Pull down resistor and voltage on DO
pinMode(pin, INPUT);           // set pin as input (no pull-up resistor)   
pinMode(pin, INPUT_PULLUP);    // set pin as input (20k-50k internal pull-up resistor)   

pinMode(pin, OUTPUT);          // set pin as output
digitalWrite(pin, HIGH);       // drive pin to Vcc
digitalWrite(pin, LOW);        // drive pin to GND


Be ESD Aware....