Spark detector uses proximity

-November 27, 2008

Hall-effect ICs find use 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.

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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Design Ideas

The circuit in Figure 2 converts the pulses from the Hall-effect IC into a dc voltage that the most basic voltmeter can read. The Hall-effect IC provides an open-collector output. You need only a pullup resistor. The sensor converts the series of generated pulses, which the LM2917 frequency-to-voltage converter from National Semiconductor converts to a voltage. The selection of C1 and R1 scales the output voltage in relation to the range of frequencies that the charge-pump section of this device will encounter. In the case of a four-stroke, single-cylinder engine, a range to 5000 rpm is more than sufficient.

The circuit provides an output voltage as high as 5V and requires a battery-supply voltage of 9V. Operation is straightforward: By pressing the Hall-effect IC against the spark-plug wire, the voltage on the DVM (digital voltmeter) can readily interpret the revolutions per minute. Because the measurement is noninvasive, this method can easily perform repeated measurements or analysis of multicylinder engines. Measurement of automobile engines differs slightly. Automobile engines have mechanical distributors that spark on every other engine revolution. Ignition systems without distributors and with one ignition coil per cylinder also spark on every other engine revolution.

Because there is no electrical contact with the ignition system, this circuit intrinsically provides isolation from the high voltage. Interfacing to microprocessors and microcontrollers thus becomes a matter of compatible logic levels. The Hall-effect IC’s power-supply voltage is 4.5 to 24V dc, which enables it to work with standard 5V processors as well as automotive voltages. You can interface multiple sensors to provide ignition diagnosis and timing analysis in automotive applications.

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