Instructables
Picture of led light using cell phone battery charger
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a salvaged 5mm led along with a 68OHMS resistor when connected to a charger having 5 VOLTS output produces what is shown below....i topped it with a pyramid made of crystal from one feng shui shop. a very decent night light,,,you could cut a hollow cylindrical translucent portion of any coloured BALL pen to give it the pinkish(mine) look or any other colour ,,,,the charger provides only what CURRENT is needed and i even use it to run a small pc cooling fan. the led along with the resistor is the same arrangement from one of my previous instructables http://www.instructables.com/id/TORCH-OR-NIGHTLIGHT-FROM-A-RECHARGEABLE-BATTERY-FR/.........set the led under the hollow of the pyramid to get your night light....yaaaaaaaaaaayyyyyyyyyy.....the charger was from an old mobile phone rated at 5VOLTS 500mA,,,,i.e, it could provide a max of 500mAmps. the red and black wires indicate -ve and +ve respectively
garygadget11 months ago
I want to use a DC charger to replace batteries on an a led series using 3 1.5 AA batteries (4.5 Volts). Say there's 10 led's on it. How do I work out the resistor rating to be used please? I assume it is a calculation to ensure the current is kept below that of the circuits lowest rating ie the led or wire? Thanks.
sbdash11391 (author)  garygadget6 months ago

when you say 10 leds you gotta be specific about the current drawn by the leds.

Typically this led draws about 20 mA of current 10 leds would need to attached parallel to each other and this arrangemnt would be in turn attached parallely to the dc charger.

It would 20mA * 10 or 200mAs or 0.2A. This DC charger is capable of providing 500mA at 5 volts. I recommend that you use 68ohms resistor in series for each led to minimize chances of led damage.

sbdash11391 (author)  sbdash113916 months ago

Assuming that the LED voltage is 3.7 volts and that it draws a current of 20mA the resistor would be ther in plac to handle 1.3V approx.

At 20mA power in resistor would be 1.3 V * 20mA = 0.026 Watt

resistance could be calculated using 1.6V/20mA= 80 OHMS

I prefer overloading my leds a liitle for that extra brightness. so i use 68ohms. for you a .25Watt 100 ohms resistor would work fine

kostya1 year ago
I gave it a try several months ago. If someone wants to try it I would recommend to use 68Ohm/1W resistor to prevent overheating.
sbdash11391 (author)  kostya1 year ago
yeah you are right,,,but that 1/4w resistor also is working fine...you see 68*(20*10^-3)^2 is the power.. quite low