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Friday, 18 October 2013

emonTH Update - Hardware

Since my last post on the emonTH wireless Temperature and Humidity monitoring node good progress has been made.

emonTH cased up


emonTH - unboxed

The most significant hardware change has been the addition of a DC-DC step-up boost converter to step-up the voltage from discharging AA batteries to a steady 3.3V. The boost converter circuit consists of a tiny (SC-70 package) LTC3525-3.3 chip a 10uH inductor and a couple of small 1uF capacitors. The step-up converter is essential for the DHT22 as this sensor does not perform well with varying supply voltage,  specifically once below 3.3V. The addition of the converter will also significantly increase battery life. The LTC3525 was chosen because of its low quiescent power consumption of 7uA and high conversion efficiency of up to 95%.

emonTH LTC3525 DC-DC boost converter circuit

The boost circuit is very impressive, given a minimum input voltage of 0.7V it boosts up to a steady 3.3V.

Using scope with AC coupled probe to examine boost converter output when stepping 2V up to 3.3V with no load: output exhibited 9.3mV RMS ripple at 333Khz

Testing  emonTH external DS18B20 temperature sensor terminal block connection

We hope to have the emonTH in the shop by December.

Stay tuned after the break for update on emonTH software, power consumption and batteries...

2 comments:

  1. I really like the look of this. I have been working on exactly the same thing, but I think I may hold off and just buy a couple of these when you are done. Your board is much neater than my breadboard and sticky tape.

    Anyway, the point of writing was to ask about the step-up. I used a similar device for a while, but I am worried about their effect on rechargeable cells. It makes perfect sense for an alkaline battery where you want to drain out every last electron, but I was worried it also virtually guarantees you will end up deeply discharging and potentially damaging a NiMH through polarity reversal. I am absolutely no expert on rechargables and was interested in others' opinions. I had the raw, un-boosted supply connected to one of the ADC input pins on the microcontroller so I could monitor the battery health and swap the cells before they depleted. Once the boosted supply drops, is it not too late?

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  2. Hi Robert, thanks for your comment. Yes it's true such a setup will discharge batteries quite deeply, this could be an issue with NiMh but like you said the system could be easily setup to avoid this. However due to their high self-discharge NIMH batteries are not very suited to this type of long term sensor node application. See my post on batteries, alkaline rechargeable batteries look like they could be just the thing: http://openenergymonitor.blogspot.com/2013/10/aa-battery-considerations.html

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