1
\$\begingroup\$

For a primary-battery powered device, that will be stored for a period of time before first use, what is the most elegant non-mechanical* way to switch it on for the first time, after which it will operate for the life-time of the battery?

The device operates in low-power mode most of the time, with bursts of activity. The only thing powered during low-power mode is a real-time clock, drawing something like 20 nA.

I have considered NFC and maybe even capacitive touch, and it seems to me that those could work to signal and power a switch to turn on, but in that case something else would have to keep the switch on afterwards. Maybe the switch could be to the RTC's VCC pin, and afterwards it could hold its own power switch in the on state, but that seems a bit dodgy.

I can't think of a really elegant way to do this.

* Meaning that the device is hermetically sealed, and it is not desirable to have a switch, button, pull-string or similar sticking out.

\$\endgroup\$
  • \$\begingroup\$ what is the primary use case of the device? why can't you have a mechanical method? And what is "mechanical"? Does this mean solid state, or is it just a tightly closed case? How long is "stored for a period of time"? \$\endgroup\$ – helloworld922 Jul 9 '16 at 9:36
  • \$\begingroup\$ Its is a wireless sensor node in a tightly closed case. Storage could be up to a year. \$\endgroup\$ – Jacob Jul 9 '16 at 9:53
  • \$\begingroup\$ NFC has possibilities, but not at 20nA - more like 500nA. Use it to power a low-power MCU (e.g. MSP430) which turns on the switch (maintaining power to the MCU and powering the load) before the MCU returns to deep sleep. The MCU could also be your RTC in that case. Alternatively, if you could find an antifuse, use NFC to blow it. \$\endgroup\$ – Brian Drummond Jul 9 '16 at 9:54
  • \$\begingroup\$ Alternatively, use a magnet to close a reed switch. Maintain power via a FET switch in parallel with the reed. \$\endgroup\$ – Brian Drummond Jul 9 '16 at 10:00
  • 1
    \$\begingroup\$ Simpler still... a latching reed switch, where a weak magnet can hold it shut. But check if severe vibration can open it again. (If so, no damage done, it can be re-closed). standexelectronics.com/products/… \$\endgroup\$ – Brian Drummond Jul 9 '16 at 10:54
5
\$\begingroup\$

Self latching power-up circuit: You seem to be overly concerned with the risks involved with a device holding itself on. This is essentially how just about any device with only push-button normally-off switches works - extremely common and standard. eg any push-on push-off LED torch with a non latching switch, and those with multiple functions with multiple presses.

Simple current transformer with half core: Core is inside case. Lay a conductor along case at correct location, pass current through cable and induce voltage in internal coupled coil. This is essentially a variant of NFC. I have transferred 1 Watt+ in this manner - from an insulated TPS coated wire to a ferrite 1/2 core. Power for eg FET turn on would be trivially easy.

You can use a processor to hold the switch on or have a very simple latch so it is processor independent.


Methods:

Capacitive power transfer can operate entirely successfully through a plastic case. Achievable power levels well in excess of "wake up" signals are possible. (Don't ask me how I know).

You could use vibration or ultrasonics with a piezo
A piezo could probably use "tap on the case" wake up signalling with a piezo pulse operating eg a MOSFET which requires only gate-capacitance charge-energy to turn on. As a bonus you could also use it for signalling to the device. A false-alarm start signal could be ignored if some sort of followup code was not sent immediately after the first signal. (As a real-world example: Coin mechanisms (illegally & covertly modified) have been used to pass false input signals to slot machines by timing the coin insertion pattern to meet a preset code).
Don't ask me how I know :-).(No, it wasn't me.) )

Magnet-activated reed switches can be very rugged and reliable. I used one in a design (that ran to 100,000++ units) with some initial trepidation, but they performed even better than the specification and have been essentially trouble free in an 'anything goes' application. (Kicking the product down a flight of concrete stairs became a standard demonstration event. It was meant to "try to" survive ANY unreasonable thing that might reasonably happen to it including full immersion and all seasons exposure. The reed didn't feature in the "interesting field failures" list). A reed-switch has genuinely zero energy consumption when off.

RF or NFC input requiring a coil and capacitor for resonance - or NFC with a coil only and less sensitivity has far more energy transfer capability than needed. It also could be used for signalling and even for external recharging if desired.


In all cases the wake-up signal is required to provide minimal energy. A MOSFET gate is one possible very low power 'input'.

\$\endgroup\$
  • \$\begingroup\$ Thanks! Some good suggestions there. You do make me wonder about the device for coin mechanisms. I feel another question coming on ;) \$\endgroup\$ – Jacob Jul 9 '16 at 15:20
  • \$\begingroup\$ @Jacob Coin ...: Someone sometime approached a group of technical people wondering if they could help him understand how some people unspecified were able to somehow take $ off the system when they had no access to the system proper. We listened. My suggestion was the right one. Where and when and who better left in the fogs of time. \$\endgroup\$ – Russell McMahon Jul 9 '16 at 17:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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