Beat Frequency Indicator

This circuit uses LEDs to display the beat frequency of two-tone oscillators. Only one LED is on at a time, and the apparent rotation of the dot is an exact indication of the best frequency. When f1 is greater than f2, a dot of light rotates clockwise; when fl is less than f2, the dot rotates counterclockwise; and when f1 equals f2, there is no rotation.

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Beat Frequency Indicator - Circuitos de Electronica