2DOF Bicore Head
click the imge for a quicktime of the head following a light source.

here are close-ups of the mechanics:
the back motor swings a long arm, which pushes a connecting rod up and down to move the head up and down. Then vertical motor pivots the entire mechanism from side to side. If the power source is attached to the head, it can pivot a full 360 degrees.

The circuit consists of two independent bicores freeformed onto one 74HC240. The timing resistor of the bicores is 2M with 3.3k resistors as buffers between the PDs and the bicores. This prevents the bicores from locking up. Here are some details of the freeformed circuit:

The photodiodes are angled out from each other at about 90 degrees to allow for some overlap as well as giving some peripheral vision.

A better alternative would be to use two independent Power Smart Head circuits to control the mechanics. This way the robot could be run for extended periods of time without worrying too much about draining the batteries.