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Do-It-Yourself Motorcycle Alarm For Less Than $40 by David Vance
basically you use the wireless doorbell. Heath Zenith Wireless Plug-In Entrance Chime (SL-6168-A). you position it so that the magnet will separate when the bike is disturb. not for all situation of course. i can see it might work for me.
from article in Motorcycle Consumernews.