Arduino connect programatically two wires of external 3.3V device

Question

MissingPuzzle

What is a missing puzzle in here, relay, transistor? I would like to pull pin of external device to the ground programatically using arduino's digital pin.

Answer 1

Probably you are missing nothing. You can just connect the pins of the Arduino and your device. Writing LOW to that pin pulls it down to ground. Of course you still need to connect the grounds of both devices, just as you did in your image.

Though we cannot be sure, since you didn't give any information about the device. Important would be, if the pin of the device is limiting the current flowing through it. Probably its an input pin with a pullup resistor, thus safe. But if its a pin that provides higher currents (more than 20mA) when connected to ground, you would basically create a short through the Arduinos digital output hardware, which in turn could be fried.

So check your device before implementing it.