Building Dark Activated LED Lights

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!

Step 1: Let's go! As a first step, we place the components on the perfboard without soldering them in yet. If you want to, you can already cut them to the right length.

Step 1: Placing the components to cut them to the right length

Step 2: With the components in place, we can use a pencil to mark the positions in which we want to drill the holes for the wires.

Step 2: Marking the spots for the holes

Step 3: For drilling, we need to remove the components again and make sure to safely clamp the perfboard on a piece of scrap wood. We can then drill the two holes.

Step 3: Drilling the holes

Step 4: With the holes made, we can now place the components to finally solder them in.

Step 4: Placing the components to solder them in

Step 5: After that, we connect the resistor and the LDR to the MOSFETs gate with a solder bridge.

Step 5: Soldering the LDR and the resistor to the MOSFETs gate

Step 6: Next, we need to attach our LED string and its battery holder. For this, cut its wire in half and tin the ends of each side.

Step 6: Cutting and tinning the wires of the LED string

Step 7: Pull the wire through the holes and fixate them in place with a knot. The knot serves as a strain relieve.

Step 7: Pull the wire through the holes, fix them in place with a knot as strain relieve

Step 8: Now for the tricky part: Take a multimeter and determine the correct polarity of the LED string and the battery holder. Connect the positive side of the LED string and the battery holder directly together (+). Connect the negative side of the string to the MOSFETs drain (D) and the negative terminal of the battery (-) to its source pin (S).

Step 8: Solder in the LED string and the battery holder

Step 9: As a final step, use some wire or the cutoffs of the components to connect the LDR to the negative and the resistor to the positive side of the battery.

Step 9: Solder the remaining connections

Finished: It's done! If you turn on the battery and cover the LDR, the LEDs should now light up. In light, they should turn off again.

Finished: The LED string lights up in the dark

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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