An LED Bargraph Optical TachometerAn LED Bargraph Optical TachometerEach LED in the bargraph indicates an interval of this size. In practice (and with practice), the resolution is higher. When an RPM reading is taken that falls between two of the LEDs, both LEDs will flicker, and you can interpolate between the two values by the relative amount of time that each LED is on.

An LED Bargraph Optical Tachometer
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An LED Bargraph Optical Tachometer