10
\$\begingroup\$

What are the options for single-use switches to turn the device on?

Intro: I am working on a project where I want to have a completely encapsulated device (no ports or opening), but it should have a long shelf life.

This means I want the device to be off the whole time before deployment.

Status: Now I am thinking what could be some options to turn it on. I found magnetic-based switches interesting, mainly reed and Hall. Hall does not seem like the best option since it draws current. Reed seems to fit, but perhaps there are more elegant and single-use options.

Question: I would like something simple, just like pulling a paper strip on batteries (not an option since the device and battery are cast in plastic.)

Do you have any suggestions, or should I really continue with the reed switch?

\$\endgroup\$
7
  • 1
    \$\begingroup\$ You need to give a better idea of the acceptable quiescent current , duration of operation of the device and perhaps general environment. Ive used a fully sealed magnetic switch in a solar light with good results. \$\endgroup\$
    – Russell McMahon
    Jan 17 at 14:50
  • 2
    \$\begingroup\$ Is there a possibility to power the device at manufacture and leave it on for life? May have to build more battery capacity into the design to cover the storage/shipping periods. \$\endgroup\$
    – Criggie
    Jan 17 at 21:24
  • 10
    \$\begingroup\$ Also, what about the end-of-life period? The end-user is supposed to trash the whole item with batteries inside? This may prevent the item being saleable in some areas of the world (California comes to mind) \$\endgroup\$
    – Criggie
    Jan 17 at 21:25
  • 5
    \$\begingroup\$ Why do you specify "single use" switches. Is there a need for it to be single use, or are you just pointing out that the switch should be expected to be used only once and thus opening the door for one shot devices? Also, what sort of reliability are you looking for? As you pointed out, on one end of the scale, a paper slip can be a switch. At the far end, Mars rovers tend to use pyros to actuate their one-time switches for extreme reliability. \$\endgroup\$
    – Cort Ammon
    Jan 18 at 17:10
  • 2
    \$\begingroup\$ @Criggie electric toothbrushes manage it, I think? When I had to throw away the last one I had (made by Philips), it had a page in the manual showing how to break it open in a certain way to remove the battery when recycling it \$\endgroup\$
    – Aaron F
    Jan 19 at 16:45

14 Answers 14

16
\$\begingroup\$

MOSFET in combination with a magnet-activated reed switch

schematic

simulate this circuit – Schematic created using CircuitLab

When the reed switch is closed, it starts passing current to the load. After about 0.5 seconds, C1 is charged up through R1 enough to turn on N-MOSFET Q2. This pulls down the gate of P-MOSFET Q1, turning power on permanently.

The R1-C1 filter ensures that short activations of the reed switch due to e.g. vibration do not activate the device. If needed, C1 value can be increased to give longer delay.

In off state, the circuit leakage is the off-state current of the MOSFETs. Common small signal MOSFETs specify maximum leakage of 1 µA, but it's easy to find parts like DMC2400 with 100 nA specification. The trade-off is between on-state resistance and off-state leakage.

In on state, the circuit adds extra quiescent current through R3 and R4. With the 1 Mohm resistors this would be a few µA. When using low leakage MOSFETs, these resistors could be increased to 10 Mohm.

\$\endgroup\$
2
  • 2
    \$\begingroup\$ In this attempt the reed switch could even be substituted for a NFC controller to avoid unintended activation if that's a concern. \$\endgroup\$
    – Sim Son
    Jan 18 at 17:52
  • 2
    \$\begingroup\$ If the case was transparent, you could use a bright light on a very small PV array (a couple of LEDs in series is common) to switch Q2 on. This is similar to a photovoltaic optocoupler, sometimes used to switch FETs with no need for a power source on the isolated side. \$\endgroup\$ Jan 20 at 1:47
14
\$\begingroup\$

Bistable flexible section on the enclosure

If you are designing a custom enclosure, whether by injection molding or 3D printing, you could add a section that can be pushed in.

