Question 12.10: You want to measure distance using an ultrasonic rangefinder......

Use a low cost SR-04 rangefinder. These devices need two GPIO pins, one to trigger the pulse of ultrasound and the other to monitor how long it takes for the echo to return.

To make this recipe, you will need:

• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• SR-04 rangefinder (eBay)
• 470Ω resistor (see “Resistors and Capacitors” on page 380)
• 270Ω resistor (see “Resistors and Capacitors” on page 380)

Fit the components onto the breadboard as shown in Figure 12-17. The resistors are necessary to reduce the echo output of the rangefinder from 5V to 3.3V (see Recipe 8.12).

Open an editor (nano or IDLE) and paste in the following code. As with all the program examples in this book, you can also download the program from the Code section of the Raspberry Pi Cookbook website, where it is called ranger.py.

The operation of the program is described in the Discussion. When the program is run, it will report the distance in both centimeters and inches, once per second. Use your hand or some other obstacle to change the reading.

Discussion

While there are a number of ultrasonic rangefinders available, the type used here is easy to use and low cost. It works by sending a pulse of ultrasound and then measuring the amount of time taken for the echo to be received. One of the round ultrasonic transducers on the front of the device is the transmitter, and the other is the receiver.

This process is controlled from the Raspberry Pi. The difference between this type of device and more expensive models is that the more expensive versions include their own microcontroller, which carries out all the necessary timing and provides an I2C or serial interface to return a final reading.

When you are using one of these sensors with a Raspberry Pi, the trig (trigger) input to the rangefinder is connected to a GPIO output, and the echo output of the rangefinder is connected to a GPIO input on the Raspberry Pi after having its voltage range lowered from 5V to a safe 3.3V.

Figure 12-18 shows an oscilloscope trace of the sensor in action. The top (red) trace is connected to trig and the bottom (yellow) trace is connected to echo. You can see that first the trig pin is taken high for a short pulse. There is then a short delay before the echo pin goes high. This then stays high for a period that is proportional to the distance from the sensor.

The code for this sensor first generates a trigger pulse (using the function send_trig ger_pulse). It must wait until the echo pin goes high and then times how long the echo pin stays high.

We can then calculate the distance using the time taken for the echo pulse and the speed of sound.

The wait_for_echo function waits until the echo pin either goes high or low, depending on its first argument. Its second argument is used to provide a timeout so that if for any reason, the echo pin does not change to the state being waited for, the loop won’t hang indefinitely.

See Also

Take a look at the datasheet for the ultrasonic rangefinder.