http://www.uoguelph.ca/~antoon/circ/uspy3.htm
http://www.uoguelph.ca/~antoon/circ/uspy2.htm
detects when someone traverses the gate, ringing the bell in a very peculiar way.
http://www.nutchip.com/progetti/annunciatore_en.htm
http://www.uoguelph.ca/~antoon/circ/uspy.htm
Another beacon
http://www.uoguelph.ca/~antoon/gadgets/locator.htm
opens the lock when you press the switches in correct sequence.
http://home.maine.rr.com/randylinscott/dec99.htm
R/C Plane lost? Try this beacon.
http://www.uoguelph.ca/~antoon/gadgets/adl.htm
great for situations where a membrane type mechanical switch is not desired.
http://www.imagineeringezine.com/PDF-FILES/fingersw.pdf