Zero Power Sensing

Zero Power Sensing
by Bernard Murphy on 02-22-2017 at 7:00 am

We’ve become pretty good at reducing power in IoT edge devices, to the point that some are expected to run for  up to 10 years on a single battery charge. But what if you wanted to go lower still or if, perhaps, your design can’t push power down to a level that would meet that goal? One area in systems where it can be challenging to further reduce power is in sensors, since these need some level of standby current to be able to sense and wake other circuitry.

Some of the lowest standby currents I have seen are ~0.3uA which is impressive (one case I found is able to support a sensor for up to 20 years on a coin cell battery) but maybe not good enough in some applications where you might need to support an array of sensors; even a 3×3 array would reduce that inspiring performance to just over 2 years, which may not be adequate for an infrequent service application.

A team at the University of Bristol in the UK have improved on this by reducing standby current to zero. They have built a voltage detector IC, called UB20M, which can use the intrinsic power generated by the sensor (for certain classes of sensor) to turn on and enable the rest of the system. They mention wireless antennas, infrared diodes, piezoelectric accelerometers, and other voltage-generating sensors as compatible with this device. The sensor must generate a ~650mV signal, but with only a few picowatts of energy to activate the UB20M. The power switch following the UB20M (to turn on power for the rest of the system) will naturally have some level of leakage (they mention 100pA in their test cases) which could perhaps be further optimized in commercial applications but is still 3 orders of magnitude better than commercial alternatives today.

So that array of sensors could become a negligible factor in the battery life of the parent IoT device. Other factors, probably leakage, would dominate. Not bad. The team is understandably cagey about how the device works since the design is patent pending in the UK. They are offering samples for testing. You can read more about the UB20M HERE.

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