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Comment by seabass-labrax | Hacker News Reader

I absolutely love that this is open source hardware; there have been multiple circuits available on the web for this kind of peripheral, but nothing even approaching the level of polish that this has.

If the designer happens to be here, I have a question regarding the circuit. Is there any protection against voltage produced by a user's rotation of the knob? The TMC6300 IC that the schematic shows is directly connected to the BDCM without any intermediate circuitry, but the only 'protections' listed in its datasheet are related to failures related to the motor being used as a motor, not as a generator.

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Designer here - I will admit that I just do this as a bit of a hobby and I'm not an EE, so I don't have a great answer for you, but generally speaking since this is a high resistance/low KV motor and there's only so fast you could spin it by hand realistically, I think the voltages you could generate that way shouldn't be so high as to cause damage in most cases. The TMC6300 datasheet does show it directly connected to the BLDC motor so that's what I based this design on.

There are probably some best practices that I'm missing though (diode on the main supply maybe?), so if anyone knowledgeable wants to chime in, I'm all ears! (Or I suppose pull requests are welcome too).

Thanks for the swift response! I've been interested in haptic feedback for a long time, but so far have not come across any general way to calculate the acceptable power dissipation for a given motor and driver circuit. Physical controls have so much to offer in ease-of-use, but the chief downside of them currently is that there is no easy 'two-way binding'.

Motors often run at high rpm and then shut off, coasting to a stop. I would expect any standard bldc controller circuit to be able to handle any potential generated by manual movements without difficulty.

Many ESCs these days have a brake function, where they short-circuit two of the phases together, stopping the motor abruptly, too.

The UX in your project is fantastic, well done. The motor kicks to simulate clicks, for example, are a really nice touch. I still don't get how the PCB detects flexing, I'll just put it down to magic.

MechE here. The way strain gauges work is with a long conductive trace that loops back on itself like a snake, or a radiator. This creates a conductive line that passes back and forth many times along the direction you want to measure strain in.

When the surface that the gauge is attached to flexes, the PCB itself actually stretches and shrinks by a very small amount, on the order of a thousandth of a percent. This causes the long conductive trace to get slightly longer, which measurably increases its resistance.

Oh wow, I had no idea about this effect, thanks!

Interactions like these are the reason why HN is so worth reading!

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