Due to plastic deformation and the shape, the protrusion will remain depressed. It can still be fully sealed, as long as the enclosure is large enough compared to the button to avoid excessive pressure, which would pop the button back up.

3D rendering of box with conical section poking up or pushed in

Then put a simple micro switch or push button underneath it.

\$\endgroup\$
2
  • 4
    \$\begingroup\$ That is cunning! Like the "diet" button in the plastic tops of soda/pop cups from fast-food joints. \$\endgroup\$
    – Criggie
    Jan 19 at 1:41
  • 3
    \$\begingroup\$ it may be possible to reset this switch using a vacuum tool like a sucker dart. \$\endgroup\$ Jan 19 at 4:18
13
\$\begingroup\$

The magnetic reed switch is a reasonable approach, but then you have to add the cost and packaging of the magnet, as well as the concern about small magnets as a swallowing hazard for children.

A simple PCB-mounted tact switch could be a good alternative, if you can implement a water-tight flexible button that covers it. A very low power accelerometer is another option, if the unused product is stored motionless.

All batteries have a shelf life as well as some slow self-discharge. You may be able to design the circuit so its quiescent current draw is of similar magnitude, eliminating the need for any turn-on device because the product life will not be compromised.

\$\endgroup\$
12
\$\begingroup\$

A reed switch sounds quite reasonable. There exist N.C. reed switches so removing a magnet could turn it on with essentially zero current (but they're relatively sparse in terms of offerings). You might be able to use a N.O. reed switch as well, depending on the exact circuitry (perhaps a high value resistor internally or an always-on circuit that wakes up periodically and checks for the switch state).

It's possible there exists a capacitive touch system that would have low enough quiescent current for your application but it would require some work to evaluate how well it would work through the potting and the battery life consequences.

Another possibility might be to use inductive or RF coupling. Things like the qi power chargers and RFID chips can operate without any external power. There would have to be some very low Iq circuitry waiting to be triggered, of course.

\$\endgroup\$
10
\$\begingroup\$

A cheap idea could be a photodiode and a black tape. Pull off the tape, the photodiode starts conducting and pulls a MOSFET/Flop/IC into the ON position.

Dark currents of < 1 nA are easily available. The bigger power hog is then the turn on circuit.

\$\endgroup\$
1
  • \$\begingroup\$ Great answer. You do not even need a photo-diode, you could use couple of normal LEDs (maybe even LEDs that are already needed in the product). You can even have these LEDs self-generate the power to activate the device. Turns out normal LEDs can make useful amounts of current. wp.josh.com/2014/03/03/the-mystery-of-the-zombie-ram \$\endgroup\$
    – bigjosh
    Jan 23 at 19:45
4
\$\begingroup\$

Use a rechargeable battery, and design the device to support wireless/inductive charging. The device would ship with the battery empty (or close to it). To activate the device, charge it. When the battery level exceeds, say, 95% charged, your charging circuit would turn the device "on" permanently.

Shelf life shouldn't be an issue since you're not trying to maintain a charge while the device is idle. This type of switching mechanism should be exceedingly difficult to activate unintentionally.

\$\endgroup\$
3
  • 2
    \$\begingroup\$ Most rechargeable batteries have significant self discharge (months, maybe a year of useful life). Primary cells like primary lithium (not to be confused with Li-ion) are often found in devices like 10-year smoke detectors. \$\endgroup\$ Jan 19 at 6:54
  • \$\begingroup\$ @SomeoneSomewhereSupportsMonica That's why you have to charge the device to activate it. The battery can discharge all it wants, but you'll still have a full charge when the device turns on. \$\endgroup\$
    – bta
    Jan 20 at 5:15
  • 1
    \$\begingroup\$ That assumes OP doesn't desire any more than perhaps 3 months of operation once activated. That's not clear. \$\endgroup\$ Jan 20 at 10:31
4
\$\begingroup\$

You can conceivably use a screw in a threaded hole (metal enclosure) or a threaded insert (plastic enclosure) so that when you insert the screw, either the screw depresses an internal switch or the screw completes the circuit between the threaded insert and another contact underneath (presumably on the PCB). To maintain a hermetic seal, just put a gasket or elastomer o-ring under the screw. My way has the benefit of no current draw when off and super cheap and easy to implement. See attached CAD file for depiction: enter image description here

