What will we learn?
Here, we’ll see how to generate PWM signals using the Raspberry Pi’s GPIO.
Prerequisites
- Raspberry Pi
- LED
- A resistor of 330 ohms
- Breadboard
- Jumper wires
What is PWM?
Power can be reduced by changing the time an electrical signal goes on and off at a specific frequency using Pulse-Width Modulation (PWM).
By adjusting the brightness of an LED using PWM, you can demonstrate how PWM works. Continuously delivering a strong digital signal to the LED allows us to maintain the LED’s full brightness level. Is it still bright enough if the LED only receives a high-quality digital signal half the time? We can observe that it’s on for half a second and off for half a second if the total period of the high and low signal takes a second, which implies that it’s on for half a second and off for half a second. What would happen if we made it so that for every 100 mS, half of the time was spent on and a half was spent off? It dims because the LED receives less power than it would if it were constantly on or connected to a high digital signal, which provides more power to the LED.
What is a “duty cycle”?
For PWM signals, the duty cycle refers to how long each cycle takes to complete, divided by how long it takes to complete one cycle of high (on) state. A percentage or a ratio is the most popular way to convey it.
To get an ideal square wave, you need a duty cycle of 50%, which is exactly what you get with a 50% duty cycle. Higher than 50%, logic high signals take longer to propagate, while logic low signals propagate quicker. The signal is always on (full-scale) with a 100 percent duty cycle, while the signal is always off (zero percent duty cycle) (grounding).
The PWM signal’s frequency
If a one-and-off cycle is completed, this signal has completed a period. The inverse of the period is the frequency, which is the number of times a periodic change is accomplished per unit of time. Speed is determined by how quickly a signal goes from high to low, which is how quickly a PWM completes a cycle. A constant voltage output can be achieved by continually turning the digital signal on and off at a high frequency.
Amount of resolution for PWM
The ‘PWM resolution’ refers to the degree of control over the duty cycle. The more ‘brightness’ levels we can display, the greater our PWM resolution needs to be. As a result, precise microcontroller timing is required because the duty cycle is 50Hz. The more powerful the microcontroller, the shorter the time intervals it can keep track of. The microcontroller must not only time the ‘interrupt,’ which generates the pulse but also run the code that controls the LED output, which must be completed before the next interrupt is called, which is another limiting issue. It’s also likely that you’ll want your microcontroller to accomplish activities other than controlling the brightness of the LEDs, so you’ll need some spare execution time between interrupts.
The fundamental benefit of greater PWM resolutions for LED PWM control is that it reduces the difference between ‘off’ and the LED’s lowest achievable brightness. Suppose we have a duty cycle of 20,000 microseconds and a resolution of 10,000 microseconds. In that case, the difference in brightness between “off” and the lowest possible brightness will be 50 percent of the total brightness. The difference would be 10% at a resolution of 2,000 microseconds. The “PWM resolution” determines the amount of brightness levels that we can support between 0% and 100% when it comes to brightness levels. (100 percent). Again, the better the resolution, the more precise the timing, and the more computing power is needed to process the information.
The above diagram shows a PWM resolution of 10%.
Depending on the nature of your application, the resolution and overall duty cycle requirements may be different. There is no need for precision control for simple displays; nevertheless, the ability to manage the brightness level may be crucial (think of the issue of mixing colors using an RGB LED, for example). More control and accuracy necessitate more microcontroller resources; thus, the trade-off is straightforward.
Applications of PWM
- Using PWM, you may adjust the screen’s brightness.
Adjusting the screen brightness with PWM does not rely on power but rather on the screen turning on and off repeatedly. The PWM dimming screen is producing no light. There is instead a continual pulsating of the display. A rapidly shifting duty cycle will always appear but with varying brightness to human eyes. The screen’s brightness increases as the time spent on it grow.
Among the additional uses for PWM technology, we can mention the following:
- Use a variety of loudness levels for the buzzer.
- Control the motor’s speed and direction with a servo.
- Provide a physical outlet for analog signals
- Create an audio output.
- Communication: encoding of the message
Raspberry Pi PWM
It has two PWM channels, PWM0 and PWM1, on the Raspberry Pi. The following are the pinouts for the two PWM channels on the 40-pin P1 header:
The Raspberry Pi 40-pin P1Header’s PWM pins are illustrated in the following figure.
On a Raspberry Pi, all of the PWM pins are used by the audio subsystem. As a result, we can choose between PWM or Audio output. It is possible to use all 26 GPIO pins on the RPi to generate PWM frequencies of up to a few thousand Hertz using software PWM. The software PWM signals’ duty cycle can be adjusted anywhere from 0% to 100%.
Hardware PWM signals can only be generated by importing the pigpio library. The RPi.GPIO library, on the other hand, can be used to generate PWM signals.
What is the Raspberry Pi’s PWM signal generation method?
