Aaron Parecki

Disclaimer: The posts in this "Ultrasonic Sensors" category are primarily a reference for myself during this project. However, I suspect some of the things I discover will be of use to others doing similar projects, so I will make them public.

Update:

We just got caller ID at my house. One of my roommates doesn't have a cell phone, so eventually everyone else stopped answering the house phone because it was always for him. But then he would come home and not know who called. Now of course, caller ID solves this problem by logging who called on the phone handset, but being me, that is not enough!

Ok, so I wasted about 3 hours on this, and have only sort of found a solution. I have a PHP script which is going to be running 24/7 checking a message queue and then interacting with a database when a message is received. It was mostly working, until it sat there for a day and I tried to use it the next day. The PHP script had quit, with the error "MySQL server has gone away." I figured there must be some sort of time limit that the mysql server will keep an idle connection alive. I couldn't find it, but luckily I was talking to nick, and he did! He found this blog entry. So I changed the my.cnf file to set the timeout to one week:

I had a thought of using a RAM disk as a message queue for a messaging application I am working on, as opposed to creating a table in a database, or using a regular flat text file. Every time I need a message to be sent, I'll add a text file on the RAM disk, then the sending process can scan for files. All of this is happening in RAM, so it won't be thrashing the disk!

This has happened once before. One day, photoshop decides it doesn't want to start up. During the loading screen, it just stops on "Reading text global resources."

One of my servers has a nasty habit of not reloading the firewall rules when it boots up. Iptables starts up just fine, but doing iptables-save shows that there are no rules listed. The weird part is that on all my other servers, I never had to do anything special to get it to remember the rules on restart.

Apparently raid arrays don't like it when you kill power to the machine. We've been doing remodeling in the house, and I've been turning on and off breakers. Apparently I forgot which breaker the servers were on, and turned it off accidentally a few times.

I recently discovered an excellent tutorial on setting up Postfix and Courier to use virtual users read from a MySQL database. This caught my attention because it is a much better setup than having real system users for all email addresses, as it allows much more flexibility. I finally set aside some time to set up a real mail server.

I was working on a website today, and I stumbled into a problem of needing to compute the brightness of a color, so that if it is a background color, I know whether to show white text or black text. My first thought was to simply add up the RGB components of the color and use that value. It turns out that is completely wrong, since pure green looks much brighter than pure blue:

I spent about an hour bashing my head against a wall with this one.

I didn't realize it before, but the subversion server running without tunneling over ssh doesn't actually send the password in the clear when authenticating users. From the subversion book:

I was just reading this webpage about shell scripting, and discovered that you can get a lot of information about your cpu and memory from the command line.

I'm reconfiguring my RAID 5 array in one of my servers (called malachite), and having some problems doing so.

This was a problem done in my CIS 315 Algorithms class in Spring 2006.

First, I downloaded the Majordomo source code from here. Following the instructions in the included INSTALL file, I was able to get Majordomo installed relatively quickly. I ran into problems when I started trying to actually send email to the new list. The first problem I ran into was that sendmail wasn't started, but that was just silly.

My VPS was restarted yesterday, and apparently Apache wasn't shut down gracefully. I tried to go to my website, but Apache wasn't running. I SSH'd into my machine to restart apache. I typed

The other day I decided to set up my iBook so that I can do web development on it.

I just found the TeX plugin for Serendipity (s9y.org). It lets you draw cool math symbols like

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.