Make a psychedelic led lamp by utilizing your old cellphone chargers. Follow the simple instructions here:
“phone chargers are usually around 5v, and I’ve found that using 2 leds in series works well. This allows the colours to be varied too, which looks nice on the finished lamp.
Sometimes the charger isn’t powerful enough for super bright leds, so I just use one and a 500ohm resister in series.
You can work out what size resister to use with Ohm’s law. V = IR. Let’s work out what resistor size we need for an LED with a forward voltage of 4V and a current use of 40mA. The phone charger provides 6V.
Subtract the forward voltage from the phone charger supply to get the voltage over the resistor = 2V. Then work out R = V/I = 2V / 40mA = 50ohms.”