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I have been trying to build the attached schematic, and my issue lies with LCD backlight.

Schematic

The designer has attached the cathode (K) pin, but not the anode (A) pin. This did not work for me. It is an old schematic, and is designed for an LCD with only 14 pins + A and K, without the newer 15 and 16 pins.

I was wondering how only using the K pin could be possible, and if there was anything I could do to get my newer module to work?

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  • \$\begingroup\$ I'd guess that the LCD backlight it actually connected to pis 3 & 4 of J6. \$\endgroup\$
    – brhans
    May 1, 2020 at 3:09

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Sometimes there is a place to mount a series backlight resistor right on the LCD display- maybe to the +5V pin 2. Probably something like 100 to 220 ohms is appropriate, sometimes as low as 50 but take care not to damage the backlight.

There is a lot of variation between different common LCD displays for the backlight connections.

For your newer module, you would connect A to +5 through an appropriate resistor and K to the MOSFET drain. The LCD datasheet should supply guidance as to which of 15/16 is A/K (usually 15 is A, 16 is K) and what resistor is required. Sometimes there's already a suitable resistor mounted on the display, but don't count on it or you could fry the display.

The original circuit appears to have had a resistor on the display, as none is shown on the schematic.

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  • \$\begingroup\$ Yes it had an internal resistor. Interesting to know how things have changed. Thanks. \$\endgroup\$
    – Ben Rosser
    May 1, 2020 at 4:05
  • \$\begingroup\$ Things have not changed. The LCD manufacturers build generic modules that can be customized to specific customer needs, for example there might be 0R backlight series resistor by default, but if you want, they can provide you 10000 displays with any resistor you want. \$\endgroup\$
    – Justme
    May 1, 2020 at 9:27

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