119: Raspberry Pi Camera Exposure

I've tried 3 cameras: the V1 (5mpix), V2 (8mpix) and a wide-angle (5mpix). I've had trouble with exposure with all of them.

One small gripe: the raspistill command --help arg lists a zillion options without a clue about what the default values are.

But that's not my story. I want to take snaps from before dawn till after dusk and I found that the many exposure-related arguments seemed to be ineffective. But I finally discovered the silver bullet. Shutter speed (-ss microseconds). That one works.

So, I average B/W pixel values in python and adjust shutter speed accordingly. My current -ss range is (bright to dark) 500 to 4000000 microseconds (1/2000th to 4 seconds).

118: A Problem with Insulation

I have several IoT systems installed in farm hoophouses (used to be called "greenhouses"). Each configuration is housed in a (mostly) waterproof plastic container -- about 12" x 18" x 8". The hoophouses are environmental trouble -- often high humidity and the daily temperature can range 50F to 120F. Processors are either Particle Photons or Raspberry Pi Zero-W and nearly all include relay boards. E.g.:

6-port relay

The boards are screwed down (loosly) to 1/4" plywood -- non-conductive, but I didn't like the back side of the boards:

many nasty soldered pins

So I decided to stick masking tape on the back of the boards. At least they ere easier to handle. Like so:

Don't do this!

At least, in the case of the particular product I used and/or the temperature range, that tape was NOT an effective insulator. I've had a few "relay board failures". But today (belatedly) I proved to myself that it was the stupid masking tape. When I replaced the tape with the black electrical type my failing relay board was fixed.

117: Overheating: The Pi's Internal Temperature Sensor

I currently have a Pi Zero W mounted inside a farm hoophouse (i.e., one of the new plastic film greenhouses). Unless the sides get rolled-up it can get over 120F in a hoophouse on a summer day. Yesterday my Pi Zero reported 154F from inside it's sealed case -- very close to the max 70C/158F for the Arm processor spec. Anyway, I protect my Pis as folows:

In cputemp.sh
h=`hostname` # I have several Pis
while true ;
do
 a=`/opt/vc/bin/vcgencmd measure_temp`
 b=`expr "$a" : '.*=\([0-9][0-9]*\)'`
 if [ $b -gt "70" ] ;
 then
  echo $h HOT | mail -s 'Rpi-HOT HALTED' some@gmail.com
  sudo halt # must power off/on to restart
 fi
 if [ $b -lt "55" ] ;
 then
  sleep 300
 else
  sleep 60
 fi
done

And in crontab
@reboot bash ...(your path).../cputemp.sh

Which means the above shell file will be executed whenever the Pi restarts.

P.S.: About overheating -- while my Pi Zero seemed to be working up to 70C, it became erratic at 66C/150F. while it still sent data to a web page, it stopped being able to snap a new image with the PiCamera/raspistill.

116: Python Threads

This isn't my usual sort of interest. And I'm not very fond of Python -- too many ways of doing the same thing (Go golang!). Anyway. I wanted to program an async task that accepts input that might modify what's happening in the main loop. I'm only posting this code snippet because it took me 20-ish Googles and a dozen source tweaks to get this to work. And the posted examples are way more arcane.

# Simple Python Threading example that puts blocking 
# I/O in its own asynchronous thread

import os, time
import threading

rv = '' # to pass data to the loop

def ck_file() : # the I/O thread
    global rv

    while True :
        if os.path.exists('cmd') :
            f = open('cmd', 'r')
            rv = f.readline()
            f.close()
            os.remove('cmd') # so it's only reported once
        time.sleep(5)

t = threading.Thread(target=ck_file) # black magic!
t.daemon = True # needed to cause the thread to exit
t.start()       # never runs without this

# main loop
ct = 0
while True :  # where you'd do something useful
    time.sleep(10)
    ct += 1
    print "ct=", ct
    if rv != '' : #data entered
        print "RV:", rv
        rv = ''   
    if ct == 10 : break # it's just an example, after all

Note: the daemon line is important. Probably should be the default. Otherwise the thread never exits.

So, in the spirit of an oyster turning a grain of sand into a pearl I have posted this pearl (?) caused by my aggravation.

115: SSH over the Internet!

(why didn't I find this sooner?)

There are several services available. I chose remot3.it. Their web page leaves certain info gaps (duh) but with my daughter's help I got 3 Pi Zeros connected. Just like local SSH! Unlike just replacing files you can interactively update cron or edit a file with nano. And the "private" account is free (so far). Unfortunately, you have to go through several web clicks to get a sort-of "session ID number" (my term) to do a remote SSH connection. Still, very nice.

114: A Better Pi Zero W (IMHO)

114: A Better Pi Zero W (IMHO)

So far I've purchased 3 Pi Zero Ws and will doubtless buy more. But this doesn't mean that I like everything about their physical layout. Here's a picture contrasting the Zero with a Particle Photon (which is very similar to an Arduino Nano or Micro).

Photon on left, Zero on right

Obviously, the Zero has to be bigger. I accept that (sort of). But I prefer the Arduino/Photon layout for 2 main reasons:

a. The GPIO pins straddle the breadboard halves. Very handy. No stiff ribbon cable required.

b. Instead of the side mount, the micro USB points out the end so a compact enclosure (e.g., hand-held) setup is possible. As is, the minimum inside width of a Pi Zero case is 2.5".

Here's my layout plan:

Note that I've left off the 2nd USB and HDMI plugs. They could be added via a daughter board connected through the above "accessories" slot. For the daughter board I'd provide a full-sized HDMI and 2 USBs.

Comments?

Re: living with current reality— Anyone know of a compact/right angle micro USB plug?

113: Better Breadboard? See post 62 at http://dicks-photon-arduino.blogspot.com/

Just pie-in-the-sky, of course.