Pushing Past Power Problems (perhaps)

Making a semi-final product today allowed me to finally start taking semi-final power consumption measurements. These were done by inserting a small resistance into the positive power supply connection and then measuring the voltage over it (my multi-meter was not sensitive enough for direct current measurements).

These were terrifying, unmodified the board pulled almost 60mA  in its idle state, giving it just over a day of life on a single 18650 lithium ion cell. This is clearly far less than I had hoped and indicates that power consumption is now something I should be worried about. Off we go on a hacky adventure!

First off was removing the 5V regulator which (given it had been shorted out anyway) was just wasting power. That saved a full 5mA of consumption.

The next step was to stutter the RF field from the reader, adding periods where it is turned off, for instance when the microcontroller is doing non-RFID things. Additionally the addition of a sleep period to the polling cycle (with the RF field off) massively reduces the RF field duty cycle and in tests did not really change the user experience (and I was looking for it). That saves another 10mA and being more aggressive on the sleep saved a further 10mA. This pushes the time on single cell up to 3 days.

Clearly I have been optimistic that the cells will actually be able to discharge their full rated capacity, so maybe 1.5 days is more like it. This still allows the proof of concept to be programmed the day before any field trials and left on over night without having any danger of it running out of power!

The interesting thing about this development is that it indicates the next big development required of the ODOTS on its path to becoming ready for certification (certification being expensive and something I only really want to do once in the future)(And something that will be necessary to bring the ODOTS, or derived system, to the mass market). Frankly the current hardware architecture is not optimised for power consumption in any way at all, the use of Arduino in particular is a bit of a liability as it does not provide fine control over AtMega peripheral activation. This may be why I eventually shift over to a different architecture in the future. While I have made an effort to tame the ravenous beast that is the RF field the MRFC522 (or one of its alternatives) will provide methods for reducing its appetite further. I have commented in the past about the ability for the ST Micro chips to passively detect tags, this would be ideal as it would massively reduce the power consumption with potentially little (randomised) impact on competitor visit recording times.

I suppose my dreams of networking the ODOTS units with LoRa and implementing a 6WLoPAN network are going to be a long way off indeed!