The HiHat machine is part of the Drum Robot MR-808. It took some experiments until I figured out how this can be crafted into a high-performance working instrument: A HiHat is normally used in any track and plays most of the time!
This robotic HiHat is based on a normal drummers hihat, with two cymbals and parts of the hihat-stand. It also contains two solenoids (electro magnets), one for opening and closing the cymbals, on for the actual beating of the cymbal.
The force needed to open the HiHat is quite high, as it has to work against the force that closes the HiHat. The solenoid used here comes from a car engine starter. They have the most powerful solenoids, are reasonably cheap, easy to get at any car junk jard (or eBay) and operate at 12V. Perfect for low budget robotics!
The disadvantage is, that they consume roughly between 30 and 50A. Un-technically spoken: This is very much! I use server power supplies which can supply a current up tp 50A@ 12V.
I started with a standard HiHat machine (the full hardware Hihat Stand) and cut the lower part off. One thing you have to consider: a HiHat is default-OPEN, meaning that the spring inside keeps the cymbals separated when no pressure is applied. Only if the drummer steps on the foot-part, the HiHat closes. Replacing the drummers-foot with a solenoid (yeeha!) would mean that you would have a constantly powered solenoid, in this case about 35A, for most of the time (closed HiHat).
As we don’t want to fry eggs with the robot (solenoids can get quite hot) its recommended to reverse this mechanics, meaning that the HiHat is default-closed, and the solenoid is only powered when you want an open HiHat. I then removed the spring and let the gravity do the closing of the cymbals. Afterward I attached a fan inside the HiHat machine as the solenoid gets really hot for off-beat disco music. The huge car solenoid is only for the opening and closing of the hihat.
The actually beater is another solenoid which just hammers directly on the lower cymbal. It is used for both closed and open HiHat.