Step 1: Datalogging for dummies
This example measures temperature, but can easily be changed to measure things like light, volts, current, speed, rotation, acceleration, soil moisture or any number of other variables. We are using the picaxe as a cheap way of interfacing multiple analog and digital inputs to an RS232 port, though these chips can do a lot more than that - see http://www.instructables.com/id/Control-real-world-devices-with-your-PC/
You will need:
1 picaxe chip 08M ($3)
Components - resistors, capacitors, temperature sensors, regulator, wires.
Free copies of the picaxe programming software and VB.Net
The picaxe 08M chip available from many sources including Rev Ed http://www.rev-ed.co.uk/picaxe/ (UK), PH Anderson http://www.phanderson.com/ (USA) and Microzed http://www.microzed.com.au/ (Australia)
The RS232 plug is a D9 female plug. If you are doing a lot of experiments and don't want to keep reaching behind the computer, you can make an "extension lead" using male and female IDC 9 pin plugs and a metre or so of ribbon cable.
The picaxe has been configured in a slightly unusual download circuit which doubles as both a programming circuit and a communications circuit. Flick the switch to go between programming and communications (or just short leg 2 to ground on a protoboard to run in communication mode).
The LM35 devices output millivolts equal to degrees in centigrade. Looking at the writing on the case, the pins are numbered 1,2,3.