What you need:
* 2 X pieces of wood 4X6. 1/4'' and the other 1/2'' thick
* 1 X 3 inches pipe. length varies depending of the deep of your pit.
* A couple of screw
* 1 X 1k resistor
* 1 X Raspberry pi
* 1X HC-SR04 ultrasonic sensor
Step 2: Create the base
Step 3: Prepare the base for the sensor.
Step 4: Add the sensor
Step 6: Connection
Pin 1 GND: connect to the ground of the rasp.
Pin 2 Echo to gpio 9
Pin 3 Trig to gpio 11. Add a 1k resistor
Pin 4 VCC to the 3.3V
Note 1: According to the manual, the sensor used 5v. But it work perfectly at 3.3v.
Note 2: Beware, we see the back of the sensor in this pic, so the order of the pin are reverse.