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I want to use LCD driver pins to light up LEDs instead of using LCD screen. This is done on a broken device so altering other pins is not possible for this case. Is this safe to CPU to connect just with limiting resistor or should I use a transistor driver for it? If so, are there any IC's to do this to avoid having a bunch load of transistors on board.

I am using EFM32 Gecko for this. https://www.silabs.com/products/mcu/32-bit/efm32-gecko

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    \$\begingroup\$ What does the GPIO output current specifications say in the data sheet? \$\endgroup\$
    – Andy aka
    Feb 14, 2018 at 10:02
  • \$\begingroup\$ 20mA per pin and 100mA total \$\endgroup\$
    – John Doe
    Feb 14, 2018 at 10:14
  • \$\begingroup\$ Try it with a low ish current to start with i.e. make the limiting resistor maybe 1 kohm. Then, as you add more LED segments you might find that the overall power supply current needed rises too high and you might need to make changes to the regulator that feeds the device to permit higher currents. If 100 mA is the total then at 1 mA drive current you can "approach" 100 LED segments. At 10 mA drive current it is far less. Do the math based on number of segemtns you have. \$\endgroup\$
    – Andy aka
    Feb 14, 2018 at 10:31
  • \$\begingroup\$ You have access to the program and can alter the function of the pins or you have a running product and want to connect some LEDs to the LCD pins? \$\endgroup\$
    – Arsenal
    Feb 14, 2018 at 12:37
  • \$\begingroup\$ Don't have access to those, so just want to replace existing custom made screen to LEDs. \$\endgroup\$
    – John Doe
    Feb 14, 2018 at 12:42

1 Answer 1

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Is this safe to CPU to connect just with limiting resistor

Safe: Yes. I recommend using at least 1kOhm since you probably need multiple LEDs.

should I use a transistor driver

That would be rather difficult since you might be unable to use VCC or GND as a potential base. The LCD is driven in AC mode using the LCD_COMx pins, so the voltage on a segment pin has only meaning when compared to the corresponding LCD_COMx voltage.

So you need to put the LED (and its limiting resisitor) between a segment and a LCD_COMx pin. Note that the LEDs light may be barely visible due to the low current and time multiplexing.

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