Making noise by hitting stuff never grows old. Percussion is also the easiest instrument type to make, so of course we have an ever growing number of unpitched instruments. So far, we only have a few, but there are many more on the way.

Instrument status: Some parts are finished and working, other parts are under construction. Currently working on woodblocks and bass drum


Our first unpitched percussion instrument was the tamborine. Later we added two old and very beat up cymbals. These are mounted underneath the EnsembleBot master cabinet and are struck by simple linear solenoids. The solenoids are actuated by MOSFET drivers through the optocoupler array described in the master circuitry page.

Much can be done to improve the pretty lame sound of the cymbals, but that has a low priority right now.


We wanted a snare drum and looked around to find a small and cheap one. But then we tried mounting a shortened drum seiding onto a tiny tambourine drum, and lo and behold! It almost sounds like a snare drum!

We mounted two parallel sticks to be able to do faster drum rolls. That way we don’t have to wait for the stick to finish its movement, and the slight but inevitable asymmetric sound only enhances the organic feel to the drum rolls.

Snare dum and the solenoids

The drum is mounted on a sheet of acrylic glass in fron the tubular bells. This plate is primarily there to attenuate/reduce the volumne of the bells to balance them better with the glockenspiel.

The sticks are pulled by 12V linear solenoids, driven by the same MOSFET driver circuit as the cymbals above.

Finished bass drum installed

We have built some sort of bass drum using a cheap bodhrán and one or two modified bass drum pedals. Instead of a foot stomping on the pedal, we are going to use some kind of heavy-duty actuator.

Our first idea was to use the kind of bi-directional linear actuators used in car locks. These are very easy to control electrically and apply mechanically, and they are very strong. But they do have a serious flaw: Overheating. A bass drum is expected to be able to beat several times per second over a longer period of time, and that is a bit too much for the internal motor and gearing of a car lock actuator.

So, instead we turn to our usual fall-back plan: solenoids! The linear solenoids we’ve used so far, are not strong enough to pull a bass drum pedal, so we have to take it to the next level.

12V 45N solenoid

We use one of these, here compared to one of the solenoids used in our glockenspiel. It’s a 12 V solenoid, with a stroke force of 45 Newton (or appx. 4.5 kg) and a 15 mm stroke length. It looks and feels like it could easily tear apart the entire EnsembleBot. On paper, it should be strong enough to pull the bass drum pedal with sufficient force, and it won’t overheat. However, as it turns out, it does disappoint a bit.


To drive the power-hungry (almost 3 A) solenoids, built a new  driver module. Based on IRL540 N-channel logic level MOSFETs and an on-board reservoir of 2 x 10,000 uF capacitors to reduce the risk of brown-outs across the system, the module is basically just an array of 4 simple MOSFET driver stages.

New MOSFET solenoid driver

Even though we technically only need two drivers, we fitted the module with four in case we’re going to need more in the future. For now, we’ll use the two spare drivers for some 1W LED light effects inside the bass drum. We made a small auxillary module accepting 2 x 12V inputs directly from the MOSFET module. Each channel has three LEDs in parallel, coupled with a 100uF capacitor.

The bass drum itself is based on actual bass drum pedals, though from a childrens’ drum kit. They were cheap, okay? We’ve removed the pedals, replaced the expansion springs and replaced the mallet arms with longer wooden dowel rods.

Bass drum pedals and solenoids

The finished drum doesn’t perform quite as well as we had hoped and planned for. The solenoids just don’t pull hard enough. We haven’t made the final calibrations and integrate them in EnsembleBot, so we have yet to evaluate their performance in the ensmemble. But if it turns out they are too weak, we will have to redesign them completely, most likely by scrapping the pedal mounts.


The next percussion instruments will be a 2-sided woodblock, placed next to the snare drum. We have the woodblock and the mallets, and the mechanical design would be rather similar to the snare drum.

Another thing missing for more modern music is a ride cymbal. We have got the two quite dreadful “crash” cymbals sounding like pot lids, but we haven’t got some more simple and delicate cymbal sound.

Next: Read about the radio-controlled bell project

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