How should I build this circuit?

I have this homemade LED-USB thingy to illuminate my desk:

simulate this circuit – Schematic created using CircuitLab

I guess it's not a very technical schematics, sorry, I'm not even near the EE area. The LED is 3V, and according "to the internet" (there was no manufactorer info on it) a blue LED usually supports up to 0.03A.

Thing is, it's not bright enough, so I want to attach more LEDs to it.

Since I have spare current from the USB I thought I should build a parallel circuit. Now, the last class on circuits I had was on high school, so I don't know if my calculations and schematics are correct:

i_total = U_wanted ( 1/r + 1/r' + 1/r'' ), r=r'=r''
i_total = U_wanted ( 3/r )
0.5A = 3V ( 3/r )
0.5A = 9V / r
r = 18 Ohms


simulate this circuit

Is this correct? If so, what material should I use as wire? A copper wire? Also, is it safe to touch it while it's on? I've been avoiding it, but I guess it's a low current and voltage...

• The convention is for red wires to be attached to "+", and black to "-", though I suppose I could believe a no-name Chinese manufacturer could get it backwards and not care. If the LEDs are lit, then you have it right, but probably the red/black labels on the schematic are reversed relative to what you actually have. – Phil Frost May 25 '13 at 19:51
• it safe don't worry it far away of dangerous limits of voltage/current . – yahya tawil May 25 '13 at 20:54

The voltage across each resistor/LED pair will be the same, so assuming that each LED has the same forward voltage, you will need to use the same value resistor regardless of how many resistor/LED pairs there are. Therefore you should use 68-ohm resistors.

That much voltage won't even be noticeable through dry skin, and copper wire is an acceptable conductor.

(Also, you don't want that wire all the way on the right side of your new schematic; that will cause a short and possibly break something.)

You can use a single 18 ohm resistor before splitting into three LEDs, or three separate 68 Ohm resistors.

Or you can use a 3.3V voltage regulator. It will output the right amount of current to keep the output voltage at 3.3V, which seems to be what your blue LED wants. Or use an adjustable voltage regulator that lets you set the specific output voltage, which will drive the output current.

Or, for better efficiency, use PWM to drive the LED/s. Just hook them to 5V, but gate their cathodes (negative end) through a N-channel MOSFET. Drive the gate of the MOSFET with a PWM signal from something like a 555 timer, or even a small microcontroller (ATTiny85 or whatever.) Vary the duty cycle to vary the brightness. with a 65% duty cycle, you'll have the same brightness as with a 3.3V voltage supply, except you won't be burning off 1/3 of all your power in those resistors, so it would be much more efficient.

I'd totally recommend doing the PWM thing :-)

• Thanks! It's just a simple DIY project, I already have the other two 68 Ohms resistors and I'm not confident enough to try anything much more complex than that. – Alex May 26 '13 at 1:26
• Don't use a single 18 ohm resistor. LEDs have a fair bit of variance in their manufacturing, and if you use a single resistor with 3 LEDs, it is likely that one of them will consume much more current than the others. At best, this will make one more bright; at worst, it will put too much current through that LED and reduce its life. – Eric Gunnerson May 26 '13 at 3:23
• -1, A single LED feeding 3 LEDs in parallel is a bad recommendation. – Anindo Ghosh May 26 '13 at 5:15

protected by W5VO♦May 25 '13 at 20:44

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