I'm trying to make a device which needs to be turned on when it's picked up and might sit in standby for several weeks before getting picked up. Right now I'm considering using a Photo resistor at the bottom of my device or using a piezoelectric sensor for this purpose, but both of them use electricity and might drain the battery before actual usage of the device. Is there any other way for a device to detect its own movement without draining its battery?
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12\$\begingroup\$ How about a switch, similar to that used in landline phones? \$\endgroup\$– HearthCommented May 31, 2020 at 10:55
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\$\begingroup\$ Simple until you starting thinking about the pulling resistors: rolling ball. Complex up front but in some ways more versatile: MEMS accelerometer with a motion alarm interrupt. You photo resistor idea may not be unworkable, for example you could have an MCU wake up every few second(s) and very put a test current through it before going back to a very low power mode. \$\endgroup\$– Chris StrattonCommented May 31, 2020 at 10:58
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11\$\begingroup\$ both of them use electricity and might drain the battery before actual usage of the device You're setting yourself the goal of zero power usage. If you change the goal to low enough power consumption, things might become much more easy. My Logitech wireless mouse lasts more than 2 years on when left alone but switches on again as soon as it detects movement. It does not use a mechanical switch. \$\endgroup\$– BimpelrekkieCommented May 31, 2020 at 11:31
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6\$\begingroup\$ More detail needed - what is the object? What is the surface? How many ways can it be picked up? What minimum acceleration is applied when picking it up? Can the surface be modified? What picks the object up? \$\endgroup\$– Andy akaCommented May 31, 2020 at 11:32
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3\$\begingroup\$ @Hearth I read your comment as "landmine" and that kinda seems suitable too. \$\endgroup\$– CriggieCommented Jun 2, 2020 at 5:36
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8 Answers
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A vibration sensor switch can be built or bought using a spring, wrapped around a central pin.
When moved, the spring contacts the pin, closing a circuit. Gravity alone isn't strong enough to close the circuit. They are surprisingly effective in all dimensions, not just in the axis perpendicular to the pin.
They are available in a variety of sensitivities and constructions, including bare and enclosed. I have seen versions made of just coiled steel wire soldered to the board.
Similar to Solar Mike's suggestion, these avoid any energy usage while not triggered.
answered Jun 1, 2020 at 8:08
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2\$\begingroup\$ You would need to combine this with something to maintain power after the sensor is tripped, a relay would work. \$\endgroup\$– rtaftCommented Jun 1, 2020 at 11:56
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3\$\begingroup\$ @rtaft Yes, or a transistor of some sort latching on, with the micro releasing it when whatever task is complete. \$\endgroup\$ Commented Jun 1, 2020 at 12:37
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\$\begingroup\$ I've seen simple versions of these built in to the design of bike lights costing next to nothing. They work well (though in the bike lights they kept getting wet and the steel spring rusted) \$\endgroup\$– Chris HCommented Jun 2, 2020 at 9:56
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Microswitches are available in a range of formats, are very small, are mechanically simple and very reliable.
Figure 1. A selection of microswitches.
The lever types are very sensitive so little force is required to operate them due to the lever's mechanical advantage.
simulate this circuit – Schematic created using CircuitLab
Figure 2. Wiring is as a simple as using the normally-closed contact (which will be usually open when the weight of the container is applied to the switch).
answered May 31, 2020 at 11:59
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2\$\begingroup\$ This is a very simple and effective option - you could simply ensure the lever arm is poking out of the bottom of the device. When the device is put down, the lever "clicks" the switch. Going further, you could possibly alter the design of the device such that it has a case design that compresses, bends or otherwise alters when picked up (engaging the microswitch). \$\endgroup\$ Commented Jun 1, 2020 at 9:51
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Should the unit be stored in a dedicated holder, a 'NC' type magnetic reed switch would suffice.
The reed switch could be mounted in the unit and wired in its supply line.
A magnet, located in the holder, would keep the reed switch open.
The reed switch would close and the unit would power up, on retrieval from it's holder.
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2\$\begingroup\$ If you have a docking station, you could arguably be supplying power to the device from the docking station (to keep it charged up). When charging stops, it's been picked up. Of course, if it gets put down somewhere other than on the docking station, that won't be detected. \$\endgroup\$ Commented Jun 1, 2020 at 9:40
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1\$\begingroup\$ Hi Ralph, I thank you for making me realize that my usage of the term 'docking station' was incorrect. I have amended my answer accordingly. \$\endgroup\$– vu2nanCommented Jun 1, 2020 at 11:32
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You could use capacitive sensing to detect when the device is picked. If it is a box, you'd need to put some conductive tape or spray conductive paint on all sides on the inside.
