Hey Reader, Happy Wednesday!

Let’s look at how streetlights work this week. I aim to write my newsletter issues in a way one can follow them while traveling on a bus, having a coffee, waiting for food, etc. Let's jump in!

How it works: 15-second answer

Street lights use cadmium sulfide (CdS) photo-resistors to turn on automatically. These photocells change electrical resistance based on light levels: low resistance in daylight and high resistance at night. This change is detected by a circuit that activates the lights, enabling automatic control without human intervention.

There are also other ways used to automatically turn on the street lights.

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How do Street lights turn on automatically?

Street lights turn on during the night time when it’s dark and they turn off when it’s sunrise or when there is enough light.

How does all this happen automatically without any human intervention? Or is there a human involved in the process? Let’s find that out by exploring different technologies used to make the same happen…

Idea #1: Light-sensitive sensors

The more commonly used technology is a cadmium sulfide photo-resistor (CdS) sensor. This is a light-sensing component used in the circuits mounted on street lamps.

The electrical resistance of CdS cells changes with the amount of light that falls on them and is hence called photo-resistors or photocells. When there is enough light during the day due to sunlight, etc., the resistance of the CdS cell is very low. But when there is not enough light in the dark, the resistance is very high. The circuit detects this change and activates the part of the circuit that turns on the street lights.

Thus, without any human intervention, the lights are controlled using a photocell sensor.

Idea #2: Using a solar panel

Solar panels are made of photovoltaic cells made of silicon, which can convert light into electricity.

A solar panel is mounted on the top of the street lights, which generates an output voltage proportional to the amount of light falling on it. When this voltage drops below a threshold (say 3 Volts for example), then we know that there is not enough light and that it’s dark. The circuit then powers the light to turn on using the solar energy it stored during the day.

This is fully self sufficient and we don’t even need a battery like in the previous idea.

Idea #3: Smart control system

Each street light tracks all the following data in real-time together to decide whether to turn on the lights or not:

1. Motion sensors to detect vehicles in the streets and turn on the lights accordingly
2. Live weather data to check for rain, fog, etc.
3. Sound system to detect a horn, voice of a human.
4. Amount of light falling on its sensors.

By taking all these inputs into consideration, a street lamp controls the light accordingly.

Idea #4: Using Timers

This is the old school and simple way of doing it. Basically we program the time in the control system to decide when we want the lights to be turned on and off. Like say, we want it to be turned on at 6 PM and turned off at 5.20 AM. But this needs constant maintenance and adjusting as weather changes often due to daylight savings, geography, and global warming. So it can get dark earlier and the timing will need some adjusting.

Question of the week

Can you come up with your own idea for how you would like to control a street light? There are so many variations out there already but it’s not very difficult for us to theoretically think of a new one. Reply to this email with your thoughts and we can have a discussion.

Also, I rebranded our newsletter from “One gadget at a time” to “How do gadgets work?” starting this week. It conveys more about the newsletter and works well in the long term.

Thank you for your constant support.

Have an amazing rest of the week, and take care!
Until next to next Wednesday,
Chendur

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