Magnetic
Vehicle Motion Sensor
By: Dave Johnson |
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| 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 earth’s 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 don’t want to make one. You start out with a
12 inch long hot rolled ½ 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, I’ll 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 I’ll see a way to determine
vehicle speed too. |
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