OpenCV, RPi.GPIO, and GPIO Zero on the Raspberry Pi

Datetime:2016-08-22 23:38:41          Topic: Raspberry Pi  OpenCV           Share

Last week we learned a bit about Python virtual environments and how to access the RPi.GPIO and GPIO Zero libraries  along with OpenCV.

Today we are going to build on that knowledge and create an “alarm” that triggers both an LED light to turn on and a buzzer to go off whenever a  specific visual action takes place .

To accomplish this, we’ll utilize OpenCV to process frames from a video stream. Then, when a pre-defined event takes place (such as a green ball entering our field of view), we’ll utilize RPi.GPIO/GPIO Zero to trigger the alarm.

Keep reading to find out how it’s done!

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OpenCV, RPi.GPIO, and GPIO Zero on the Raspberry Pi

In the remainder of this blog post, we’ll be using OpenCV and the RPi.GPIO / GPIO Zero libraries to interact with each other. We’ll use OpenCV to process frames from a video stream, and once a specific event happens, we’ll trigger an action on our attached TrafficHAT board.


To get started, we’ll need a bit of hardware, including a:

  • Raspberry Pi:  I ended up going with the Raspberry Pi 3 , but the Raspberry Pi 2 would also be an excellent choice for this project.
  • TrafficHAT: I purchased the TrafficHAT, a module for the Raspberry Pi consisting of three LED lights, a buzzer, and push-button from RyanTeck . Given that I have very little experience with GPIO programming, this kit was an excellent starting point for me to get some exposure to GPIO. If you’re just getting started as well, be sure to take a look at the TrafficHat.

You can see the TrafficHat itself in the image below:

Figure 1:The TrafficHAT module for the Raspberry Pi, which includes 3 LED lights, a buzzer, and push button, all of which are programmable via GPIO.

Notice how the module simply sits on top of the Raspberry Pi — no breakout board, extra cables, or soldering required!

To trigger the alarm, we’ll be writing a Python script to detect this green ball in our video stream:

Figure 2:The green ball we will be detecting in our video stream. If the ball is found, we’ll trigger an alarm by buzzing the buzzer and lighting up the green LED light.

If this green ball enters the view of our camera, we’ll sound the alarm by ringing the buzzer and turning on the green LED light .

Below you can see an example of my setup:

Figure 3:My example setup including the Raspberry Pi, TrafficHAT board, USB webcam, and green ball that will be detected.

Coding the alarm

Before we get started, make sure you have read last week’s blog post on accessing RPi.GPIO and GPIO Zero with OpenCV . This post provides  crucial information on configuring your development environment, including an explanation of Python virtual environments, why we use them, and how to install RPi.GPIO and GPIO Zero such that they are accessible in the same virtual environment as OpenCV.

Once you’ve given the post a read, you should be prepared for this project. Open up a new file, name it pi_alarm . py , and let’s get coding:

# import the necessary packages
from __future__ import print_function
from imutils.videoimport VideoStream
from gpiozeroimport TrafficHat
import argparse
import imutils
import time
import cv2
# construct the argument parse and parse the arguments
ap = argparse.ArgumentParser()
ap.add_argument("-p", "--picamera", type=int, default=-1,
 help="whether or not the Raspberry Pi camera should be used")
args = vars(ap.parse_args())

Lines 2-8import our required Python packages. We’ll be using the VideoStream class from our  unifying picamera and cv2.VideoCapture into a single class with OpenCV post. We’ll also be using imutils , a series of convenience functions used to make common image processing operations with OpenCV easier. If you don’t already have imutils installed on your system, you can install it using pip :

$ pipinstallimutils

Note: Make sure you install imutils   into the same Python virtual environment as the one you’re using for OpenCV and GPIO programming!

Lines 11-14then handle parsing our command line arguments. We only need a single switch here, -- picamera , which is an integer indicating whether the Raspberry Pi camera module or a USB webcam should be used. If you’re using a USB webcam, you can ignore this switch (the USB webcam will be used by default). Otherwise, if you want to use the picamera module, then supply a value of -- picamera 1 as a command line argument when you execute the script.

Now, let’s initialize some important variables:

# initialize the video stream and allow the cammera sensor to
# warmup
print("[INFO] waiting for camera to warmup...")
vs = VideoStream(usePiCamera=args["picamera"] > 0).start()
# define the lower and upper boundaries of the "green"
# ball in the HSV color space
greenLower = (29, 86, 6)
greenUpper = (64, 255, 255)
# initialize the TrafficHat and whether or not the LED is on
th = TrafficHat()
ledOn = False

Lines 19 and 20accesses our VideoStream and allow the camera sensor to warmup.

We’ll be applyingcolor thresholding to find the green ball in our video stream, so we initialize the lower and upper boundaries of the green ball pixel intensities in the HSV color space (be sure to see the “color thresholding” link for more information on how these values are defined — essentially, we are looking for all pixels that fall within this lower and upper range).

Line 28 initializes the TrafficHat class used to interact with the TrafficHat module (although we could certainly use the RPi.GPIO library here as well). We then initialize a boolean variable ( Line 29 ) used to indicate if the green LED is “on” (i.e., the green ball is in view of the camera).

