Drum Solenoid Interface for Tap-Tempo Metronome

We added an external piezoelectric element to set on the bench top, making it easier for kids to reach. We simply soldered this external piezo element to the same pads as the Tap-Tempo Metronome`s built-in piezo element. Here`s the large piezo we used. For the drum striker, we used an electromagnetic solenoid, which consists of a coil of wire wound around a movable metal core.
Drum Solenoid Interface for Tap-Tempo Metronome - schematic

When current is sent through the coil, the metal core moves a short distance. Unfortunately, the pins on a microcontroller are relatively weak, meaning that they can only supply 20-40 milliamps (mA) of current, compared to the 1 amp draw of our solenoid. This means that we can`t directly connect the microcontroller to the solenoid, as not enough current would be supplied. The usual solution to this drive strength problem is to use a transistor as a switch. Here`s a very common way to drive a large inductive load from a microcontroller. Since the solenoid requires a higher supply voltage, 12v in our case, we need to account for the extra power supply. In this circuit, we use a transistor that can easily handle the current through the solenoid (1A). We used a TIP120, which is a kind of power transistor that can be treated like a NPN bipolar junction transistor (BJT) and can handle up to 5 amps of current, more than enough for our 1A solenoid. While this circuit is very common, the diagram is based on an example diagram from the Arduino Playground (PDF). When the output pin from the microcontroller is driven high, the path between the top and bottom terminals of the transistor are connected, allowing current to flow through the solenoid. When the output pin is driven low, no current flows through the transistor, stopping the solenoid. The large electrolytic capacitor is very important, as it is used to help supply the large...

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