Circuit emulates mechanical metronome

Jim Kocsis, Allied Aerospace, South Bend, IN -- 8/19/1999

The circuit in Figure 1 produces timing signals with a sound like that of a mechanical metronome. IC₁ is a 555 timer that oscillates at approximately 3200 Hz. The two 3-kW resistors and the 0.047-µF capacitor set the frequency. IC₂ divides the frequency of IC₁'s output by 2. IC₂ produces a square wave with an exact 50% duty cycle. The frequency of the output of IC₂ determines the sound of each beat. A higher frequency yields a sharper sound, like beating on a small drum; a lower frequency produces a deeper sound, like beating on a large drum. You can adjust the sound by changing the values of the 3-kW resistors or the 0.047-µF capacitor. The square wave from IC₂ is always present on Pin 1 of IC₅. IC₃ is a low-frequency oscillator that runs from approximately 0.5 to 3 Hz. This oscillator determines the rate or speed of the beat. The two 100-kW resistors, the 1-Mµ potentiometer/resistor pair, and the 1-µF capacitor determine the oscillator frequency.

For every rising edge from IC₃, the IC₄ one-shot produces one low-to-high-to-low output pulse. The 30-kW resistor and the 0.15-µF capacitor determine the length of IC₄'s output pulse. This pulse is approximately 2 msec long with the values shown. The output from IC₄ allows three to four pulses from IC₂ to pass through to the output of IC₅. When IC₅'s Pin 2 is high, the pulses from IC₂ appear at IC₅'s Pin 3; when Pin 2 is low, no pulses appear at Pin 3. From IC₅'s Pin 3, the series of pulses routes to the volume control and then to the power audio amplifier. All the circuitry except IC₆ uses a 5V supply. IC₆ needs a 12V supply to drive the speaker loud enough to hear over the sound of an instrument. If you want a visual indication of the beat, you can connect an npn transistor, with a 470W series base resistor, to IC₄'s Pin 6. An LED in series with a 470W resistor from the collector to 5V produces the visual indication. (DI #2404).

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