Internet Of Mailbox

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Abstract.

This post summaries the test I made to produce a mail presence sensor. It is aimed to be wireless, connected to the Pilogger logging station. It is interesting to use a load cell in order to know the weight of the object in the mailbox. But we will discuss the difficulties to implement a reliable solution.

The sensor.

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Figure 1 – NTC inserted in the load cell

It could be found very cheap load cell on Ebay, with a small board featuring a 24-bits ADC. The ones I used for this test is a 10Kg cell with a HX711 board.

As the principle of sensing is based on the physical deformation of the metallic rod, the system is affected by temperature change. So I decide to use a thermistor (NTC), placed directly within the metallic rod, by placing it in a small drilled hole. (Figure 1, on the left)

The electronic

The HX711 is an interesting chip for its price range. It is design to measure very tiny resistance change using a Winston bridge. It have two channels, one with two gains (64 and 128) and another with a fixe 32 gain. While the first channel is connected to the load cell, the second one is perfectly usable for measuring the thermistor. To do so we have to do another Winston bridge including this thermistor. (Figure 2, on the right)

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Figure 2 – Wireless mailbox probe schematics

I chose to use the same micro-controller and RF link as the previous wireless temperature probe. In other words, a PIC16F1825 and a nRF24l01+. The difference is the HX711 module and a voltage regulator. The HX711 needs a stable 3.3V, and then I use an analog input of the PIC to have a battery voltage information.

Code.

The micro-controller code is rather simple as well, it sleeps for around 8 seconds, then sequentially get 8 values of HX711 channel ‘A’ and average them, the same for channel ‘B’, then get the ADC value of the battery, and finally send the data with the nRF24l01.

The XC8 C compiler file for the PIC16F1825 is here in Github.

Installation.

I fixed one end of the load cell directly to the mailbox bottom, and the other end to an aluminium plate of approximately the surface of the box. For the tests, I just fixed the proto board on a side with tape, and the battery below.

Tests and data.

The system is surprisingly sensible, a letter is clearly visible on the weight data.

But sensitivity is not precision. And things start to be more complex when I try to figure out how to calibrate the system. There is a link between temperature and weight data, but not only. The entire mechanical system plays a role.

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Figue 8 – One month of data

Figure 8 shows a complete month of temperature and weight data, with arbitrary units (ADC output). While points marked with ‘1’ are the time where a letter was on the plate, the others are interesting. Point ‘3’ shows that a heavy parcel was kindly placed in the box by the postman. But after that, the weight base line did not recover to the same value. Apparently a mechanical bias appeared, it could be a permanent distortion of the load cell.

I could not fully understood the events marked as ‘2’. These drops should reflect a resistance change, so I imagine it could be linked with humidity and water condensation. It seems it disappeared once I cover the HX711 pins and copper tracks with varnish.

I tried to establish a correlation between the temperature values and the weight values in order to compensate the changes. As the event ‘3’ put an offset I have two data groups. I agree the data looks funny, and it is hard to see a true correlation. (Figure 9&10) We could barely see a kind of exponential curve such as Weight = a + b. Temp², but not enough to compensate correctly.

Conclusion.

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Figure 11 – Battery discharge in volts

To summarize, the concept is clearly working. I first though the battery would not last so long, but event not fully charged, the 4000mAh li-ion is still alive after a month. (Figure 11).

The HX711 used strait out of the box without shielding gives quite good results. On the 24bit resolution, around 19 to 20 are out of noise, which is remarkable for such a draft system.

As it is, I have at least an idea if there is something present (and roughly how big) in the mailbox, and this everywhere I could have an internet access. To be continued !

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Image dithering

Abstract.

I had to convert images to black or white points. This post is a summary of my tests and learning. My main source is the nice ‘Dither’ Wikipedia page. The major issue I had to face is when converting an image with an undefined length coming from a scanning process.

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Conversion.

To print pictures on thermal paper with reasonably good rendering, they should be converted according the printer limitations. It’s quite simple, it can be printed only black pixels (and whites as the paper is usually white).