\$\endgroup\$
1
  • 4
    \$\begingroup\$ I mean technically it is a CAD file :D though I would not call paint CAD software \$\endgroup\$ Jan 19 at 12:38
2
\$\begingroup\$

How about a ("one-way") memory metal contact?
Mind the upper limit on storage temperature…

\$\endgroup\$
2
\$\begingroup\$

If you have an MCU inside you could connect the power supply input through a reed switch in parallel with any other, low-leakage switch controlled by the MCU.

A short burst provided by sweeping a magnet would wake up the MCU and turn the other switch on. The controlled switch can be a load switch IC, discrete FETs, or even be a bistable relay.

Perhaps a bistable relay coil could be turned on directly by the reed switch (without any MCU).

\$\endgroup\$
1
\$\begingroup\$

You could combine one of these with a fuse that would blow and be a permanent switch with some simple transistor logic.

Alternatively, you could just use a fuse port to be the 'switch'; my mind comes to those mini-breakers you see on like power strips, when they blow, they are pushed out and reset when pushed back in (and stay down). Dual benefit of safety. Example: https://www.amazon.com/RFXCOM-microswitch-Breakers-Overload-Protector/dp/B0C1GK74BS?th=1

Obligatory note to try to make your battery replaceable, reduce e-waste :) (though will admit usecase dependent, etc, etc, still an important consideration)

\$\endgroup\$
1
\$\begingroup\$

A possible idea: use mechanical microswitch that is actuated by a sliding ramp. Use a magnet from the outside to move the ramp. As the ramp slides to the 'on' position, it falls into a small 'pocket' to hold it there.

Another idea: a snap 'clicker' style strip that is actuated by a mechanical button. Once pushed, the strip snaps to 'on', and it can't be pulled back to 'off'. You could even use a plain toggle switch.

\$\endgroup\$
1
\$\begingroup\$

You can embed a marble maze inside the product with a conductive ball bearing at one end and a pair of contacts at the other.

To activate the device, the user executes the specified series of rotations which could be printed on the device or in the manual.

If the pathway and instructions are designed correctly, it will always activate after the correct sequence but it will be exceedingly unlikely to activate by a series of random movements. You can add more turns, t-bones, chuttes, and make it 3D to reduce the chances of accidental activation to any arbitrary limit.

\$\endgroup\$
0
0
\$\begingroup\$

Do you need to guard against unintended activation? Perhaps a combination requiring simultaneous activation of multiple modalities: Simple reed switches could be triggered by incidental magnetic fields (including electromagnetic). Tilt switches and shock sensors would be immune to magnetic fields, as they are triggered only by particular motions. Bimetallic thermostatic switches are triggered only by temperature change.

\$\endgroup\$
0
\$\begingroup\$

What about creating your own spring-loaded one-time-use relay, where you remove a bolt (or something else) to release the spring? Keep it simple and stupid, makes for an easy journey. It could, at the same time, release some two-component glue to "close up" the device from any openings (e.g. by the spring also breaking some [glass?] container inside the device).

Or make it fully electric: Create two prongs, where one needs to connect a power supply, to "melt" a thin metal plate that's holding a spring relay off, if acceptable. If it needs to be portable then use the devices own power source to quickly melt something.

If you can use flexible sealed rubber membranes, then it should be doable to make one time physical push buttons (just like emergency stop buttons on machinery).

\$\endgroup\$
2
  • \$\begingroup\$ (something else, considering pulling [an isolating] strip on batteries was ruled out as contradicting hermetically sealed.) \$\endgroup\$
    – greybeard
    Jan 18 at 6:41
  • \$\begingroup\$ If it's not possible to seal the device "later" then this probably wouldn't be an option, true. A complicated mechanism with (one-way) rubber seals might also be possible. \$\endgroup\$
    – Raf
    Jan 18 at 6:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.