The popular TC1602A-01T LCD features an LED backlight. The backlight is nominally 5 V but reading the datasheet gives some warnings.
It appears that the LED has a forward voltage, \( V_f \) of 4.1 V at 100 mA but that there is no current limiting built in. If connected directly to a 5 V supply then excessive current would flow (as predicted by the IV curves), the backlight would be destroyed and the display ruined.
A simple LED current limiter will suffice.
We can calculate the value of R1 from the voltage drop across it and the current through it:
\( R_1 = \frac {V_{SS} – 4.1}{0.1} \)For a 5 V supply this would give R1 = 9 Ω. The nearest standard value is 10 Ω and this would be fine.
Q1 is a standard NPN switch. If used it will drop about 0.2 V when on and this could be subtracted from the R1 calculation above. If switching is not required then it can be omitted and the LED cathode connected to ground.
