I want to connect K pin (or LED-) of LCD to the NPN transitor, so i can control it. The question is how can i calculate resistor value? I found this schematic online, do you think it would work?

Backlight supply voltage is 3V, current is 16mA.

enter image description here

  • \$\begingroup\$ Beware that BC327 is a PNP type transistor, yet the schematic symbol is NPN type which is what you need for the circuit to operate. What is the backlight current, backlight forward voltage, and what is the supply voltage to backlight? \$\endgroup\$
    – Justme
    Jun 7, 2020 at 14:39
  • \$\begingroup\$ There is no K pin on the diagram. \$\endgroup\$
    – Transistor
    Jun 7, 2020 at 14:48
  • \$\begingroup\$ It is LED- pin on diagram \$\endgroup\$
    – cerouno
    Jun 7, 2020 at 15:07
  • \$\begingroup\$ You could calculate this from the transistor current gain. Typically you'd want the transistor to be soundly "on" otherwise it ends up doing part of the current limit resistor's job, but for your low current needs things aren't too critical. You might measure the voltage drop across each of the 100R, LED, and transistor. And given the resistor you can determine the actual current from the voltage across it without breaking the circuit to insert an ammeter. For some LED colors you might need a higher voltage to use such a scheme... Or especially backlights that have LEDs in series. \$\endgroup\$ Jun 7, 2020 at 15:24

1 Answer 1


Yes, I think the transistor needs to be NPN type. This is how a finished product usually look like, I think you can borrow the concept. how to adjust contrast

And it depends on whether your LCM is single voltage supply or double. For double, you may need to adjust both voltages. A simple explanation is here.

  • \$\begingroup\$ You must provide attribution if you post an unrelated link that promotes a product, that means you need to provide your relationship to the link or it is considered spam. If you wish to edit this post contact me and I will turn off the lock \$\endgroup\$
    – Voltage Spike
    Aug 8, 2020 at 6:00

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