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project: traffic loop car park barrier

andychallis

Jul 4, 2011
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Jul 4, 2011
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hi, im new to this and electronics so sorry if i get the wrong page, i am a college student and i study physics maths further maths and chemistry (ie not electronics) but i am doing a challenge for the silver crest award and our project is to make a car park barrier lift when a car distorts the electromagnetic field.....

anywho we have made the breadboard up to the point of where we need the motor to change but we have a speaker set in at the moment and as the magnetic field changes the frequancy changes.... so i was wondering how we make the arm on the barrier lift when the frequancy changes, something like i would call a frequancy dependant resistor? :s hope this is enough info please help! we are a team of 4 and are stuck!

Thanks Andy
 

duke37

Jan 9, 2011
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You could use a high pass filter (or low pass filter) so that when the frequency gets high enough (or low enough) then it passes an output. This could be rectified and used to power a relay or fet to switch the motor.
There are also frequency to voltage chips which could be used (LM2971N), this would be the simplest solution.
 

davenn

Moderator
Sep 5, 2009
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Hi Andy
welcome :)

the usual system used, and our company use it for all their vehicle detection loops, is you have a loop of cable in the ground. This loop is the inductor in an oscillator circuit that is located nearby. When the metallic body of the vehicle comes over the loop it changes the inductance of the loop and therefore the oscillator freq.
This change in freq is detected and triggers relays etc to activate the other barrier circuitry

cheers
Dave
 
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