There are three sensors; a pair of Sharp GP2D12 IR (infrared) distance sensors and a custom IR circuit. The GP2D12's will detect objects in the range of 10 cm to 80 cm. Unfortunately, objects in the range 0 thru 10 cm return the same values as objects in the 10 to 80 cm range. The close up IR sensor can detect objects in the 0 to 20 cm range. Bigger...

This is the circuit board for the close IR circuit. The IR LED (lower right) is connected to one of the PWM outputs on the PIC. The PWM output is setup to generate a 38 kHz square wave. The Panasonic 4602 IR demodulator (black sqaure in the middle) detects the bounced IR signal. This allows Marauder to detect when another robot is immediately in front of it, and hence ignore the line sensors. This is particularly useful if the other robot has a white or shiny scoop and gets under Marauder's scoop. Marauder doesn't want to detect the opponent's scoop as the white line at the edge of the ring. Bigger...

This picture shows the back of the circuit board. It really isn't that difficult to create custom circuits, as long as they aren't too complicated. Bigger...

This is the schematic for the front sensor. It's also available in Eagle format. The green LED is in series with the IR LED, and this gives positive feedback of when the IR sensor is lit up. The red LED is connected to the output and gives an indication of when an opponent is detected. Bigger...

This shows one of the GP2D12 sensors. The case is made of a conductive plastic and I read somewhere that grounding the case was a useful thing to do. So I drilled and tapped a #2 screw in the side and connected a wire from ground to the screw. Bigger...

From the back you can see the wire from ground (the middle wire from the connector) to the screw. When I first did this, I didn't make the screw short enough, and it shorted against the trace closest to the screw. You can see a little shiny spot where this happened. Needless to say, the sensor didn't work when this happened. Bigger...