How I Feed My Cats with Linux
Cats love toys. Our cats, Cotton and Tulip, slowly have taken over our house with their little plastic doo-dads—ping-pong balls, furry mice, bells, springs and things to scratch. The cats are rarely bored. On the weekends, my wife and I oblige the kittens by tossing their toys around the house, flinging strings and jingling bells. We scratch their backs and feed them treats. They're both in love with these little stinky fish treats; all we need to do is shake the can, and they stop whatever they're doing and dash to the kitchen. Their English lexicon now includes their names and the words good and treats.
Monday through Friday, nine to five, however, the cats are responsible for their own entertainment. While we're away, we're sure the cats have a good time with their toys. Our rugs almost always are moved around, ping-pong balls end up in water dishes and fur covers our chairs. The only real difference between the weekday and the weekend is our presence and the lack of treats.
We have to work, but that doesn't mean our cats should have to go without stinky little fish, right? Why should our economic necessities have a negative effect on their treat times? Isn't it our responsibility to build them an Internet-enabled, Linux-based, cat-feeding device?
Where do we start? Three ingredients are key to our Linux-based Internet cat feeder: logic on the system, a way to talk to a device and a device to talk to. I chose Python for the logic piece, talking over a serial port to a microcontrolled cat feeder of my own design. Let's start at the bottom, the device, and work our way up to the top, the logic.
I first heard about the BASIC Stamp microcontroller from an article on Slashdot in which three guys were using a BASIC Stamp to control a bolt gun. They had taken some great pictures of bolts destroying fruit. Microcontrollers, I soon learned, are everywhere. They're the bits of logic in our microwaves and our remote controls. They are tiny and often difficult to use.
Parallax, Inc., specializes in making microcontrollers for non-engineers, specifically for students and hobbyists. Parallax products are well documented, easy to use and relatively inexpensive. I bought the Homework Board, the most inexpensive starter kit, from Radio Shack for around $75 US. It came with a book, a bag of electronic components for the experiments in the book and the board and chip.
The Stamp itself actually is a PIC microcontroller with some memory. Typically, you need to program microcontrollers with a low-level language, such as Assembly. What sets the BASIC Stamp apart from a typical microcontroller is the programming language you use to make it do stuff. Parallax developed a superset of BASIC, called PBASIC, that makes it easy to build expressive, useful programs quickly. In addition, the Homework Board has an integrated solderless breadboard, which makes for quick rewiring of projects.
The BASIC Stamp has 16 I/O pins. Each pin is set to high, +5V, or low, 0V, based on programs you create. Say you want to make an LED blink. You attach one end to an I/O pin and the other to a ground pin. You write a program that says, every second, turn the I/O pin to high (on), wait for a second, then turn it to low (off). Now replace the LED with a servo, and we've got the start of the cat feeder.
The I/O pins also listen for +5V or 0V. PBASIC even has a built-in function that allows an I/O pin to read serial data, the basis of which are high/low charges that make up binary words. Don't worry too much about serial connections yet; we cover them more in the next section. For now, understand that the BASIC Stamp can receive a command easily from a Linux system over a serial cable and turn on a servo that drives our cat feeder.
Parallax has done a great job of creating a fun community of hobbyists. Two mailing lists are devoted to its products, and dozens of sites have ideas for projects. Although the best integrated development environment for the BASIC Stamp is available only for Microsoft Windows, a tool called bstamp has been created, with Parallax's help, to program a BASIC Stamp with Linux. An example of tokenizing a program and running it, follows:
# bstamp_tokenize catcode.bs2 catcode.tok PBASIC Tokenizer Library version 1.16 # bstamp_run catcode.tok Model: Basic Stamp 2 Firmware version BCD = 16 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 Ack = 0 DEBUG OUTPUT: (Press [Control]-[C] to complete sequence) _________________________________________________________ Waiting for Command Received Command: B Feed the kitty! Waiting for Command Received Command: B Feed the kitty! Waiting for Command __________________________________________________________ Received [Control]-[C]! Shutting down communication!