Ultrasonic sensor to Pixhawk Can't be Detected in Mission Planner

I have tried to make an avoider on a quadcopter drone using the HC-SR04 ultrasonic sensor. I use the aux out pin to connect ultrasonic and arduino with pixhawk. However, when I set it in mission planner and check proximity, the sensor can’t be detected in mission planner. What is the cause of the sensor can not be detected in the mission planner?