Over at Wayne and Layne our good friends, well, Wayne and Layne (really Adam and Matt, who we recently profiled) have published a build guide for upping the oomph of their excellent Tactile Metronome kit.

If you stopped by our table at the Maker Faire, you got to play around with our big drum demo of the Tactile Metronome. Lots of people had questions about how we drove the solenoid for the drum striker, so we decided to write it up once we got back home.
For the drum striker, we used an electromagnetic solenoid, which consists of a coil of wire wound around a movable metal core. 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.


Drum solenoid interface for Tactile Metronome

In the Maker Shed

Wayne and Layne make the excellent, open source Tactile Metronome kit available in the Maker Shed.