Full-wave Rectifier
The half-wave rectifier chopped off half our signal. A full-wave rectifier does more clever trick: it flips the - half of the signal up into the + range. When used in a power supply, the full-wave rectifier allows us to convert almost all the incoming AC power to DC. The full-wave rectifier is also the heart of the circuitry that allows sensors to attach to the RCX in either polarity.
A full-wave rectifier uses a diode bridge, made of four diodes, like this:
At first, this may look just as confusing as the one-way streets of Boston. The thing to realize is that the diodes work in pairs. As the voltage of the signal flips back and forth, the diodes shepard the current to always flow in the same direction for the output.
Here's what the circuit looks like to the signal as it alternates:
So, if we feed our AC signal into a full wave rectifier, we'll see both halves of the wave above 0 Volts. Since the signal passes through two diodes, the voltage out will be lower by two diode drops, or 1.2 Volts.
AC Wave In: 
AC Wave Out (Full-Wave Rectified):
If we're interested in using the full-wave rectifier as a DC power supply, we'll add a smoothing capacitor to the output of the diode bridge.