This is a cool little circuit that i made for a school project. It can be used as one of those things that is usually seen on the door of small shops so that when someone walks in it goes "ding dong". It could also be used as a burglar alarm (just need to get a loud enuf siren).
A light has to be pointed at the LDR. It can be anything, just as long as its bright enough. Im pretty sure that an infrared LED would work, a regular light, or my personal favorite: a laser pointer, just like in the movies 8) . You could encircle your house w/ a laser so wen someone crosses the beam you know.
R1 can be adjusted for sensitivity of the circuit to light. The lower R1, the more sensitive, and vica versa. The value of R1 will not allow the circuit to work in direct or indirect sunlight. R1 would have to be increased to make the circuit less sensitive to light (even then i doubt it would work in direct sunlight). An alternative to this is putting a tube on the LDR that is a couple centimeters long so that only light from the light source could get in.
R4 can be adjusted for how long the buzzer stays on. 100k makes it stay on for about 9 seconds. Or the value of C1 could be adjusted.
Someday i plan on putting a transmitter on the circuit so that it could be put out in the yard, and a receiver inside would go off. HAVE FUN :lol:
Laser or IR beam alarm system
This is a cool little circuit that i made for a school project. It can be used as one of those thin