The socially responsible among us wishing to save the planet from a carbon dioxide doom should quickly move on to the next blog post now, as I have a confession to make: I own and occasionally get to drive a car with non-catalysed V8 engine. Of course my love of V8s is all down to my having been the weedy kid with glasses who always got sand kicked in his face. I couldn’t kick ass but I sure can give a throttle pedal a good beating, getting the reward of g-forces nearly dragging my hands of the steering wheel and severe back pain from being crushed back in the seat. Heavenly sensations accompanied by the sounds of a low rumble rushing to a deafening roar. Fortunately all these agreeable noises drown out the disturbing sound of Niagara Falls coming from the direction of the fuel pump.
It was while driving to work on one of the rare days this summer in the UK when it wasn’t raining – the V8 experience is only complete when cruising in a convertible with the roof down and Ray-Bans on - that I was struck by an uncharacteristic twinge of guilt. True its lifetime was similar to that of a new sub-atomic particle created in that other great user of energy, the Large Hadron Collider, but it was detectable. Could I use energy more efficiently?
Most of my time is spent in front of a computer screen and while I have to use what I’m given at work, it’s my choice at home. I relaxed, because for reasons not connected with Green Issues I have recently replaced our two lumbering desktop PCs with a pair of relatively tiny nettops: Acer Aspire Revo. The old ATX-case machines had to go because they just took up too much space. Opening up one of these steel boxes reveals a large amount of nothing: much of the space reserved for expansion cards that will never be fitted. The outer casing supports fans necessary to shift large amounts of wasted energy in the form of heat.
The nettops on the other hand are tiny: about the size of an average hardback book and can even be bolted to the back of the monitor thus disappearing completely. Of course the real issue is the power consumption and the dual-core 64-bit Atom processors consume very little. We’re talking tens rather than hundreds of Watts and while not exactly sparkling in performance compared to their ‘gas guzzling’ relatives, they are able to handle 1080p HD video thanks to the Ion graphics chip. There is a processor fan, but this revolves so slowly it’s barely audible. I use a current-sensing mains socket strip that’s supposed to turn on the five other sockets when the master detects that the PC has been switched on. The Revo takes so little current the socket strip doesn’t even notice!
Coincidentally, some interesting ‘Green’ products came into the office the other day: a range of low-capacity solid-state batteries and associated energy harvesting demo kits. Energy harvesting is the recovery of small amounts of energy from the environment sufficient to power an electronic device such as a wireless sensor. This ‘free energy’ varies from the obvious, sun or lamplight to the less obvious stray RF EM energy picked up by an antenna, to the ingenious, piezo-electric transducers stuck on moving parts of the human body. Even a temperature gradient can yield energy via a Peltier-type device. ‘Free energy’ has always existed but only recently have the power requirements of useful devices such as microcontrollers come down to make ‘harvesting’ it worthwhile. Examples of this new generation include the Renesas RL78 microcontroller family and ARM Cortex-M0 core devices from NXP.
The Cymbet Universal EP Energy Harvester board can take inputs from a variety of transducers to charge a pair of 50µAh ‘chip’ batteries. The charging system is managed by an Energy Processor chip which performs the vital task of maintaining an impedance match between the transducer and the harvester system. This impedance matching is necessary to achieve Maximum Power Transfer from the solar panel or piezo-electric device, wringing out every last microamp. Impedance matching often gets forgotten in a low-frequency digital world but has always been of fundamental importance in analogue RF systems and high-speed digital communications eliminating reflections and other horrors. The energy processor chip is even able to handle photo-voltaic panels whose output impedance varies with output current. The on-board battery consists of a pair of 50µAh chips in parallel; hardly enough capacity to drive even a very low-power microcontroller. However they can supply milliamps in bursts, so a typical application could be a wireless sensor which periodically wakes up, takes some readings, transmits them and then goes back to sleep.
Now this scenario fits perfectly with the new generation of energy efficient microcontrollers which feature a variety of snooze/sleep/hibernate low-power modes. At last we may get a significant increase in the number of truly ‘Green’ (little or no impact on the environment) applications, as opposed to those needing to be powered indirectly by a distant power station. I will be writing about my experiences interfacing the Cymbet EP kit to a Cortex-M0 mbed module in a few weeks time. Has it stopped raining yet? No, dammit. Oh well. I think I’ll just put my feet up and conserve some energy.
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