That is not very clear.
I think you mean that you have another non-Arduino device that you want to control with an Arduino, and to do so the Arduino has to change a voltage from HIGH to LOW and keep it low for 500ms.
Please confirm that this is correct, or explain where it is incorrect.
You must ensure that the connection to the other device does not cause more than a few mA to flow into or out of the Arduino pin to which is is connected. The absolute max for an I/O pin is 20mA but the max for several pins together is lower. You need to read the Atmel datasheet for the MCU in your Arduino.
You must ensure that the other device NEVER puts a voltage outside the range 0v to 5v on the Arduino pins.
You also need a GND connection between the Arduino and the other device.
the other device is the Adafruit soundboard. and i was having problems using TX/RX communications it was slowing the touchscreen right down to a crawl for some reason. but it also works off GPIO triggers. the board is powered by 3.5v independently but the whole system has a shared ground.
//pinMode(40,OUTPUT); should go in setup.
works fine and i have 6 out of 11 trigger pins connected and all work fine