Hobby Projects‎ > ‎Projects‎ > ‎

USB powered Led Smiley

Parts Required :

1> Red leds (11)
2> Any standard USB Cable
3> Cardboard
4> Resistor (33 ohms 1/2 watt)
5>Cirular case ( I used the electric tape case)

  • First cut a circle of the size of the case out of the cardboard
  • Mark the positions of leds on the cardboard using pencil to roughly represent the shape of a smiley
  • Heres a rough circuit diagram that we will implement (Pardon me for the appearance I did it in 5 min in Paint)

  • Solder all anodes of leds to each  other.
  • Do the same for all cathodes

  • Preserving the end of the USB cable that goes into the computer cut the cable leaving enough length so that you hang your smiley or velcro it to ur desktop.
  • The USB cable has four smaller cables, the computer provides 5V, 500mA max on all usb ports between the red and the black cable with the red being the positive.

  • Make a small notch in the Plastic circular case for the usb cable to pass through.
  • Solder the Red cable to the Anode via the 100 ohm resistor and the Black cable to the cathode
  • Plug it in and test it......

I had tested my circuit on the breadboard before soldering it and it worked.  I did realise a flaw in my design later which I want to share with you guys.  Since all the leds are in parallel we have the same current flowing through all the branches once the leds are on. So the current flowing through the resistor is approximately

= 100 mA

So the current flowing in each branch is approx 9mA, while the leds forward current is 20mA. Hence though the leds are on there are at half their brightness. So one soln to the problem is to use a smaller resistor. Yes that is possible but just dont forget that you will have to go for a 1 watt or higher rating resistor. Because currently the power desipated by the 33ohms resistor is

P = I *I *R
 = 100mA * 100mA * 33

 =0.33 watts.

Hence we are using a 0.5 watt resistor

Hey if you didnot make sense of this note dont worry our SMILEY still looks cool, go ahead make it..........