Raspberry Pi Status

I've done much of the simple GPIO stuff. LEDs, buttons, temperature sensors, PIR motion sensor, photosensor. I've also learned (the hard way) about protecting the Pi and backing up the work I've done.

Now I'm about to break into transistors, higher voltage and a more interesting device, e.g., a solinoid-operated water valve.

If any of this confuses you, you should scroll back a few episodes in my blog.

Raspberry Pi Photo Resistor (light sensor)

I ordered 2 of these a couple weeks ago along with the capacitors listed at Adafruit plus heat shrink tubes and extra breadboard wires. Anyway, these things are so tiny I thought they'd been left off the order. But before complaining, I went through my Pi stuff again and found them.

(a dozen of the sensor bit would fit on a dime)

I had a couple problems getting this device to work:

1. I wired it up like the Adafruit lesson shows (1 mF capacitor and GPIO on the ground side), straight to 3.3V pin on the other side. That didn't work. Looking around some more, I found--

   http://www.raspberrypi-spy.co.uk/2012/08/reading-analogue-sensors-with-one-gpio-pin/

Their example added a resistor on the ground side of the circuit. This eventually worked.

2. My batch of assorted resistors from Radio Shack comes with a list of how many of each kind in the package but the taped-together strips are not labeled. The "pi-spy" interface to the photo sensor added for a 2.2k resistor on the 3.3V side of the circuit. I did a google for "resistor 2.2k" but the color coding shown didn't match anything from my Shack assortment (after a non-trivial search using a magnifying glass). The on-line color-band "calculators" seem difficult, but then I found a "resistor widget" app for my Mac.

The user interface to the widget could be better: you click on the bands repeatedly until it adds up to the value you want. But it's better than the alternatives. Guess what? I found that code among the Shack's jumble.

Here's my Python test program ("adapted" from various sources):
#!/usr/bin/python

import RPi.GPIO as GPIO, time, os

PHOTO_pin = 22

GPIO.setmode(GPIO.BCM)

def PStime (pin):
        reading = 0
        GPIO.setup(pin, GPIO.OUT)
        GPIO.output(pin, GPIO.LOW)
        time.sleep(0.1)

        GPIO.setup(pin, GPIO.IN)
        # Takes about 1 millisecond per loop cycle
        while (GPIO.input(pin) == GPIO.LOW):
                reading += 1
                if reading > 1000: # don't loop forever!
                        return -1
        return reading

for x in range(20):
        print PStime(PHOTO_pin)
        time.sleep(2)
GPIO.cleanup()

The printed results are kind of counter-intuitive. The darker it is, the more loops it takes to get a reading. My setup prints the following values--

  100+ a darkish room

  50     medium interior

  30     light interior

  10     pretty bright

What am I going to use this for? I want my Pi to send a warning if the main 110V power fails. So I'll set it up as follows:

a. UPS backup battery plugged into 110V.
b. Pi and Internet modem plugged into UPS.
c. Photosensor (above) pointed at an LED that's plugged into non-UPS 110V.
d. When external power fails a little Python program (which runs once per minute) notes the sensor change and sends out a text message and an e-mail before shutting itself down.

BTW: Are you familiar with the Linux (and UNIX before it) scheduling service "crontab"? If not, you should learn about it.

Door Sensor Working

I finally got the Magnetic Switch / Door Open sensor working. My wiring isn't "exactly" like anything I saw on the Web.

There are 2 undifferentiated wires out of the device. In the try that worked I connected:

wire 1 to 10K resistor to GND

wire 2 to a GPIO pin (e.g., 25 for code below)

Here's a messy pic:

Here's my crude (but working) Python code:

#!/usr/bin/python

import time

import RPi.GPIO as GPIO

GPIO.setwarnings(False)

GPIO.setmode(GPIO.BCM)

DOOR_pin = 25 # 1 wire->10kRes->GND, other->GPIO25

LED_pin = 17

GPIO.setup(DOOR_pin, GPIO.IN)

