In the initial version (1997), the Leak used a wire mesh sensor to sense the water, connected to a BASIC Stamp 2, which was connected serially to a Macintosh 7200 running Opcode's Max. The sensor design was based on Dan Cummings' Rainbox sensor.
The next few years, I looked into various sound chips and amplifiers in hopes of eliminating the macintosh. There were two problems: first, I had about 15 minutes of audio. Second, I needed to be able to play any segment of the 15 minutes independently on any of 8 audio channels. The latter made me eventually abandon the chip solution.
The current version (2000) runs on the guts of a powerbook 540c, in a projector created in Macromedia's Director 6.5. I took everything but the motherboard and the hard drive away, and put it in the base of the machine. The only problem with this was getting the machine to work without a monitor. To solve this, I shorted one of the monitor sense pins on the external monitor port to ground. The mac therefore believes it's got a monitor, and everybody's happy.
In addition, the wire mesh sensor, which proved unreliable when it was saturated, has been replaced with a laser and phototransistor, wired directly to the serial input line. The phototransistor is normally shorted to ground, with a weak pull-up resistor to 5V. The drops of water pass between the phototransistor and laser, breaking the beam, and causing the resistor to send voltage to the serial in pin. The stamp reads this as data. Any incoming byte means water has passed the beam.
Running directly to the serial port dramatically improved the response of the computer to the sensor. On the old sensor, if a drop passed while the stamp was communicating serially to the mac, it was not sensed. In this version, that problem is no longer present.