Next, we have the main video processing loop of our script:

# loop over the frames from the video stream
while True:
 # grab the next frame from the video stream, resize the
 # frame, and convert it to the HSV color space
 frame =
 frame = imutils.resize(frame, width=500)
 hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
 # construct a mask for the color "green", then perform
 # a series of dilations and erosions to remove any small
 # blobs left in the mask
 mask = cv2.inRange(hsv, greenLower, greenUpper)
 mask = cv2.erode(mask, None, iterations=2)
 mask = cv2.dilate(mask, None, iterations=2)
 # find contours in the mask and initialize the current
 # (x, y) center of the ball
 cnts = cv2.findContours(mask.copy(), cv2.RETR_EXTERNAL,
 cnts = cnts[0] if imutils.is_cv2() else cnts[1]
 center = None

On Line 32 we start an infinite loop that continuously reads frames from our video stream. For each of these frames, we resize the frame to have a maximum width of 500 pixels (the smaller the frame is, the faster it is to process) and then convert it to the HSV color space.

Line 42applies color thresholding using the cv2 . inRange function. All pixels in the range greenLower <= pixel <= greenUpper are set to  white and all pixels outside this range are set to  black . This enables us to create a mask representing the foreground (i.e., green ball, or lack thereof) in the frame. You can read more about color thresholding and ball detection here .

Lines 43 and 44perform a series of erosions and dilations to remove any small “blobs” (i.e., noise in the frame that does not correspond to the green ball) from the image.

From there, we apply contour detection on Lines 48-50 , allowing us to find the outline of the ball in the mask .

We are now ready to perform a few checks, and if they pass, raise the alarm:

 # only proceed if at least one contour was found
 if len(cnts) > 0:
 # find the largest contour in the mask, then use
 # it to compute the minimum enclosing circle and
 # centroid
 c = max(cnts, key=cv2.contourArea)
 ((x, y), radius) = cv2.minEnclosingCircle(c)
 M = cv2.moments(c)
 center = (int(M["m10"] / M["m00"]), int(M["m01"] / M["m00"]))
 # only proceed if the radius meets a minimum size
 if radius > 10:
 # draw the circle and centroid on the frame, (int(x), int(y)), int(radius),
 (0, 255, 255), 2), center, 5, (0, 0, 255), -1)
 # if the led is not already on, raise an alarm and
 # turn the LED on
 if not ledOn:
 th.buzzer.blink(0.1, 0.1, 10, background=True)
 ledOn = True

Line 54makes a check to ensure at least one contour was found. And if so, we find the largest contour in our mask ( Line 58 ), which we assume is the green ball .

Note: This is a reasonable assumption to make since we presume there will be no other large, green objects in view of our camera.

Once we have the largest contour, we compute the minimum enclosing circle of the region, along with the center (x, y) -coordinates — we’ll be using these values to actually  highlight the area of the image that contains the ball.

In order to protect against false-positive detections, Line 64 makes a check to ensure the radius of the circle in sufficiently large (in this case, the radius must be at least 10 pixels). If this check passes, we draw the circle and centroid on the frame ( Lines 66-68 ).

Finally, if the the LED light is not already on, we trigger the alarm by buzzing the buzzer and lighting up the LED ( Lines 72-75 ).

We have now reached our last code block:

 # if the ball is not detected, turn off the LED
 elif ledOn:
 ledOn = False
 # show the frame to our screen
 cv2.imshow("Frame", frame)
 key = cv2.waitKey(1) & 0xFF
 # if the 'q' key is pressed, stop the loop
 if key == ord("q"):
# do a bit of cleanup

Line 78-80handle if no green ball is detected  and the LED light is on, indicating that we should turn the alarm off.

Now that we are done processing our frame , we can display it to our screen on  Lines 83 and 84 .

If the q key is pressed, we break from the loop and perform a bit of cleanup.

Arming the alarm

To execute our Python script, simply issue the following command:


If you want to utilize the Raspberry Pi camera module rather than a USB webcam, then just append the -- picamera 1 switch:

$ --picamera 1

Below you can find a sample image from my output where the green ball is not present :

Figure 4:Since the green ball is not detected , the LED and buzzer remain off.

As the above figure demonstrates, there is no green ball in the image — and thus, the green LED is off.

However, once the green ball enters the view of the camera, OpenCV detects it, thereby allowing us to utilize GPIO programming to light up the green LED:

Figure 5:When the green ball is detected , the LED lights up and the buzzer goes off.

The full output of the script can be seen in the video below:


In this blog post, we learned how to utilize both OpenCV and the RPi.GPIO/GPIO Zero libraries to interact with each other and accomplish a particular task. Specifically, we built a simple “alarm system” using a Raspberry Pi 3 , a TrafficHAT module, and color thresholding to detect the presence of a green ball.

OpenCV was used to perform the core video processing and detect the ball. Once the ball was detected, we raised an alarm by buzzing the buzzer and lighting up an LED on the TrafficHat.

Overall, my goal of this blog post was to demonstrate a straightforward computer vision application that can blend both OpenCV and GPIO libraries together. I hope it serves as a starting point for you future projects!

Next week we’ll learn how to take this example alarm program and make it run as soon as the Raspberry Pi boots up!

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See you next week…


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