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Build Diaries / Re: Belly Tank Build Diary
« Last post by Mike Brown on Today at 07:22:41 AM »... I really like Hall Effect sensors...I am using a reverse biased gear tooth sensor...Gear tooth sensors generally have only two wires. Hall effect transistors require three wires (as do all transistors). And Hall effect transistors respond to a magnetic field, not to ferrous metal. So- how do these "gear tooth sensors" function?
Jack, I believe the two wire gear tooth sensors that you are referring to are simply coils of wire called Variable Reluctance sensors. The target is magnetic and as it moves past the sensor an AC signal is created. They are very rugged and the least expensive of the sensors. There are problems with VR signals as I see them. The signal is a slowly rising and falling AC output so the input typically has a comparator to help shape the signal. The VR output is also a linear AC alternator, the faster the target moves the larger the signal and conversely no movement equals no signal. The sensors that I like use Hall Effect elements that do detect a magnetic field however in this particular sensor there is a magnet opposite the actual sensing element so as a ferrous target passes by the sensing element it detects the change in the magnetic field. It is three wire as it requires 5-24Vdc power for operation. The output is open collector in that it acts like a switch contact to the negative lead. The output requires a "pull up" resistor. In my case I use a 2.2k Ohm resistor to +12V. This gives me a 0V output when the target is detected and +12V with no target present. The output is an almost perfect square wave and works with no movement. This is helpful when setting up. I use a LED on the output that will illuminate when the target is in range. It is a good idea to use an oscilloscope to set up a VR sensor in a new application. I hope that this helps.
