led count

hi guys, can i know what is the max nos. of led in series that i can connect to a single arduino pin? tnx

It depends on the forward voltage of each LED. A LED doesn't start to light until its forward voltage is exceeded, so to light two LEDs in series, you need to put at least twice the forward voltage across them both.

Forward voltages differ by colour. The numbers below are not exact and it’s best to check the data sheet if… you can find one, but they may give you an idea of what to expect.

IR 1.5
red 2.0
orange 2.0
yellow 2.1
green 2.2
true green 3.3
blue 3.3
white 3.3
UV 3.5

If you want a longer string, use a higher voltage and an NPN transistor at cathode at the bottom of the string. Another option is replace the NPN with part like ULN2803 (equal to 8 NPN drivers & base resistors in1 package) for controlling multiple independent strings.