In this instructable, I will show you how to make your own LDR-LED bracelet. The LDR (Light Dependent Resistors) activated LED bracelet is a based on a simple circuit. The bracelet turns ON when it goes in dark. When it's comes in daylight, the LDR sensor makes off the LEDs (Light-emitting diodes). This bracelet is very compact, light in weight and looks cool. Let’s get started!

Step 1: About Light Dependent Resistors (LDR)

Light Dependent Resistors (LDR) are also called photoresistors. They are made of high resistance semiconductor material. When light hits the device, the photons give electrons energy. This makes them jump into the conductive band and thereby conduct electricity.

The circuit shown above shows a simple way of constructing a circuit that turns on when it goes dark. In this circuit the LDR and the other Resistor form a simple 'Potential Divider' circuit, where the centre point of the Potential Divider is fed to the Base of the NPN Transistor. When the light level decreases, the resistance of the LDR increases. As this resistance increases in relation to the other Resistor, which has a fixed resistance, it causes the voltage dropped across the LDR to also increase. When this voltage is large enough, it will cause the Transistor to turn on. The value of the fixed resistor will depend on the LDR used, the transistor used and the supply voltage. This makes the LEDs light up.

Step 2: Material Required

One LDR Sensor
One RGB LED
One NPN Transistor BC547
One Copper string LED Lights (Consists of 7 LEDs)
One Circular Plastic Box
One Resistor 10kΩ
One On/off Push Button
RO System Water Pipe (length = 18 cm, Outer diameter = 6mm)
One Battery (3.7V, 200 mAH)
PCB piece (size = 15mm X 15mm)
Plastic translucent sheet piece/ Scrap Material
Metallic piece from Hard disk drive
Wires
Heat Shrink Tubes

Tools used

Soldering iron
Drill machine
Wire Stripping Plier
Super glue
Adhesive Tape

Step 3: PCB Board

Take a PCB Piece of size 15mm X 15mm. Solder all the Components: - LEDs, LDR sensor, transistor and Resistance on the PCB according to the circuit diagram. Now solder battery wires: - Red wire for positive terminal and Black wire for negative terminal. The Basic circuit is ready. Make all the Connections carefully and check the circuit working or not before assembling all parts.

Step 4: Main Body and Wrist Band

Now, take a small Plastic circular Box of size (Diameter = 33.6mm, Height = 13mm). I found this in my small junkyard. You can use 3d printed Box. The plastic box consists of two parts: - upper part (light green) and bottom part (light blue).

Make 2 holes on opposite sides of the box of (diameter = 8mm each) with the help of Drill machine or with the help of Soldering Iron. These holes are made to hold gripping parts of wrist band.

A plastic pipe (length= 18cm) of RO water system and its gripping parts were used which acts as wrist band. See the images for help.

Along with this, make a hole (diameter = 3mm) adjacent to one of the 8mm hole for the ON/OFF push button and make a 4mm hole for LDR sensor near it.

On the upper part of the plastic body make a hole of (diameter = 15mm) and paste a black plastic piece over it and a white plastic translucent sheet below it with the help of super glue. So the partial light can pass through it.

Now take Copper string LED Lights consists of 7 LEDs. Turn its wire into spring shape with the help of metal rod or needle so it can be easily inserted into the plastic pipe (internal diameter = 3.4mm). One end of the pipe is covered with glue and other end has LED lights wires. Check the LED string wire before assembling all the components.

Step 5: Assembling All Parts

Take a tiny ON\OFF push button (size= 7mm X 7mm X 7mm). Solder it with main circuit board along with String LEDs. Take out the head of LDR Sensor from the 4mm hole.

Now solder a 3.7V battery wires with the main circuit board. Battery size is (21mm X 10mm X 3mm). The battery is rechargeable so you can charge it. Cover the bottom part with upper part. I added a metal piece from my old hard disk drive and looks good.