The perfect defense

[r_w]

Built an array of cheap passive infrared sensors (like used in motion-sensor lights).  Number of sensors needed depended on the accuracy and range of motion wanted.  Controller would find largest heat signature by comparing sensors and move the servos to match.  Now that you jog my memory, the $100 pricetag included some serious student discounts and stupid good deals on surplus items.

Later projects were in signal processing to make target acquisition from standard video--needed supercomputers to do in real time then but now is built into most digital cameras.  Once you found the target the ranging and lead were easy. 

[Solus]

Hmm...wonder if the sensors have enough "resolution" to have the array run both horizontal and vertical, figuring center of mass would be the "hottest" sensor in the vertical plane. 

Not only would it track horizontal movement, it would also compensate for squatting, crawling or laying down.

[fightingquaker13]

Hmm...wonder if the sensors have enough "resolution" to have the array run both horizontal and vertical, figuring center of mass would be the "hottest" sensor in the vertical plane. 

Not only would it track horizontal movement, it would also compensate for squatting, crawling or laying down.

Just hook it up to a Saiga 12 gauge with a 20 round mag loaded with #4 buck. The elevation problem will take care of itself.
FQ13

[Solus]

Just hook it up to a Saiga 12 gauge with a 20 round mag loaded with #4 buck. The elevation problem will take care of itself.
FQ13

Well, sure, if you want to settle for the easy solution 

[tombogan03884]

Even if it was as much as a $1000, it's still pretty darn cool

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