This adds a feature: the device can know where it is touched. For example if it is a box, it will be able to detect fingers on all sides independently. So it will know if it is touched (one finger detected) or picked up (at least two fingers, on opposite sides).
For example, MSP430 microcontrollers implement capacitive sensing with very low power:
The few µA required are lower than battery self-discharge. If you design the circuit for lowest standby power, it doesn't really matter if it is drawing a few µA from a couple AAA batteries or if it is fully off.
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\$\begingroup\$ But that would be self discharge PLUS the extra to run the capacitative sensing... \$\endgroup\$ Commented May 31, 2020 at 11:48
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4\$\begingroup\$ Sure... but if it draws only a couple µA the AAAs will die old age before all the capacity is used up ;) \$\endgroup\$– bobfluxCommented May 31, 2020 at 16:19
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A metal ball that moves and connects contacts - no movement no connection no current drain.
answered May 31, 2020 at 10:55
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1\$\begingroup\$ That's by far the simplest where it works; the catch is you need to carefully manage pulling resistors so they don't become a current drain if at rest in a position where the ball is completing a contact against them. \$\endgroup\$ Commented May 31, 2020 at 10:56
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\$\begingroup\$ You could use movement of the metal ball, in combination with energy harvesting for a complete battery less solution. \$\endgroup\$ Commented May 31, 2020 at 13:12
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\$\begingroup\$ You could possibly replace the ball with a tilt switch (much the same idea, but made for you). Both solutions require that when the device is put down that the switch is in the off (disconnected) position though. \$\endgroup\$ Commented Jun 1, 2020 at 9:42
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I am assuming that the object picked up is an eCommerce package left at the door step or something similar.
Base on the above assumption, I recommend look at solutions that is based on energy harvesting. This should address the requirement for no battery, lack of power or low power. Take a look at solutions offered by enocean self powered IOT solutions
Below is an example of a push button transmitter module. The following is used as light fixture switch. The system uses on/off motion of mechanical light switch motion to generate sufficient electrical power. The electrical power is generated using the built in electro dynamic generator.
Functional Principle
The electro-dynamic energy transducer is actuated by a bow, which can be pushed from outside the module on the left or right by an appropriate pushbutton or switch rocker. When the energy bow is pushed down, electrical energy is created and a RF telegram is transmitted including a 32- bit module ID (PTM 210J optional 48 bit). Releasing the energy bow generates different telegram data, so every PTM telegram contains the information that the bow was pressed or released.
I suggest taking a look at PTM 215 Push button multi-channel switch module or something similar
Alternatively you could use a pressure sensor, but this would require power.
answered May 31, 2020 at 12:59
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A magnet near a coil of wire only generates a current when the magnet is moving. Put a spherical magnet in a short and wide box with a coil on the lid (by "short", I mean similar height to the width of the magnet). When the device is moved, the magnet will roll around, generate a current in the coil, and that could be enough to power a startup circuit that turns on the main one.
If the device is overall flat in shape you might be able to make the coil out of a corner of the main PCB and just clip a plastic cover over that area to keep the magnet in the appropriate area, or even design it into the case.
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\$\begingroup\$ I thought about this, but the problem is that it's a low-voltage noisy AC signal. A magnet sandwiched between two coil springs in a tube might be more efficient. The mass-spring system will have a characteristic vibration frequency, and the coil springs can double up as wire coils. The tube is used to constrain the system to one dimension \$\endgroup\$– MSaltersCommented Jun 2, 2020 at 12:16
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\$\begingroup\$ The low-voltage noisy AC bit is a fair criticism, though I figured some of the super-low-power electronics out there might be able to deal with it. I did consider a magnet in a tube, but was concerned the single-dimensional constraint might sacrifice too much sensitivity, whereas childhood memories of those "roll the ball through the maze" puzzles and their associated frustration painted a pretty clear picture of something mechanical and sensitive to motion. \$\endgroup\$– RohanCommented Jun 3, 2020 at 13:01
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Possibly not quite what you're after, but you could tether the device with an easily removed string. When you pick up the device, the tether comes undone, and the device knows it's been picked up. This would be sort of the reverse of a "tether kill switch" that you might find on a motorbike or jetski: https://www.mpsracing.com/products/MPS/hc01.asp