The first step consists to get a gray-scale image. There is plenty well known method, from the simplest averaging of the Red/Green/Blue channels to advanced ones where the pixel intensity is scaled to a color wavelength to simulate a particular response (such as black & white films).

michelangelo27s_david_-_bayermichelangelo27s_david_-_floyd-steinbergThe second step could be achieved by several algorithms. To convert gray-scale pixels to only black or white, a rather easy method is the Ordered Dithering. But it there is noticeable patterns on the resulting image. Another common method is the error diffusion, and in particular the Floyd-Steinberg dithering algorithm. It is this last one that I choose to study as I found the resulting image interesting.

Codes.

The codes we can find over Internet usually rely on a buffer to store and propagate the errors of each converted pixel. This buffer have the same size of the image and is completed during the iteration over all the image lines.

As receipt thermal printer use paper roll, there is virtually no image length limit. If we want to convert on the fly the image and strait print it, we cannot rely on a fixe full image sized buffer. That’s why I experimenting conversion algorithm based on Floyd-Steinberg, but with only a small rotating buffer. The size of this ‘line’ buffer could be as small as a line size plus one.

Other than the code itself, I try to explain the principle with this Gif :

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https://pierremuth.files.wordpress.com/2016/04/dither.gif

And the Java code use to generate it :

https://github.com/pierre-muth/selfpi/blob/master/src/tests/Dither.java

Very good readings about dithering :
http://bisqwit.iki.fi/story/howto/dither/jy/
http://www.tannerhelland.com/4660/dithering-eleven-algorithms-source-code/

Icebreaker Game

Abstract.

12026610_10207455771046540_1864584773_nAfter experimenting with the small receipt thermal printers and manage to make a portable camera, my brother came up with a very clever idea. He is organizer of a juggling convention, and he looked for a kind of funny game around beer selling and their receipt. The aim was to entertain and offer a beer every lets say 50 sold. He said we can do more than print pseudo randomly a winning ticket based on certain ratio.

The idea.

beerWe imagined a machine standing on a bar, with on the client side a camera pointing at you, a screen showing the camera output and few instructions. On the back side, a single button allowing the barman to launch the lottery for one ticket. What the machine do once you press the front big button (I really love these Adafruit massive buttons!), is taking a picture of you. And then there is two possibilities. If you loose, a souvenir receipt is printed with the picture of your face and a random funny quote. If you win, a winning receipt with the face of someone else is printed, and instructions telling you that if you find the random guy on the printed picture, you both won a free beer.

The printer.

2321The printer choice should consider reliability, speed and print
quality. Even if the last one is relative to what we can expect for black or white thermal printing. I moved from the affordable and small serial Adafruit 58mm printer to a more professional one. After some non conclusive test with a cheap 80mm Chinese printer, I finally get an Epson point of sale printer. The TM-T20 is USB and commands are very well documented on their website. If not set to the maximum print speed, they could make surprisingly good prints for bitmap pictures. It depends a bit of the paper quality as well. I get very nice results with BPA-free recycled paper roll.

The system.

Raspi_Colour_ROnce again, the raspberry pi is a nice card and of course perfectly capable of doing such a process. There is the official camera module, even if a basic webcam would make it as well, USB ports for the printer, HDMI for a screen, GPIO for buttons and leds. But most importantly, I can reuse a lot of code form the Polapi.

The software is written in Java and uses two external libraries. One for GPIO – Pi4J and an other one for USB – usb4JavaIn addition, the native raspbian program raspividyuv is used to get the camera frames. The camera output is on top of everything and always visible on the screen. The all Java software part is hosted on github. The runnable Jar file must be launch on the Raspbian LXDE environment and could be easily auto-launched. There is some logs here, but I must complete them a bit.

The case.

In the view to avoid yet another device taking dust in the garage, I made the case looks like an arcade. I just have to exchange the front panel to have a raspberry pi Mame machine ! (That will instead probably take the dust of the attic, but with style, surrounded by retro consoles)

Wood sawing 

Case filling

Resulting device

Outlook.

