At 03:41 PM 4/8/2013, James Felder wrote: >What's the difference between the Hall Effect sensor in a computer >keyboard and the one in a Vanagon distributor? I don't think keyboards typically use Hall-effect devices as that would be a very expensive way to do it, but in principle, no difference. In the distributor the Hall-effect sensor and a magnet are mounted on a common base with a small gap between them. The distributor shaft carries a steel wheel like a jar lid with equally spaced tabs and openings in the vertical portion, one tab and opening per cylinder (so four each for us). These pass through the gap in the Hall generator assembly. When the gap is open, the sensor "sees" the magnet and generates a tiny voltage which closes a transistor switch path to ground. When the gap is closed by a tab, the sensor no longer generates a voltage and the switch opens. Because the spacing is equal, the result is a square wave. On the Digijet system the distributor has centrifugal and vacuum advance/retard mechanisms, and the Hall generator output goes first to the idle stabilizer module which advances timing if the idle drops too low. Otherwise it passes the pulse straight through to the ignition module on the firewall which generates the ignition primary pulse that drives the coil. The distributor output also feeds rpm and rough timing info to the ECU so it can compute and time the injection pulses. On the Digifant system the distributor simply reports rpm and TDC to the ECU. From these and other inputs the ECU computes ignition advance or retard and outputs a primary ignition impulse at the correct time, as well as controlling the injection timing and quantity. Also on the Digifant system idle stabilization is accomplished by a much fancier system that operates a tiny auxiliary throttle (the Idle Stabilizer Valve) instead of fiddling with timing. Because this system depends heavily on the throttle position switch, the Digifant engines will not idle down properly if the switch does not close. A Hall-effect keyboard would probably have tiny magnets mounted on the keys, with the sensors and amplifier circuitry beneath them, one set per key. It could be used to make a keyboard where the electronics were totally sealed up, but it would be sensitive to nearby magnets (and expensive). Yours, David |
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