What is the use of LDR in automatic street lights?

You might have noticed some street lights that automatically get turned on in the night and turned off in the morning.  This happens because the lights have photoresistors or light dependent resistors (LDR) that sense the sunlight and control the street lights accordingly.

How does LDR work?

LDR working

LDR is a light-dependent resistor that changes its resistance when different amounts of light fall on it.  They work on the principle of photo conductivity where it gives less resistance in high light intensity and high resistance in low light intensity.  In other words, it gives high resistance at night and low resistance in day.  LDRs are made from semiconductor materials like cadmium sulphide, which help the lights to have their light sensitive properties.  When light falls on the surface of LDR, the conductance of the element increases or the resistance of the LDR in the control circuit falls.  When it becomes dark, the resistance of the LDR increases and switches the light on.

LDR has been used for years in a variety of applications such as camera light meters, automatic exposure controls, motion-detector lights and street lights.  Traditional street lights consume a lot of power and many times, the lights remain on even after sunrise.  Automatic street light systems using LDR have a lot of advantages. Street lights placed in LDR system do not demand any monitoring as they function on their own.  They are reliable, cost-effective and effective.  They do not require the labour force to switch on and switch off the lights.

Automation in solar street lights

LDR in Solar Street Light

Solar street light usually work at night and stay off during the day time.  Operation of a solar street light depends upon the presence or absence of light in the atmosphere; therefore, LDR works very well in controlling the solar light.  LDR is usually available at a reasonable price and this is an intelligent way of introducing automation in the street light system.  This also improves the overall operation efficiency and power consumption of the street light.

During sunset, visibility starts to decrease and levels of natural light reduce.  Therefore, when the visibility drops, solar street lights are expected to turn on.  Automatic solar street lights switch on by themselves by analyzing the visibility data delivered by a sensor.  During daybreak, the visibility level starts to rise due to the presence of natural light in the atmosphere.  There is obviously no need to operate the solar street light during the day and it is only feasible to conserve energy by turning it off. An automatic solar street light turns off the light automatically by sensing the presence of sunlight.

An automatic solar street light circuit must consist of LDR, transistor, resistors, breadboard, battery and wires.  In combination with a light-dependent resistor, transistor acts as a controller that controls the operation of the solar street light.  The working of a transistor is dependent upon the working of the light-dependent resistor.  The input of the transistor should be the output of the LDR.  Transistor sends a signal for the light to turn off and when no signal is available, the light turns on.  When the light available in the atmosphere is below the limit necessary for the operation of the LDR, then the resistor cannot provide input to the transistor.  When there is sufficient light present, then the LDR sends a signal to the transistor, which then sends a signal to stop the operation of the solar street light.

Modern solar street lights have energy-saving motion sensor and dimming features which help the charge to last longer and the lights to continue their operation even during non-sunny days.  Individuals and businesses have been recognizing solar-powered street lights to be a convenient outdoor lighting option and becoming more responsible in terms of their contribution towards the society.  With latest technological advancements in the solar sector, solar street lights have become more affordable and accessible to everyone.