Circuit sensor shadow (twilight) with ldr – lighting control


Serves to control a load according to the lighting in this case in the absence of the light circuit is activated. One application is good practice to light a lamp so dark, like those used in lighting systems for public lighting.

The circuit is quite simple, the trimpot is used to adjust the operating point according to the light falling on the LDR, which changes its resistance depending on the amount of light, the LED serves to indicate ro circuit state and can be removed in many cases.

About light dependent resistor (LDR) photoresistor

A photoresistor or light dependent resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor.

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A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Schematic of the shadow sensor circuit with LDR

Schematic Of The Shadow Sensor Circuit With Ldr

Suggested of the printed circuit board for mounting circuit shadow sensor

Shadow Sensor Pcb Circuits, Energy, Light / Led Circuit Sensor Shadow (Twilight) With Ldr - Lighting Control
Pcb Layout
Shadow Sensor Componnents Circuits, Energy, Light / Led Circuit Sensor Shadow (Twilight) With Ldr - Lighting Control
Sensor Pcb Layout
Shadow Sensor Silk Screen Circuits, Energy, Light / Led Circuit Sensor Shadow (Twilight) With Ldr - Lighting Control
Pcb Silk

Component list

Resistors 1/4 watts 5%
R1, R3 1.8KOhms – Black, Gray, Red, Gold
R2 680 Ohms – blue, Gray, Brown, Gold
P1 10k – Trimpot
Q1 BC548 or equivalent – Transistor NPN
D1 1N4004 or equivalent – Diode
VCC AK500/2 Connector power supply
CN AK300/3 Connector Load
K1 Relay Onrom G5LE or equivalent
PCB, , Box, 12 volts x 500mA power supply, etc.

Download PDF Files for this circuit
Download PDF     Download PDF

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About the author blog author. Electronics technician for the technical school of Brasilia - Brazil. Interested in electronics, circuits and technology in general.

5 thoughts on “Circuit sensor shadow (twilight) with ldr – lighting control”

  1. Hello friends. I made this circuit and realized that the transistor is reversed. The connection of said component to be reversed. The transmitter is attached to the pickup position and vice versa. To fix the circuit simply reverse the transistor and everything will work as expected. Hope this helps.

  2. Hi Toni,

    Thanks so much for your circuit – it saved my college electronics project!

    But just a quick query, did you build this exact circuit? Because I followed your instructions as closely as I could, checking and rechecking, and the circuit doesn’t seem to work. The relay is always on.



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