GPIO.setup(LED_pin, GPIO.OUT)

tm = 0

changes = 0

last = 0

h = 0

for x in range(20): # tries

        m = GPIO.input(DOOR_pin)

        if m:  # Mag. sw open detected

                if last != m:

                        GPIO.output(LED_pin,GPIO.HIGH) # LED on

                        tm = time.strftime("%y, %m/%d:%H:%M:%S")

                        print tm #temp -- should log

        else:

                if last:

                        GPIO.output(LED_pin,GPIO.LOW) # LED off

        if m==1 and last==0 

                changes += 1

        h += m

        last = m

        time.sleep(2)

print "m's:", h

print 'changes:', changes

GPIO.cleanup()

The 2 halves of the sensor have to be about .5" apart to register "ON" (HIGH).

Now for my photo sensor.

My Raspberry Pi — Extending Sensor Circuits

I've been wondering what the best way to do this is — and how far you could go. Here's my inelegant solution:

- I bought a 3' 3-wire extension cord
- I cut the cord part in half
- I also cut 3 M/F breadboard patch wires in half (a black, a red and a yellow)
- I stripped all cut ends (12, in all)
- I soldered the female patch wires to the female half of the of the 3-wire cord
- Ditto, the male with the male
- I chose to color code as follows: red=current, black=ground and yellow=GPIO control
- Heat-shrink tubes and electrical tape finished the job.

Above: female end connected to a PIR motion detector.

After I made sure that the short cord worked I added a 50' 3-wire extension between the 2 plugs shown above.  Guess what? That worked too.

So much for good news — what are the down sides?

1. These cords and plug are heavy and stiff. I have to be careful not to bend patch wire pins.
2. 14-gauge wire is over-kill.

But, so what.

After Disaster!

My Raspberry Pi

(if you are new to the site you might want to scroll back a ways)

Lady Bracknell: To lose one parent, Mr. Worthing, may be regarded as a misfortune.
To lose both looks like carelessness.
The Importance of being Ernest,
by Oscar Wilde

Lady B. would have chided me, too. I managed to crash the Linux filesystem on my Pi's SD card. Again! But this time I had a backup image of the entire 16gb card and I have copied individual files back to my iMac, occasionally. But not often enough. My SD image was a month old and the last time I copied files was a week ago.

You might ask, "Why did your Pi's filesystem go bad?" Because I was plugging wires from the GPIO pins to my breadboard while the Pi was powered up. Bad idea! I saw the Pi reboot -- must have grounded a power wire. Afterwards I couldn't talk (via SSH) to the Pi. When I say I "saw" it restart, I mean that the status LEDs went dark and then only the PWR light stayed on. Here's what my LEDs look like, normally:

The REAL info on the Pi's status LEDs:
http://elinux.org/R-Pi_Troubleshooting

Anyway, my SD card backup/restore procedure:

From my Mac's Terminal app (PCers use Putty)--

Backing Up

- Shutdown your Pi
- Pull the SD card and plug it into your USB flash memory reader
- Determine what the device name that has been assigned to your SD using diskutil:
   $ diskutil list  (you should be able to tell by the size in GB)
- Then you have to unmount it:
  $ diskutil unmountDisk /dev/disk2 (yours may not be "disk2")
- Now the copy -- you have to be superuser (be very careful):
 $ sudo dd if=/dev/rdisk2 of=raspberry-pi-backup.img bs=1m

Note I copied from the "raw" device (rdisk2) instead of the "block" device -- it's faster. Also, I used "bs=1m" (block size = 1MB), also faster. When I say faster -- the difference for a 16gb SD versus using "bs=1024" is 6 hours!

Restoring (with care you may never have to)

- Plug the same sized SD card into your flash reader (it's good to have a spare)
- Find the /dev name (diskutil list, as above)
- Also like the above, do the unmount
- Now the copy swapping "if" and "of" (in-file & out-file)
 $ sudo dd of=/dev/rdisk2 if=raspberry-pi-backup.img bs=1m

More about the limitations of copying individual files back to the SSH host: You can't usually copy system updates or things like crontab entries. If you don't know about crontab, you should! Try--

  $ man crontab