Need help building a motion detector

[Jassi]

My group for a design class is building a completely automated draw-bridge but we need to build a motion detector to sense vehicle activity. Anyone ever built one that can help us out? RadioShack has them for a lot more than our budget.

A simple light sensor that gets tripped and detects when the light coming to it gets tripped would be the ideal solution. We couldnt find a place to buy it so we are stuck building it.

Any help will be greatly appreciated.

//Jassi

Edit : spelling error (I am an engineer in training... )

 

[jagec]

photodiode, LED?
Use a coil of wire under the roadway, like they do with some stoplights?

 

[DaMenace]

laser LED and photediode would be my guess too

there are proximity sensors that vary their voltage with distance.....

 

[PottedMeat]

how much is your budget?
here are some options:

1. for 12.95 you can get a infrared intrusion sensor (range 30 ft) - the kind they use in stores to trigger chimes or counters from BGMICRO.com - its already got a normally open/normally closed output
BGMICRO Infrared Intrusion Sensor

2. ultrasonic sensor maybe - for 10 bucks senses objects in 5-7 ft
Bgmicro Ultrasonic link

3. A capacitive sensor? two strips of conductive material spaced out by a few mm, when an object passes over it, the capacitance changes - you could do the triggers in software

4. like above a laser LED and a sensor - one of those cheap laser pointers and a sensor, there maybe a way to do this really cheaply with a commonly available sensor + filter

 

[Jassi]

SWEET!!!!!!

Thanks a lot PottedMeat, I ordered 2 of each

Thanks