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Spark detector utilises proximity

04 Feb 2016  | Robert Most

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Hall-effect ICs are employed as proximity sensors in applications such as proximity detection and angular-velocity measurement on rotating machinery. Hall-effect devices can detect mechanical motion without mechanical contact. This noninvasive detection is due to the magnetic nature of the Hall effect. A current flowing through a semiconductor in the Y direction produces a negligible potential difference in the X direction (figure 1). In the presence of a magnetic field at a right angle to the current flow, the Z direction, a displacement voltage appears across the semiconductor in the X direction. This effect is the Hall voltage, VH.

Figure 1: A current flowing through a semiconductor in the Y direction produces a negligible potential difference in the X direction.

Hall-effect ICs detect, signal-condition, and add hysteresis to the electrical displacement. In essence, the devices measure the electric field, which the magnetic field causes, across the semiconductor in the X direction. Therefore, if you subject the semiconductor to an electric field of sufficient magnitude in the X direction, the Hall-effect device would detect the electric field, as well.

Internal-combustion-engine designs require precise control of spark timing. The microcontroller that controls engine parameters not only changes the spark relation relative to the piston position, but also, in more advanced engines, requires feedback for variable valve timing. In addition, diagnostic aids and engine-troubleshooting hardware can benefit from an easy way to measure spark timing using this novel approach. Even the most basic carburetor adjustments on a lawnmower require a method to measure an engine's revolutions per minute. Four-stroke small engines create a spark on every engine revolution. Therefore, the detection of this spark is a direct indication of engine revolutions per minute.

By simply placing the Hall-effect IC against the spark-plug wire using the correct orientation, you can detect a spark-plug pulse using its electric field. Simply attach the device with electrical tape to the spark-plug wire's insulation. Because the Hall-effect IC incorporates internal signal conditioning and hysteresis, no additional components are necessary to read a basic frequency from the device, unlike with the traditional current-transformer method.

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