There are two ways to generate PWM signals with the Raspberry Pi. It is possible to generate PWM using either hardware or software.
We’ll need to resort to third-party devices to generate PWM with the Raspberry Pi’s PWM connectors. As a result, only four PWM pins are available on the Raspberry Pi. GPIO12 and GPIO 18 share a PWM channel, whereas GPIO 13 and GPIO 19 share the other. This means that the pi has only two PWM channels that can be controlled independently of each other.
The Raspberry Pi’s GPIO resources can be controlled quickly and precisely to generate a PWM signal using the software. Because it may be generated from any GPIO pin, software PWM is more adaptable than hardware PWM. However, software-based PWM has the major disadvantage of being less accurate than a hardware-based PWM channel. Because software PWM generation consumes CPU resources, your Raspberry PI’s processing capacity will be limited.
Even though hardware PWM is the preferred approach for generating PWM from the Raspberry Pi, we will use software PWM in this article.
Circuit connections
After the LED is plugged into GPIO21, the anode is linked. After that, connect the LED’s cathode to a 330 Ohm series resistor and ground the resistor’s other end.
Pins 2 and 6 of the Pi board can be used to supply the circuit with DC power and ground.
Creating a Python program
The thorny Python IDE on raspberry pi will be used here to write our python script. If you haven’t already done so, please go back to Chapter 2 and read about how to get started with this IDE before reading on.
To keep things simple, we’ll create a file called PMW.py and save it to our desktop.
Putting the project into action
Using a rectified sine wave as a rough approximation, you can fade an LED using an analog signal. For additional information, check out the Arduino LED fading recipe.
We’re using a 50 Hz software PWM signal to generate a customized sine wave with RPi. It has a 20-millisecond window at this frequency. During the application, the frequency does not fluctuate.
Increasing the software PWM duty cycle from 0 to 100 is required to produce a rectified sine wave. The PWM signal is applied to the LED in five-pulse trains every 0.1 seconds, with each train lasting 0.1 seconds.
As a result, the duty cycle is lowered from 100 to 1 in steps of minus one. Five PWM pulse trains, each lasting 0.1 seconds, are applied to each increment. Iteration continues indefinitely until a keyboard interrupt is received, at which point the user program terminates.
Code
Import RPi.GPIO then time libraries. Then a simple script is run to begin. The GPIO.setwarnings() method is used to disable the warnings.
To set the RPi’s PINs to the number of board, use the GPIO.setmode() function to set the pin numbering. The GPIO.setup() method configures pin 40 of the board as an output. However, the GPIO.PWM() technique is used to instantiate board pin 40 as a software PWM.
It is possible to write a user-defined setup() function to ensure that the software PWM has no duty cycle when it is first started. Only one instance of this function is ever called.
The duty cycle of the PWM signal is altered from 0 to 100 and then back to 0 in a user-defined loop() function. This occurs in increments of one, with a 0.1-second gap between each. For an endless number of times, the LED lights up and fades back down.
The PWM signal is turned off when a keyboard interrupt is received by calling the endprogram() method. The GPIO of the Raspberry Pi is then wiped clean.
Setup() and loop() are the two methods in a try-exception statement, and they are each called once.
Code Syntax
The GPIO.PWM() method
A PWM instance can be created with the help of this function. This is a two-step process:
- PWM signal must be generated on a specific channel number
- The frequency in Hertz of the PWM signal. The method must be assigned to a variable before an instance can be generated.
The syntax for this method is:
The number of the channel must be given in accordance with the user-Board program or BCM numbering.
The start() method
This technique can be used with a PWM software instance. PWM duty cycle is all you need to know about this.
PWM instances can be accessed by calling this method from a Python program. A software PWM signal with the specified duty cycle is started at the supplied channel.
The syntax is as follows:
The ChangeFrequency() method
This technique can be used with a PWM software instance. There’s only one thing needed: a new Hertz value for the PWM signal’s frequency.
The frequency of the PWM output is changed when this method is used on a PWM object in Python.
The syntax is as follows:
The ChangeDutyCycle() method
An instance of PWM software can use this technique. One reason is all that is required: the launch of a new cycle of service.
The duty cycle ranges from 0.0 to 100.0. The duty cycle of the PWM signal is changed when this method is called on a PWM instance in Python.
Here is the syntax of the method:
The stop() method
This technique can be used with a software PWM instance. It doesn’t need a response. An instance’s PWM signal is paused when this method is called on it.
The syntax for this method is:
Output
Conclusion
Congratulations! You have made it to the end of this tutorial. We have seen how PWM is generated in the raspberry pi. We have also seen how to set up our raspberry pi pins with LEDs to be controlled and wrote a python program that controls the output of these pins. The following tutorial will learn how to control a DC motor with Raspberry Pi 4 using Python.
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