Light Dependant Resistors: The Fundamentals of Automatic Street Light Design

I’m not usually afraid of darkness. In fact, I can’t fall asleep at all if any lights are left on. But a few nights ago, I desperately missed the familiar glow of street lights while attempting to observe sea turtles laying their eggs at a local beach. I missed out on an experience of a lifetime, hidden in pitch darkness.

Of course, street lights might sound dull compared to the exciting electronics behind technologies like the Internet of Things (IoT). But without a solid design, the street lights we take for granted may not turn on when they’re supposed to. Let’s start with the basic science that makes street lights intelligent.

The Evolution of Automatic Street Lights

Back when I was a kid, street lights were not as intelligent as they are now. They always turned on at a specific time in the evening and turned off early in the dawn. In cities where the daylight didn’t vary much, there weren’t any significant issues, except when clouds blocked sunlight.

These days, street lights are far more intelligent and can turn on as soon as ambiance decreases. The street light’s control mechanism senses brightness using a light sensor and decides whether to switch on the light. In electronics, a light sensing component is the Light Dependent Resistor (LDR).

How Does an LDR Work?

As the name implies, a Light Dependant Resistor functions by varying its resistance with respect to the light intensity that it is exposed to. The resistance decreases as light intensity increases. A typical LDR has resistance in the range of megaohms when it’s dark, and hundreds of ohms in daylight.

LDRs are made of semiconductors that contain very few moving electrons. This explains their high resistance in darkness. When exposed to light, the semiconductor absorbs the light photons, which results in more electrons being released. Naturally, the resistance decreases.

LDR is the heart of an automatic street light

Key Aspects of Automatic Streetlight Design

Besides the light sensor, one of the main considerations in designing a street light is choosing the correct light bulb. The light bulbs used in street lights are usually high-voltage and high-luminance. The brightness of the bulbs is usually decided using roadway width and the distance between the lights in a specific formula.

Once the light bulb’s specifications have been chosen, you have to design the control circuit. With the LDR at its core, there are two options to approach the design: with or without a microcontroller.

The principle of using the LDR to turn on the street light is to capitalize on the changing resistance on a voltage divider. The LDR is connected to another resistor, and the varying voltage between both components is used to activate a transistor. The transistor is then used to switch on the light bulb through a mechanical or solid state relay.

If you’re not fond of designing with transistors, a microcontroller can also be used in an automatic street light design. The same principle of voltage divider applies, and the varying voltage is connected to the Analog to Digital Converter (ADC) pin of the microcontroller. The microcontroller then uses a digital threshold value to decide on switching on the light bulb.

A microcontroller based design allows more flexibility. Editorial credit: spaxiax / Shutterstock.com

Naturally, using a microcontroller offers more flexibility, in terms of adjusting the threshold value of switching on the light. This may be useful when the LDR suffers degradation in time and its dark resistance increases. A configuration module in the firmware will allow the offset value being taken into account by the street light algorithm.

Optimizing Your PCB Design for all Environments

When designing and manufacturing the Circuit Board, designers also have to consider the environment where the PCB will be installed. If it is installed in a marshaling cabinet that may be exposed to the elements, the Printed Circuit Board needs to be designed to operate reliably in a hot and humid environment.

A professional PCB design tool like Altium’s Altium Designer^® can help you optimize the key aspects of street light design and develop functional street lights for any environment, humid or otherwise.

Not sure if your street light will turn on when it’s dark? Talk to an expert at Altium.