New here and to robotics and have encountered my first 2 problems. Following instructions in a book, I have fitted 1 HR-SR04 ultra sound detector but could not get the code given to work, seems to get stuck in the first while loop, so never getting a return time :

import gpiozero
import time

TRIG = 23
ECHO = 24

trigger = gpiozero.OutputDevice(TRIG)
echo = gpiozero.DigitalInputDevice(ECHO)

robot = gpiozero.Robot(left=(17,18), right=(27,22))

def get_distance(trigger, echo):

    while echo.is_active == False:
        pulse_start = time.time()

    while echo.is_active == True:
        pulse_end = time.time()

    pulse_duration = pulse_end - pulse_start

    distance = 34300 * (pulse_duration/2)

    round_distance = round(distance,1)


while True:
    dist = get_distance(trigger,echo)
    if dist <= 15:

I then found some sample code using RPi.GPIO - which worked,returning values down to 3-4cm, so I know the sensor is ok. I also found the GPIOzero docs and tried the following code:

from gpiozero import DistanceSensor
from time import sleep

sensor = DistanceSensor(echo=18, trigger=17)
while True:
    print('Distance: ', sensor.distance * 100)

This code worked in that it would detect an obstacle, but wouldn't return a value less than 10cm.

So my 2 questions (after 2 days of trying to figure this out myself!):

  1. Why does the first code not work? I have seen almost exactly the same code repeated elsewhere as how to use the sensor but I just cannot get it to work for me.
  2. Why does the second code, using gpiozero not return a value less than 10cm?

I know this is probably very basic stuff, but would greatly appreciate some advice/explanations/pointers.


2 Answers 2


Your code looks fine but maybe the problem is about your circuit. I dont know if you did but you should connect a voltage divider to HC SR04's Echo pin. The reason for this is that the sensor works at 5V but your Raspberry is working at 3.3V. You can use 2K and 1K resistor with 2K is the one connected to GND. enter image description here

  • $\begingroup$ Thanks for the response. I should have stated that I had already connected the sensor using a voltage divider as you have described above, it's more that I can have the sensor working using rpi.gpio but the code using gpiozero doesn't seem to work - I just cannot see why that code doesn't work and can't understand why the second code works, but only measures down to 10.0cm. $\endgroup$
    – Talvalin
    Commented Feb 21, 2021 at 20:53
  • $\begingroup$ Did you check if you are physically set an output using gpiozero? What I mean is can you measure any voltage at the output pin when you set it to high? In the second code, is it actually 20cm when it says that the distance is 20cm? $\endgroup$ Commented Feb 22, 2021 at 8:16

In addition to what Yahya said, An arduino does a way better job of controlling sensors than an RPi does. I would personally use an arduino to get the sensor values and send that to the RPi for processing.


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