Assembling circuit.

Bernard

Joined Aug 7, 2008
5,480
A passive infra red. PIR, sensor module might be a way to go, from scratch it's a little harry. I've used the Radio Shack one.
 

BSomer

Joined Dec 28, 2011
434
How large of an area do you want to monitor? PIR would be better for larger areas. If you just want to know when something passes by a certain point, you could go with something as simple as a photocell and laser/LED. A little more information on what you are trying to achieve/monitor would help.
 
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