Posted on Mar 6, 2013

In machine and equipment design. some applications require measurement of both the shaft speed and the direction of rotation. Fig. 56-4 shows the circuit of a tachometer, which also indicates the direction of rotation. The flywheel has two magnets embedded in the outer rim about 45° apart. One magnet has the north pole toward the outside and the other magnet has the south pole toward the outside rim of the flywheel.

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Because of the magnet spacing, a short on pulse is produced by the TL3101 in one direction and a long on pulse in the other direction. A 0-50 JLA meter is used to monitor the flywheel speed while the LEDs indicate the direction of rotation. The direction-of-rotation circuit can be divided into three parts: TLC372 device for input conditioning and reference adjustment. Two 2N2222 transistors which apply the Vee to the two LEDs when needed. The two TIL220 LEDs which indicate clockwise (CW) or counterclockwise (CCW) direction of rotation.

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