are three types of LDRs: – Ultraviolet LDR: more active to ultraviolet light, including cadmium sulfide, cadmium selenide LDR, etc., used for UV exploration. – Infrared LDR: Lead sulfide, lead telluride and lead selenide are the main ones.

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