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Magnetic Vehicle Motion Sensor
By: Dave Johnson
Many months ago one of my engineering associates (actually my software guy) showed me a neat vehicle motion detection system he designed.  To demonstrate how well the thing worked, he gave me a powerful bar magnet and told me to go out into the hallway, outside his laboratory office.  He told me to hold the magnet still while he turned on the system.  He then closed the door.  When everything was set, he told me to wave the magnet up and down.  I waited a few seconds, and then moved the magnet rapidly up and down, from my knees to over my head.  The alarm sounded.  I stopped moving the thing and the alarm turned off.  I then stepped back a few paces and tried again.  I slowly moved toward the lab office.  I was about 10 feet from the motion sensor unit when the alarm started to turn on again.  I was amazed.  My associate told me how he was using the sensor to detect the movement of metal vehicles from about 20 feet away.  The large metal mass of the vehicle distorted the earthís magnet field, which is then picked up by the sensor. He used is in his security camera system, which turned on TV cameras when his system detect vehicle motion.  I told him that I just had to have one of these.  He told me how to make one.
These magnetic sensors are available commercially if you donít want to make one.  You start out with a 12 inch long hot rolled Ĺ inch diameter steel rod.  He said that hot rolled steel works better than cold rolled rods.  You then wind 15,000 turns of 36 gage magnet wire onto the rod.  This forms a coil with an inductance of about 7 henries.  The coil is then connected to a sensitive Amplifier circuit, which boosts the voltage picked up by the coil and also filters out the higher frequencies that need to be ignored.  In my country 60Hz power line noise would certainly be picked up by the coil from nearby power lines.  The filter rejects those higher frequency signals and concentrates on signal between 0.1Hz and 10Hz.  With some additional voltage comparators, acting as alarm thresholds, the system can be turned into a nice vehicle motion detector.  The coil and the preamp circuit can be mounted inside a piece of PVC pipe, to protect it from the weather.  This sensor can be mounted on a pole or even buried in the ground.  Perhaps with some additional signal processing the system could not only detect vehicle motion but determine vehicle speed.
In coming weeks, I will walk you through the construction of this sensor and show you some actual signals produced by it.  Then, Iíll complete the project with the rest of the circuits to turn this into a motion alarm.  Since my house is next to a busy street, I should be able to capture some scope images of various cars and trucks moving by.  With some luck, maybe Iíll see a way to determine vehicle speed too.
 

Please send comments to me


December 2009     Issue 4

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