Over the last couple of weeks the pace of development for Python programmers who like to dabble in GPIO has gone up a gear. Both main GPIO systems for Python on the Raspberry Pi have been getting some enhancements.
WiringPi for Python version 2 is in Beta testing. I discovered two bugs while I was trying it out. Both of those have been squashed. One by Gordon “Drogon” Henderson in the C source code and another by Phil “Gadgetoid” Howard in the Python install script. There’s a lot more stuff to test, but that’s not what I’m writing about today, so I’ll save that for a future post. Suffice it to say that the Python Gertboard software all works with WiringPi2 (at least it will when I release it soon with very minor tweaks).
So what? Why would I care about PWM?
It’s kind of useful. Let’s back up for a minute in case anyone doesn’t know what it is/does. PWM is pulse-width modulation. Put simply, this is a signal that is switched between on and off, usually rather quickly.
Do you remember when you were a kid, how you sometimes flicked the light switch on and off repeatedly, really quickly, to see what would happen? And you were told not to do it in case something bad happened (and it never did)? That was crude PWM.
It’s a “square” waveform (on or off most of the time) and looks something like [the photo at the top of this post]….
It’s used for controlling a variety of devices – motor speed, servo* positions and other things. You can even use PWM to vary the brightness of leds by switching them on and off at varying frequency and duration.
There are two important parameters that determine PWM…
Frequency, in Hertz (Hz) is the number of times per second that a pulse is generated. This counts from the start of one pulse to the start of the next. i.e. from when the pulse starts to rise, to the next time it starts to rise. So it includes all the “on” time and “off” time and “in between” time for one complete wave cycle.
Duty cycle is the percentage of time between pulses that the signal is “high” or “On”. So if you have a frequency of 50 Hz and a duty cycle of 50%, it means that every 1/50th (0.02) of a second a new pulse starts and that pulse is switched off half way through that time period (after 1/100th or 0.01s)….
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There will be a part 2 to this article with some practical examples. This PWM isn’t accurate enough for servos (too jittery), but should be fine for led dimming and motor control (where it doesn’t much matter if it skips a beat or two).