LEDs in series
LEDs can be placed in series providing some features are taken into account.
The main item to include is a current-limiting resistor. A LED and resistor is called a string.
A string can have 1, 2, 3 or more LEDs. Three things must be observed:
1. MAXIMUM CURRENT through each string = 25mA.
2. The CHARACTERISTIC VOLTAGE-DROP must be known so the correct number of LEDs are used in any string.
3. A DROPPER RESISTOR must be included for each string.
The following diagrams show examples of 1-string, 2-strings and 3-strings: