Tach Pulse Multiplier

A car tachometer is a device, which produces a signal conveying the engine speed. This information then feeds the engine management system, which drives the instrumentation to display the speed in rpm.

The signal from the tachometer, known as "tach" signal, consists of a series of pulses where each pulse represents a fraction of rotation of the main engine shaft. A Hall sensor usually generates this signal by detecting the change in magnetic field as the shaft rotates.

An automotive firm contacted me to provide a solution to a unique problem that occurs when an engine is changed. In a custom build, they often install a more powerful engine; however, the engine management system may not recognise the tach signal from the new engine, therefore causing an incompatibility issue. If the Hall sensor was sending out four pulses per rotation of the engine shaft, and the engine management expected 64 pulses per rotation, how would one solve such a problem?

Solution

One solution is to convert the tachometer signal, by multiplying the pulses by a factor of 16, which would produce a compatible 64 pulses per rotation. To multiply the pulses by a factor of 16, one possible solution is to use an LM565 phase lock loop IC with a divide-by-sixteen counter in the feedback loop.

If you are not a digital electronics engineer, you might find this confusing; however, a divide-by-sixteen counter will multiply the input pulses by 16 when it is within the feedback loop of the PLL chip.

Block Diagram of Solution

The block diagram in the header section shows a frequency divide-by-sixteen counter inserted between the output of the voltage-controlled oscillator (VCO) and the phase comparator (PC). It forms a feedback loop to the comparator. Since the feedback is divided by 16, the phase comparator compensates by multiplying the input by 16.

This IC uses a closed loop feedback system, where the output feeds back to the input to determine if it is correct. The internal phase comparator performs the comparison.

When the phase comparator detects that the output is 16-times lower than expected, it compensates by driving the VCO 16-times faster, therefore the output frequency will be multiplied 16 fold.

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