Since my last post on the emonTH wireless Temperature and Humidity monitoring node good progress has been made.
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| emonTH cased up |
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| 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%.
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| 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.
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| 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 |
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| Testing emonTH external DS18B20 temperature sensor terminal block connection |
We hope to have the emonTH in the shop by December.
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