distant alarm

[farshad_momtaz]

hi,

i want to make a 2 electronic boards that if 2 peaces get about 20 feet apart one of them do something(for example turn on a LED or start an alarm)
i was wondering where should i begin? and is there any electronic board design out there that can do that?

thank you

 

[Tsalagi]

Hi,

here you can get an idea:

 

[Tsalagi]

http://www.electronic-circuits-diagrams.com/alarmsimages/alarmsckt12.shtml