It was quite a success on the Juggling convention, it brought fun and few free beers ! It ran smoothly without any reboot despite I expected some bugs.

After being used on a week-end for around 500 prints, it will be used on a Hackathon in Geneva with a slightly different mode.

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The BIG_CLOCK

We need clocks. Once you have passionate activities which absorbs all your attention, if you want to keep contact with the social society surrounding you, it’s crucial to know when we are.

From this observation, it started this project with the wish to use the big LCD panels which take the dust somewhere. They are Lumex LCD-S101D30TR, 7 segment digit 3″ tall. The micro controller is a PIC18F87K90, and it does everything. So the hardware comes down to only a battery, a micro (with surrounding capacitors and an oscillator) and 6 LCD panels.

 

The digits are soldered on prototyping boards, which are screwed to two long rods. A basic white surrounding plastic frame, and I think it does the job!

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No relief now to say “Oh god, I didn’t see the time passing while I’m playing….”

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For details, go to the hackaday.io project page : https://hackaday.io/project/9687-bigclock

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Wireless temperature probe

Here is an attempt to give some information on the wireless probes I’m using with my Raspberry pi logging station (for the full story). The wireless link is performed by nRF24l01+ modules.

1- Reception

The data reception is done by directly connect a nRF24l01 module on the raspberry pi SPI port. And the pilogger java project  configure and handle the data reception/treatment, thanks to the Pi4J library.

2- The probe

To sense temperature without using long wires, there is a constraint to consider: the wireless module powering. It could be for example from a solar panel, or from a battery. Using a solar panel is not always possible. In case of measuring outside temperature, the solar panel should be deported to the sensor, as the sensor should not be exposed to the sun. Moreover, the charging circuit efficiency is critical for small sized module.

Powering only using battery simplifies the module but requires very low consumption component to last a reasonable time. The nRF24l01 wireless transceiver is very efficient on that way. To keep a very low consumption, I choose the MAX31723 temperature sensor. It is precise and uses the same SPI communication protocole as the nRF24l01 module. To manage these components, I choose the microchip PIC16F1825.

The schematics files of the module below could be find on the github pilogger repository.

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As well as the PIC code in C. The code put the micro-controller in sleep mode for roughly 4 seconds between each temperature measurements and transmissions. And the all circuit is powered directly from two AA Ni-MH batteries. Each components could still work down to the lower battery voltage of roughly 2 volts, and charged batteries are around 2.6 volts which is below the max 3.3 volts of the RF module.

I didn’t yet make printed circuits, then I use prototyping board such as :

In addition of the temperature, I used the internal voltage reference of the PIC to get the battery voltage and send it to the pilogger.

For the moment, I did two of these probes. One is outside in a waterproof case powered by two 1000 mAh AAA  Ni-MH batteries, and  one on the attic, powered by two 2000 mAh AA batteries. I don’t have yet a full battery life-time. I can see the battery voltage decreasing very slowly over a month, but Ni-MH discharging curves are rather flat for low current. I will update this post once probe stopped working !

Edit:

After 100 days of sending 12 bytes of data every 4 seconds, I think we can conclude the overall circuit does not consume too much energy. The two AAA 1000mAh battery voltage only drop by 40 mV. Let see if this can last one year !

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Instant-Printing-Point-and-Shoot camera

polapi01Here another project finally finished ! The Bluetooth thermal printer was fun. Unfortunately I didn’t get it work with apple i-stuff. A stand alone camera, as the Polaroid is in fact even more fun !
I used brass angle rods, glass fiber plates and screws for the case. A raspberry pi 2, this time with its own camera module. A TFT screen, the thermal printer, and a lipo battery. More details here:

hackaday.io/project/7176-polapi

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During a party, the 3.6Ah lipo was able to keep running the raspberry pi for around 6h, plus around 100 prints !
I found a large aperture CCTV lens to get maximum sensibility. The counter part is it need to be manually focused.

It is worth to mention few facts about the paper. Thermal paper is very cheap and could be easily found without BPA, but it is sensible to sunlight and heat. Exposed to sun, the prints will fade with time.

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