Introduction: Dark Detecting LED Flowers

You could use an Arduino to make a dark detector (nightlight), or you could just use a transistor.

You could make a regular nightlight, or you could turn a decorative object into an interactive light display.

That was my thinking going into this project. Once you understand the circuit, you can apply this to your own design.

But, here's what I did.

Step 1: Gather Materials

For the circuit you'll need

  • 3 LEDs
  • 3 330 Ohm resistors
  • 1 1 kOhm resistor*
  • 1 photoresisitor
  • 1 2N2222 transistor
  • 1 multipurpose PC board
  • Wires

* Depending on your specific photoresistor and how responsive your lights are, you might need a different resistance. Use a breadboard to play around and test.

For the flowers

  • Felt
  • Flowerpot
  • Cardboard


  • 2 AA batteries
  • 1 AA battery holder


  • Soldering iron and solder
  • Wire cutters
  • Pliers, needlenoose
  • Handsaw or Dremel tool for cutting the PC board

Optional: breadboard and a multimeter for prototyping and testing

Step 2: Understanding the Circuit

This project builds from your basic dark detector or nightlight. Evil Mad Scientist Windell Oskay shares the basics of how the transistor works here.

Note: These components are different. We're using a 2N2222 transistor and a photoresistor, and we're using 3 LEDs instead of just one.

The transistor works like a switch between the photoresistor and the LEDs. When it's light out, the phototransistor's resistance is lower than the resistance from the LEDs, so the current skips the LEDs. They never turn on.

When it's dark, the photoresistor's resistance is higher than the LEDs, so the current flows to them instead. The lights turn on.

So what does our circuit look like?

Step 3: LEDs in Parallel

If we want to light just one LED, that's easy. But multiple LEDs need to be in a parallel circuit. See how I have it here.

I'm new to electronics, so I like to use Autodesk Circuits to test whether things will work. It does, so let's get building.

Step 4: Measure, Cut, and Strip Wires

Get your 3 LEDs, 3 330 Ohm resistors, the photoresistor, and two different colors of wire.

Cut the wire to get 8 pieces 3-4" long. The length is based on whatever it is you're lighting up. I want my flowers to be close to top of the flowerpot, so I'm using shorter wires. If you want to wire purchased fake flowers, you'll need longer wire.

Strip the wire ends about an inch.

Step 5: Solder the LEDs and Photoresistor

Wrap one end of the resistor to the LED anode (positive side, the longer leg).

Wrap the other end to the stripped wire. Wrap a second stripped wire to the LED cathode (negative side, the shorter leg).

Do this for all 3 LEDs.

Take the remaining 2 wires and wrap the ends to the photoresistor.

Step 6: Prep the PC Board

I tried to stay as close to my digital design, so I used this PC board from RadioShack that looks like a breadboard.

You won't need all of it. Before you cut it down to size, lay out the transistor, photoresistor, 1 kOhm and resistor to make sure everything fits. Cut and strip wires to connect the components.

Mark where you will cut. Then use your handsaw or a Dremel with a cutting tip to cut it.

Step 7: Cut Felt and Dart Petals

For each flower, cut 7 circles of felt for the petals and one yellow circle for each flower's center.

Lay out the circles to design your flowers. For each petal, sew a dart at the bottom. angelichigo has a great Instructable on felt flowers. Use it to help you sew the petals and rows of petals. Stop at Step 7 when you sew four petals together. You'll want to keep the middle loose so you can slip a wire through.

Step 8: Assemble the Flowers

I've tried this project by (1) sewing the flowers together first and then adding the LEDs, and I've tried (2) sewing the flower around the LED. Number 2 works best for me, because it's incredibly difficult to thread the wires through the sewn flower (1).

Start at the center. Arrange your LED at the center of the yellow circle. Then slide the wire through sewn petal layers (step 7).

When you're finished, you should have three flowers ready to pot!

Step 9: Solder the Transistor, 1 KOhm Resistor, and Wires

Use the picture notes to see how I connected everything.

Just a reminder: With the flat side of the transistor facing you, the left leg is the emitter, the middle is the base, and the right is the connector.

I'm no soldering expert, so if you need help with this, check out Colin's Lab: Soldering.

Step 10: Flip the Board Over

Remember those holes you left for the photoresistor?

Now we're going to use them!

With the back of the board facing up, insert each end of the photoresistor into the holes you left open.

To get a clean look at the base of the flowers, and to prevent the flower wires from getting tangled with the transistor, flip the board over. Then insert the stripped wire base through the bottom of the PC board and solder the wire to the top.

Make sure you get a solid connection to the copper.

Step 11: Solder the AA Battery Pack to the Board

Depending on your pack, you'll need to solder wires to it, too.

Step 12: Create a Base for Your Pot

I wanted my flowers close to the edge and the battery pack easily accessible, so I created a cardboard base for the PC board to it on.

Measure the diameter of the inside lip of the pot and cut a cardboard circle to rest on top.

Then cut a hole so the lumpy parts of the circuit stick through.

Step 13: Assemble the Base

Glue or screw the battery pack through the screw holes in your board to the cardboard.

See how clean that looks?

Step 14: Finishing Touches

Place the cardboard base inside the pot. Arrange the flowers how you want, insert batteries, and admire your work!

Lights Contest 2017

Participated in the
Lights Contest 2017