I decided to have a small christmas tree in my office this year. And every christmas tree needs lights, right? I didn't want to use AC powered lights, I wanted to use battery powered string lights, but these was hopelessly sold out. So I decided to make my own lights.

Step 1: Material and Tools

Material

LEDs (I used SMD LEDs from LED "bulb")
resistors for LEDs
polyswitch 300-500 mA
cable
copper tape
cheap universal pcb
micro USB to DIP adapter (seach on eBay or AliExpress) (optional)
USB cable

Tools

hot glue gun
wire strippers
cutting pliers
utility knife
soldering iron
helping hand

Step 2: Circuit

USB in computer give 5V and can give current up to 500 mA. (In USB version 3.0 can give up to 700 mA)

For protecting computer USB is appropriate use a polyswitch fuse with value about 300-500 mA. Polyswitch is clever resettable fuse.

For every LED is necessary to use a resistor.

How to calculate resistor value for one LED

First thing you need to know is the Ohm's law. Famous equation: R = V / I (R is Resistance; V is Voltage; I is Current)

Second thing you need to know are Kirchhoff's circuit laws.

Third thing you need to know is VA characteristic of a LED. From this graph you can get voltage drop on the LED for recommended current. For common LEDs is recommended current 20 mA. White LEDs have voltage drop typicaly about 3.2 V for 20 mA.

Calculation:

We know that voltage on the USB is 5 V. Voltage on the white LED is 3.2 V, so voltage for the resistor must be 1.8 V, because 1.8 + 3.2 = 5 (this implies from Kirchhoff's voltage law). We also know that the current is 20 mA, because we choosed this value. Because components are in series, current is equal for all components including the resistor. Now we can simly use the Ohm's law: R = V_resisor / I = 1.8 / 0.02 = 90 Ohm.

Equation: R_for_LED = (V_power_source - V_LED_at_20mA) / 0.02A

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Why we have to use resistror? Because if voltage on the LED had reached 5 V, then the current would have been too high and would have destroyed the LED instantly.

I decided to use 120 ohm resistror, because I used superbright LEDs and I decided to lower the brightness.

How many lights can be in one string? It is maximum current divided by current of one LED. In this case: 500 mA / 20 mA = 25. Or 300 mA / 20 mA = 15.

Step 3: String Lights

Cut three copper strips for every one light. Then glue the strips on a base as you can see in the picture above. As a base I used a margin of universal PCB, but a popsicle stick for example can be probably used as well.

Solder LEDs and resistors on the prepared bases. Take care of LED polarity. After this use hot glue gun to put some hot glue on the top of every LED to make light diffused. This is a bit tricky, because the glue is too liquid. Put glue out of gun slowly, turn LED to the down and then cool the glue by blowing. After some practise you will be able to made it. Finally use cutting pliers to remove a glue remainders.

Now solder lights to a cable. Take care of LED polarity again.

Finally solder cable to the polyswitch and to the "micro USB to DIP adapter" or to a USB cable according to the schematic in the picture above.