The Parallax RFID reader is inexpensive and easy to hook up to an Arduino or Wiring board. It has four connectors which can be plugged right into a bread board and there is code for it available at the Arduino Playground. One thing that I really like about the reader is its small footprint. It is about the size of a playing card and just 2-7mm thick. The reading distance is dependent on the tags that are used, I got maximum distance of about six centimeters on the axis that is orthogonal to the reader and 2-3 cm on the parallel axis. A short reading distance can be a good or a bad thing depending on the application. If you like me want to identify a shoe with an embedded RFID tag when the wearer enters an area, then it’s a not so good thing. If you know of another inexpensive and easy to interface reader that has a greater reading distance I’d be very interested in hearing about it…

Update: I have posted some Arduino code that improves stability and fixes a couple of errors in the Arduino Playground code for the Parallax RFID reader.

Mechanical rotary shaft encoders such as the one pictured to the left are probably the cheapest way to measure rotation. This particular encoder, which costs about 2 euro, has 24 steps per revolution. The resolution is a bit of the low side for many applications but it can be increased with gears or wheels. Shaft encoders only measure relative rotation. In other words you can’t know at which position the sensor is when you first read it, but you can keep track of how much and in which direction it has turned since you started to read it.

Rotary encoders are not only cheap, they are also simple to hook up to Arduino or Wiring boards. To put it short you monitor a pin on the encoder and when it changes state you check the state of another pin to determine in which direction the encoder was turned. Paul Badger has posted some nice Arduino code for rotary encoders on the Arduino playground.

Alternatives to mechanical encoders are optical rotary encoders and 360 degree potentiometers. These encoders often have significantly higher resolution than the mechanical ones and some (such as potentiometers) can be used to measure absolute rotation. The downside is that they are more expensive (usually around 50 euro or more). A budget alternative is to go trash hunting for an old printer with a motor that has an optical encoder that you can reuse (check out this Make video). Not all printers have these motors though, I’ve trashed three so far without any luck…