Saturday, February 23, 2013

Mini lanterns on a LED series

I made a set of small paper lanterns for a LED light series we have on a wall (indoors!). The paper lanterns are stylish and easy to make - they are a standard type of origami called 'fusen', which means balloon. I learned about them from the book The Simple Art of Japanese Papercrafts by Mari Ono. These little cubes are inflated after folding, and the air hole was very convenient for putting the fusen on a LED lamp.

The ones I made for this project are quite small. I used 7.5x7.5 cm paper, and the side of the cube is always a quarter of the side of the paper you started with. Since the LEDs don't give off any heat, the small size is not a problem. The origami paper diffuses the light very nicely, and the colors of the papers show up very bright.

Best of all, the color scheme infinitely adjustable, just by adding and removing colors according to seasons, holidays and moods!

Tuesday, February 19, 2013

WiFi access point with Raspberry Pi

Raspberry Pi as a WiFi router

In a previous post, I talked about using the Raspberry Pi as a router. Now I wanted to add a WiFi device, and use the Pi as an access point as well. The Pi has three network interfaces: wlan0 is the newly added WLAN device, eth0 is my LAN, and ppp0 is a USB GSM modem connecting everything to the internet.

The primary reason for all of this is to have an access point for an Android phone. It turns out that an ad-hoc network is not enough - Android connects only to real access points (unless rooted and configured to be less suspicious). So the task now is to set up a Linux access point.

Software-wise, the access point functionality is provided by hostapd, which can be installed on the Pi easily.
sudo apt-get install hostapd
The hardware part is quite tricky though - not every wlan card works in access point, or managed, mode in Linux. The state of support in the various drivers can be found at Among the USB WiFi sticks, ath9k_htc seems well supported. I ended up buying a TL-WN722N, since it is supported by the ath9k_htc drier, and the external antenna looks efficient. It has worked very well so far, and is in daily use. Below, I describe the steps required for setting up everything. I am using Raspbian, but most of the things should work on any distribution.

The steps
Set up a DHCP server. Edit /etc/dhcp/dhcpd.conf and add the following lines.
subnet netmask {
option domain-name-servers;
option routers;
interface wlan0;
If you already have an entry for DHCP on another subnet on another interface, make sure that the old subnet declaration also specifies for which interface it applies. Next, execute the following commands in a terminal, as root.

ifconfig wlan0    #bring up the interface
/etc/init.d/isc-dhcp-server restart #restart the DHCP server
echo "1" > /proc/sys/net/ipv4/ip_forward #turn on IP forwarding
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE #add a routing rule.

Replace ppp0 on the last line by the interface connected to the internet. For me ppp0 is a GSM modem.

Edit  /etc/hostapd/hostapd.conf . Add these lines (as a starting point):
wpa_pairwise=TKIP CCMP

Next, start the access point by running hostapd.

hostapd -d /etc/hostapd/hostapd.conf

-d is for debugging output. -dd shows even more. Can be useful while setting up.

Now you should be able to connect to your new access point! Note that when hostapd quits, wlan0 looses it's IP address. So the IP must be set each time before hostapd is run.

A TP-link TL-WN722N used as an access point

If everything works with this setup, it is convenient to automate the startup procedure, so that the access point is enabled at each boot.

To bring up wlan0 at boot, add the following to /etc/network/interfaces
auto wlan0
iface wlan0 inet static
To start hostapd automatically, add the command to /etc/rc.local:
hostapd -B /etc/hostapd/hostapd.conf
-B is for running in the background, as a daemon. Messages are logged in /var/log/syslog.

For setting up the IP forwarding stuff permanently, see my previous router post, and this nice guide.

So far I am very pleased with this setup. The only drawback is that the LAN on eth0 and the wireless network are separate and isolated from each other. Perhaps it's possible to construct a network bridge between the LAN and the WLAN interface. Then the two networks would appear as one, but this I leave for the next hacking session.

Add randomness!
Finally a note about a puzzling message from hostapd. When run with the flag -dd, hostapd works, but outputs
Add randomness: count=1 entropy=0
Add randomness: count=2 entropy=1
Add randomness: count=3 entropy=2
and so on, with a new line every second or so. I thought this had to do with /dev/random running out of random bits, but this is not the case.

Reading the source code of hostapd (version 1.0, since this is what I have on the Pi), this turns out to be a normal and harmless debugging message.  Hostapd keeps an internal pool of random bits (for encryption), in addition to the random numbers it reads from /dev/random. In different places in the program, random (or at least unpredictable) data is mixed into this pool. For example the signal strength of each received packet is used in this way. The "Add randomness" message is printed each time data is added to the pool. This is done in the function
random_add_randomness (const void *buf, size_t len)
in the file hostapd-1.0/src/crypto/random.c. So, in summary, the message can safely be ignored, and goes away if one runs hostapd without the -dd flag.

However, on a small system like the Pi, there is a risk of depleting /dev/random, especially just after a boot. I observed these messages from hostapd:
random: Got 18/20 bytes from /dev/random
random: Only 18/20 bytes of strong random data available from /dev/random
At this point I installed the program haveged,
sudo apt-get install haveged
Haveged is a program that helps with providing randomness or entropy, which it collects faster than the kernel does by default. I have not seen the message since then.

Saturday, February 16, 2013

A Pibow case!

My Raspberry Pi in its new Pibow housing.
For Christmas I got a Pibow case. I like it very much! The Pi in the Pibow case feels nice and robust. The Pi gets slightly warmer in the case than without one, but this has not been a problem at all.

Thursday, February 14, 2013

Happy Valentine!

We wish you all a happy Valentine's day with origami hearts and an equation.

Let's make a plot in polar coordinates, of \( r = |\phi|^p\). The angle \(\phi\) goes from \(-\pi\) to \(\pi\),  and \(p \approx 1 \). Here is the plot, in processing, with \(\phi = 0 \) pointing up. Move the mouse to change \(p\)!

Mathworld has more equations for hearts, but none of them is as simple as ours. As \(p = 1\) gives a nice shape, \(p\) can be left out for an even simpler equation.

Sunday, February 10, 2013

Storage system for wrapping paper

Gift wrapping paper has always been a pain to store. Inspired by several similar arrangements on Pinterest, we made our own gift wrapping organization and storage system, on the inside of a clothes cabinet door.

For storing rolls of gift-wrapping paper, we used one of the (in Finland) ubiquitous Lanka & Muovi bathroom shelves, which consist of metal wire covered with a softer white plastic. We removed the lower parts of the two upper shelves with a wire cutter. The leftover wire stubs were smoothed down with a Dremel.

The result was two square ring-like supports over a deeper shelf, good for an almost infinite number of paper rolls!

Next, we used two Lanka & Muovi bars to make a holder for gift wrapping ribbon rolls. The bars (I really don't know what they're meant for in their original form) needed to be separated from the wall to make space for the ribbon rolls, we did this with some pieces of wood we had lying around.

We sawed off one end of the metal bars, and put in a hook for that end to rest on. This makes the ribbon rolls re-arrangeable without having to remove the entire thing!

Another bathroom shelf serves as storage for other gift-wrapping miscellanea: scissors, tape, gift bags, cards, markers...

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