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I have an Arduino Uno and I'm working on a project that requires me to use six MCP23017 chips on a I2C bus plus a LCD 16x2 monitor. I need MCP23017 chips to power 36 LED's at the same time (8 LED's per four chips, 4 LED's on the fifth one + sixth for some input pins). So far I figured to drop the current with resistors for LED's to about 5 mA per LED. So that's about 180 mA for LED's and at least 120 mA for LCD monitor. I believe that Arduino Uno has a max of 200 mA through 5V pin. So is there any way for me to drop the current below the required limit? Also I do need to adress each pin with LED individually.

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Use an external 5V regulator to increase the amount of current you can use. The built in linear regulator for the Arduino is limited based on the input to output voltage difference causing high power loss and heat. Keep in mind that the Arduino also uses about 50 mA or more depending on your code.

The other option is scanning, that is turning the leds on and off rapidly so that it looks like they are all on but only one is really on at any given time. This would reduce your average current draw.

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