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Step 2(optional) solder leds onto resistors
If you already have the LEDs soldered onto resisters from one of our kits, you can skip this step.
Otherwise, 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 2.5V and a current use of 20mA. The phone charger provides around 5V, but can vary so if you have a multimeter you can test it.
Subtract the forward voltage from the phone charger supply to get the voltage over the resistor = 2.5V. Then work out R = V/I = 2.5V / 20mA = 125Ohms.
Alternatively
It often works to just put the LEDs in series, so that the voltage is divided across them both. To do this, twist together the long leg of one LED with the short leg of the other, then test with the phone charger and check that they are not too bright or too dim.
For more info on serial vs parallel circuits, see the wikipedia page here.
Otherwise, 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 2.5V and a current use of 20mA. The phone charger provides around 5V, but can vary so if you have a multimeter you can test it.
Subtract the forward voltage from the phone charger supply to get the voltage over the resistor = 2.5V. Then work out R = V/I = 2.5V / 20mA = 125Ohms.
Alternatively
It often works to just put the LEDs in series, so that the voltage is divided across them both. To do this, twist together the long leg of one LED with the short leg of the other, then test with the phone charger and check that they are not too bright or too dim.
For more info on serial vs parallel circuits, see the wikipedia page here.
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