For my home-monitoring setup I would like an Arduino to measure the supply voltage it is getting from a DC battery UPS (Uninteruptible Power Supply). Unfortunately (actually by design, but that’s another story), the power supply is 24V, which means it will put out anywhere from 21.3V-29.8V (according to the manufacturer), which is far too much to measure with the Arduino’s 0-5V input range. For simplicity’s sake, lets assume we want to measure a 20-30V voltage.
The immediate answer is to use a voltage divider, which will bring a voltage in the 0-30V range into the 0-5V range. The general formula for the resistor divider is:
We want to give , so
Now, just as a sanity check we should calculate the current of the resistor divider, to make sure we’re not converting too much electricity into heat. Ohm’s law gives us
which in this cases gives
No problems there.
This works okay, but we lose a lot of precision, as only ~1/3 of the Arduino’s range is actually used: the Arduino’s ADC has 1024 different readings between 0-5V, so when reading the 0-30V range the precision is just about
over the range.
If only we could move the lower bound, so that 20V would map to 0V on the Arduino. A wild Zener Diode appears! One use of a Zener diode is as a voltage shifter.
The closest Zener diode I could find was an 18V of the BZX79 series. This resulted in the following circuit:
which I hacked into my Arduino box.
Now, theoretically the formula for translating an voltage at the Arduino to the supply voltage should be:
I then did some quick measurements of various input voltages and the resulting voltage at the Arduino pin:
Plot it into a spreadsheet, create a graph and add a linear regression gives:
Now, this formula is a bit different compared to the theoretical one, mainly in the Zener diode drop. However, the datasheet for the BZX79 actually has the 18V C-type () as between 16.8-19.1V, so this is well within spec. Since this is just a one-off, I’m happy to just use the measured formula, as this will be more accurate.
The final precision should be . The current should be around , which again is ok.
You need to modify the variables at the top: set currentDay, currentHour, currentMinute according to the present time.
The pre-programmed schedule is to clean at 03:00 on Sunday, Monday, Wednesday and Friday. You can change this if you wish, by altering the daystorun and times variables.
If you don't modify the schedule, the Roomba should start automatically after 2 minutes.
Put it all together
You should now have a partially undressed Roomba, and a programmed Arduino. Now it is time to connect them. With both unpowered, connect the following:
Arduino GND to Roomba ground (pin 6)
Arduino TX (pin 1 on e.g. Uno) to Roomba RX (pin 3)
It should look like this:
Now, the moment of truth. Press the "CLEAN" button on the Roomba, the light should go on. Plug in the USB for the Arduino. The Roomba light should turn off briefly, and after a few seconds the Arduino should blink it's LED. The schedule is now programmed, all done!
The local hacker space here in Aalborg (HAL9k) has moved from Rapsgade 1 to a permanent location at Sofiendalsvej 80. For a number of weeks we he been moving boxes of all our stuff, painting walls and other tasks related to the move. One of the things that I was involved in was the electricity installation and the computer network.
We have been so lucky to inherit a large number of metal and plastic cable trays including a number of Ethernet jacks and power outlets reducing the budget for establishing a new infrastructure at the new place.
After a lot of work, 50 m metal, 20 m plastic cable tray and 1,2 km Ethernet cable later we now have 74 ethernet jacks connected to our distribution frame with our two 48 port gigabit Dell switches connected to an OpenBSD firewall in VMware on our Dell R710 server, routing IPv4 and IPv6 (through a SIXXS tunnel). Thank you to Xena for the switches and the server.
The wireless network is handled by a 802.11a/b/g access point. The access point and the directional antennas are actually meant for outdoor usage, so we mounted it on an IKEA table leg to give it a touch of the rustic outdoor life on a large pole…
If you are curious about the details of the network setup check out the hal9k wiki here.
Besides the network installation we have also installed a new distribution board and a lot of power outlets. We have a dedicated power meter for the server room besides the three other power meters (which according to our plan will be monitored by our LIAB) and relays for the groups used in the workshops making integration with the physical access control system and emergency buttons easily done.
I had this old POS (Point Of Sale) receipt printer laying around for some time and thought it could be fun to play a bit with it. Printing plain text is simply done by sending it as characters via a serial connection. Cutting the paper is some ESC code which i found in a manual online. Printing images was a bit more tricky, since it is requires some specific codes, depending on how the dots in a vertical bar, equal to the height of a character, must be composed. I wrote a small Python module integrating the ability to print images, text etc. since I could not find anything online simliar to what i wanted.
At the local hacker space (HAL9k) we have now received a large box full of used receipt printers, VFDs etc. and some of the members have asked for the module for printing images so I have uploaded pyPOSprinter to GitHub. It makes use of pySerial for communicating with the printer and Python Image Library for reading image files. The included example also shows how to use pyQRNative to print QR-codes (bar codes are soo last centory :-) ).
from pyqrnative import PyQRNative
qr = PyQRNative.QRCode(5, PyQRNative.QRErrorCorrectLevel.Q)
At the hackerspace in Aalborg (HAL9k) we wanted to mark equipment etc. with various information such as the owner of the equipment, possible instructions on how to use it and other relevant information.
We decided to make something based on stickers with QR codes where the user is redirected to the relevant subpage on HAL9k’s website. One requirement was furthermore that it should be possible to change the name of a wiki page or possibly change to a new wiki/CMS system without putting on a bunch of new stickers, i.e. something more dynamic than a direct link.
With regards to the stickers we ended up with a label printer attached to an Ubuntu Linux PC. We got 1300 stickers (resulting in 2600 QR codes) on Ebay for 350 DKK – enough stickers for the next foreseeable time..
Mikael from the hackerspace wrote some PHP code that we run locally on an Apache webserver on the PC, which basically generates a QR code (well actually two since we have two individual QR codes per sticker in order to end up with the right size), adds some (optional) text, saves the resulting image to a file and then calls the lp printing command.
The label printer was made to work with CUPS by using the instructions on this site.
Now back to the requirement about the desire not having to put new stickers on all the equipment when we change wiki/CMS, rename pages etc. We solved this by letting the QR codes point to http://qr.hal9k.dk/HQRxxx where ‘xxx’ is some number. The webserver at qr.hal9k.dk automatically redirects (307 Temporary Redirect) to the relevant page. In order not to have a manually maintained list that nobody would update anyway i wrote a Python script that every 5 minutes goes through all the pages on the HAL9k wiki using the page hal9k.dk/qr as an index. Every time it sees the string ‘HQR’ followed by a number on a given page, a reference is made and a .htaccess file is then updated so that the user is redirected to the correct page. The mappings between the keys and the wiki pages, logs etc. generated by the script is available at qr.hal9k.dk.
So the end result, besides having fun making it all, is that now users can easily print labels and put them on equipment at the hackerspace. If they forget to create the wiki page the user is automatically redirected to this page for a reminder about just that