In 2019 [Simen] and [Amud], two students from the University of Oslo, set out to design a unique open source display. The result was Fetch, a screen that uses electromagnets to suspend the ferrofluid on 252 “pixels” across the screen. After some delays due to COVID, they recently unveiled version 2.0 of the display on their project page.
While the duo managed to overcome the mechanical challenges associated with using ferrofluids fairly easily, they were quickly bottlenecked by their electronics. The use of electromagnets supporting a liquid presented a unique challenge; the magnets could not be turned off even for a millisecond, otherwise the “pixel” would fall to the bottom of the screen. This immediately ruled out any sort of multiplexing and meant that everything had to be driven in parallel. As if that weren’t hard enough to get around, the effect of having multiple electromagnets activated next to each other would alter the way the ferrofluid flows. This meant that the strength of each electromagnet would have to be adjusted based on what is currently displayed, rather than just being turned on or off.
All of this, coupled with other overheads like generating pulse width modulation for the inputs, was just too much for a single microcontroller to handle. So the pair set out to design a better version of their electronics that would offload a lot of the hard work. At the same time, they decided that a bit of mechanical optimization was in order; they redesigned the boards to be longer and thinner, allowing them to fit snugly behind the row of electromagnets they controlled.
The new boards feature a PCA9685 integrated circuit, which can control up to 16 channels of 12-bit PWM on i2C, perfect for the screen size. Since this integrated circuit cannot supply enough current to drive the electromagnets, it has been paired with a Darlington ULN2803 transistor array, capable of delivering up to 500mA to each electromagnet.
With prototypes in hand (and a few bodge wires here and there), [Simen] and [Amud] the new driver cards performed wonderfully, displaying text in a fascinating way that no ordinary display could match. Watch the video after the break for a demonstration of the new controllers in action, as well as a deeper dive into the controller development process.
You want to know more ? Check out our previous article on Fetch! Or if you’re looking for another cool way to use ferrofluids, how about making it dance in a custom speaker!
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