Skip to main content

setting up pwm

There's always an obstacle..

I implemented the pwm module as described in the link in the previous post, but oddly enough, it didn't work.

The error I received:

AttributeError: 'module' object has no attribute 'PWM'

After some time scratching my head, I came to the realization that somehow my python was running an older version of the RPi.GPIO library, as PWM control was a pretty recent addition to the library. 

And indeed it was. 

Not really sure what's happening in detail, but it seems that there are two different... entities? that manage libraries for Python. One is through the usual "apt-get" command and the other is "pip." It appears that Python looks at "pip" first, and it currently linked RPi.GPIO to a really old version of the library. 

Uninstalling that older version ("pip uninstall RPi.GPIO") and using the version I obtained through apt-get ("apt-get install python-rpi.gpio") solved this. 

Now, a less-hacky implementation of PWM. 

Note that the PWM offered in this library is a software-driven PWM, meaning that the cycles it counts are based on the cycle count of the process (program). This can be inaccurate because programs can be put "on hold" by the operating system during heavy load, and during this time on hold, it stops counting. Because PWM relies on timing, this could cause "jitters" in the devices that read the signal. A better implementation is a hardware-driven PWM, which uses the clock cycles in the physical board to count. 

For now, I think the software-driven PWM will be fine.

Time's up for working on this today; I'll change messages into JSON next time, I guess.

Comments

Popular posts from this blog

nodejs migration

Having been playing around with NodeJS recently,  I (naturally?) started re-writing some of the OpenSesame code using Node last night. I think I've only worked on it around 2 hours so far, but I've already set up a basic client interface (a socket.io chat tutorial rip) with a server that the Raspberry Pi can connect to and receive requests to open the door.  This is probably due to socket.io's socket management (socket.io is the WebSocket module for Node); for my first implementation, I had to manually write up a structure that managed sockets, but that is pretty much handled by socket.io. Also, the servo control logic is pretty much recycled (and the Raspberry Pi code is still Python), and I do remember spending some good time figuring that out.  Cool neverthelss. I'll probably keep both versions around.

quick recollection on setting things up

If I can swear that the password is correct, then it's possible that I'm getting the ID wrong. Hadn't logged into the web server in a couple of months and was unable to SSH into it. After an hour or so, I ended up deleting the instance and recreating a clean instance (read: erasing everything). I then realized I'd been putting the wrong ID.   apt-get install sudo apt-get install Anyways, some links I visited when setting things up (pretty much a dump of whatever I copy-pasted to notes) : Raspberry Pi http://www.makeuseof.com/tag/optimize-the-power-of-your-raspberry-pi-with-raspbian/ Logging http://docs.python.org/2/howto/logging-cookbook.html Tornado - Python webserver that supports Websocket http://stackoverflow.com/questions/2924991/what-popular-webservers-have-support-for-html5-websocket http://lowpowerlab.com/blog/2013/01/17/raspberrypi-websockets-with-python-tornado/ http://stackoverflow.com/questions/11695375/tornado-identify-tra...

alcohol sensor (and some patience)

Soldered the alcohol sensor into something that is connectable: I tried to connect this to the Arduino, as I had the appropriate circuitry, but I did not get any legitimate output from it. 5V going in, 5V coming out with no variations. Nothing seems to be awry in wiring, as the circuit seems to be grounded properly (and the 5V current is flowing).  There are a couple of potential factors as to why I'm not seeing any results: - I'm using a 10k ohm resistor, while some guides (and the datasheet for the sensor) asks for 100-200k. However, there seems to be a good amount of people using 10k and getting at least some kind of result. A batch of 100k ohm resistors I ordered is on its way, so I guess I can try with them when they come. - This site  claims that these sensors take 24-48 hours for its signals to be stable. It also tells me that I should not be powering the sensor directly from the Arduino, which I have been doing, out of concern that the power draw of ...