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Given in the title, I'm looking to design a small embedded system that only turns on when the part it's inside is spinning.

The electronics is completely enclosed.

I've done some googling for rotary switch, or gyroscope solenoid but to no avail. Do these things exist?

I'm looking for some kind of relay to make a battery connection.

Needs to be miniature i.e. MEMs sized.

At a guess, maybe 100RPM turn on speed, max 24,000RPM.

Thanks in advance

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    \$\begingroup\$ Is it OK if the system uses a couple µA while off? (don't say "no" without considering whether that's below your battery's self-discharge current; do say "no" if it is a problem.) Also, what are size, vibration hardness, power usage, switching speed, and current ability restrictions? \$\endgroup\$ Oct 21, 2019 at 18:50
  • \$\begingroup\$ I'd say that'd be OK I'm not looking for crazy battery life..Are you suggesting I use a standard MEMs gyroscope with some kind of comparator? \$\endgroup\$
    – foldone
    Oct 21, 2019 at 18:53
  • \$\begingroup\$ Try googling "governor electrical". Most switch off while spinning, but I imagine its action could be reversed. \$\endgroup\$
    – glen_geek
    Oct 21, 2019 at 18:54
  • \$\begingroup\$ Why a gyroscope? All you need is a cheap accelerometer \$\endgroup\$ Oct 21, 2019 at 18:54
  • \$\begingroup\$ Gravity would affect an accel, when the product is not spinning it can be stored in any number of orientations.. Though thinking about it obviously even 100RPM might produce higher Gs than 1.. need to do some calcs \$\endgroup\$
    – foldone
    Oct 21, 2019 at 18:56

6 Answers 6

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If I liked mechanics, I'd just have a weight attached to some spring, mounted so that the centrifugal force will pull it radially and hit a tactile switch.

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  • \$\begingroup\$ Needs to be miniature I'm afraid, MEMs type size. Not something I can do in the workshop! \$\endgroup\$
    – foldone
    Oct 21, 2019 at 18:54
  • \$\begingroup\$ mention that in your question by editing your question. \$\endgroup\$ Oct 21, 2019 at 18:56
  • \$\begingroup\$ What's MEMS size? I get a ton of garbage when I Google that. \$\endgroup\$
    – user233390
    Oct 22, 2019 at 1:53
  • \$\begingroup\$ @user233390 MEMs size, in my opinion, is SMD chip size, around 5mmx5mmx3mm ish maximum \$\endgroup\$
    – foldone
    Oct 22, 2019 at 11:43
  • \$\begingroup\$ Oh, OK. I was thinking there was some set standard that was MEMS. Can I ask what that stands for? It sounds like a fun project whatever you are doing. \$\endgroup\$
    – user233390
    Oct 22, 2019 at 14:11
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I'd go with something with no moving parts if you can. Maybe an optical reflector, with a reflective surface mounted on the spinning part. You can use a retriggerable one-shot to keep the output high until the spinning stops, and you can use the output to drive whatever type of relay/switch/FET you need.

One thing to watch out for is if it stops with the mirror on top on the optoreflector. The one-shot would need to be edge triggered.

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  • \$\begingroup\$ The electronics is going inside the spinning part. Good idea if it wasn't though! \$\endgroup\$
    – foldone
    Oct 21, 2019 at 18:57
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    \$\begingroup\$ works the other way, too. Shine a laser from the inside to the outside. \$\endgroup\$ Oct 21, 2019 at 18:58
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Instead of a gyroscope, just get an accelerometer and mount (one of) its sensing axes radially – being spun causes a centrifugal force, which is measurably an outwards acceleration.

Many accelerometer ICs come with an interrupt pin that you can use to wake up a microcontroller.

Gravity is not really a problem – if you can mount your accelerometer at a radius sufficiently sized that the centrifugal force outshines gravity.

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  • \$\begingroup\$ Yeah, decent idea, cheap and easy to integrate. Just need to make sure 1g (gravity) is smaller (probably FoS of 2x) than the min acceleration caused by rotation \$\endgroup\$
    – foldone
    Oct 21, 2019 at 18:58
  • \$\begingroup\$ that's just a problem of mounting the sensor close to the axis of rotation \$\endgroup\$ Oct 21, 2019 at 19:01
  • \$\begingroup\$ absolutely, best idea yet \$\endgroup\$
    – foldone
    Oct 21, 2019 at 19:03
  • \$\begingroup\$ @foldone And that 24,000RPM doesn't go over the maximum rating. \$\endgroup\$
    – Jeroen3
    Oct 21, 2019 at 19:58
  • \$\begingroup\$ @MarcusMüller - actually, you get more centripetal acceleration for a given rate of angular rotation by being far from the axis, not close to it. On a given rotating body, the centripetal component will overcome the gravitational one at a lower rate of rotation the further out you manage to mount the accelerometer. \$\endgroup\$ Oct 22, 2019 at 2:11
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You are looking for a centrifugal switch.

A centrifugal switch is an electric switch that operates using the centrifugal force created from a rotating shaft, most commonly that of an electric motor or gasoline engine. The switch is designed to activate or de-activate as a function of the rotational speed of the shaft. Wikipedia

Just go to Amazon and have at it using that terminology. Looks like they are pretty reasonably priced. $9.00!

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  • \$\begingroup\$ That's exactly what I need, except much smaller! Will have a look around \$\endgroup\$
    – foldone
    Oct 21, 2019 at 22:25
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Another solution is to use a microcontroller's sleep mode. It is a cool feature that even turns off the system oscillator. You would have a permanent magnet fixed on the outer casing and then when the circuit board spins, the inductor will sense a change of magnetic flux and create a voltage that gets sensed by the wakeup pin of the microcontroller. It would not turn on unless the pcb is spinning and even if the pcb were stopped exactly on the permanent magnet, it still wouldn't turn on because there is no change in flux. Basically a one-coil motor and the output voltage from the coil turns on the MCU. No mechanical switches or springs. Super low energy consumption when not in use. Very compact. You'd have to size the inductor and magnet to generate the proper turn-on signal, but that's easy to do. You'd probably use a 3.3V MCU so the inductor/permanent magnet would be as small as possible.

Here us a quote I found: PIC microcontrollers’ Sleep feature is an extremely useful mechanism to minimize power consumption in battery-powered applications. In Sleep mode, the normal operation of a PIC microcontroller is suspended and the clock oscillator is switched off. The power consumption is lowest in this state. The device can be woken up by an external reset, a watch-dog timer reset, an interrupt on INT0 pin, or port-on-change interrupt.

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Vibration Sensor will make when the external spring touches the internal rod. If this was placed parallel to the rotational axis but non-concentric, a low rotation would produce enough force to make it.

I assume that gravity however is not enough to make the switch!

photo of vibration sensor from Adafruit

(Image source: Adafruit: Fast Vibration Sensor Switch)

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