Building Dark Activated LED Lights

light christmas DIY transistors

Let's learn how to build dark activated LED lights to brighten up the dark time of the year.

Building Dark Activated LED Lights

It's winter, it's cold, and it's dark outside. What's better than bringing some light into this darkness. In this year's DIY project, we are going to build our own dark activated LED lights from a cheap battery operated LED string.

Here is what we need:

  • a battery-powered LED string
  • an LDR
  • a resistor between 1 kΩ - 1 MΩ (100 kΩ recommended)
  • a MOSFET (FQP30N06L)
  • a piece of perfboard (14x5 holes)

Required material

Additionally, you are going to need to following tools:

  • a soldering iron and solder
  • a wire cutter
  • a 3.3 mm drill
  • a pencil

The Circuit

Let's have a look at the circuit for this project. The LEDs shall light up when it gets dark outside. We can achieve this by using a simple transistor circuit which is shown below.

Circuit diagram for the dark activated LED lights

The LDR and the resistor R1 form a voltage divider. The resistance of the LDR increases the darker it gets. A high resistance of the LDR means a higher voltage at the MOSFETs gate. As soon as the gate voltage crosses the 1 V mark the MOSFET starts to turn on.

To control at which brightness level that happens we can tune the value for resistor R1. The optimal value depends on how you want to use your LED string. If it you want it to only turn on, if it is fully dark you require a high value like 470 kΩ or more. If you want it to also light up on darker days, you need a lower value like 10 kΩ.

I can only encourage you to experiment, until you find the value that fits your use case. If you don't feel like experimenting, I can recommend you to use 100 kΩ. It should fit for most indoor use cases.

Let's build it!

The Result

And there it is. Using our little circuit, we now have made our own dark activated LED lights. May they be of joy to you!
Have a merry Christmas and a happy new year. Be blessed!

The result: Our LED string now lights up in the dark and turns off in light

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