Ever notice how your refrigerator light turns on the moment you open the door? That simple, satisfying glow is thanks to a sensor or switch—and today, we’re building our own version of that using a Light Dependent Resistor (LDR), also known as a photoresistor.
Whether you’re working on a solar tracker, robotic eye, or just want a light to fire up at sunrise, this beginner-friendly electronics project is a great way to get hands-on with responsive lighting.
What You’ll Need
To build this light-activated LED circuit, you’ll need the following:
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1x Photoresistor (LDR)
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1x LED (or a pre-wired LED with a resistor)
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Resistor (if not already wired to the LED)
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Battery and battery connector
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Quick-connects or soldering iron and solder
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Helping hands tool (optional but highly recommended for soldering)
Quick-Connect Build (No Soldering)
Let’s start with the simplest version—using quick-connects. This is perfect for prototyping without having to break out the soldering iron.
Step-by-step:
- Connect the LDR: Take two quick-connect clips and attach one lead of the photoresistor to each.
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Wire the Negative Side:
- Connect the negative (black) wire from your LED to one quick-connect.
- Connect the negative (black) wire from the battery connector to the same quick-connect.
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3. Wire the Positive Side:
- Connect the positive (red) wire from your battery to a second quick-connect.
- Connect the positive (blue or red) wire from your LED to this same quick-connect.
Once everything is connected, attach the battery—and just like magic, the LED lights up. Now try covering the LDR with your hand or an object. The LED dims or turns off, depending on the amount of light. That’s responsive lighting in action!
Soldered Build (For a More Permanent Circuit)
Want a stronger, more permanent connection? Let’s solder it up.
Soldering Steps:
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Strip the Wires: Carefully remove a bit of insulation from each wire.
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Twist Wires Together: Pair the appropriate wires (positive to positive, negative to negative).
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Solder the Joints:
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Use a soldering iron to heat the twisted wires.
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Apply a drop of solder until it flows and covers the joint.
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Let it cool, and make sure it’s solid (a “cold solder” joint won’t hold).
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Once you’ve soldered everything together, add the battery and watch your LED respond to light just like before. The circuit is now much more secure and durable.
How It Works (and Why It’s Cool)
The photoresistor changes resistance based on how much light it detects. More light = lower resistance = LED turns on. Block the light, and the resistance increases, which dims or shuts off the LED.
This is basically how your fridge works! Either a physical switch or a sensor like this detects when the door is open and turns on the light.
You can use this concept in all kinds of projects—from sunrise alarms to outdoor garden lights that only come on at dusk.
Want the Opposite? Lights That Turn ON in the Dark
If you’re looking to build a circuit where the LED turns on in the dark instead of the light—don’t worry, I’ve got a video coming for that soon.
This light-activated LED project is perfect for makers looking to get into basic electronics or for anyone who wants their creations to respond to the environment. Whether you’re using quick-connects or going full solder, it’s a super satisfying build with lots of creative applications.