středa 4. března 2015

Meteo node

After that experiment yesterday that helped me to confirm that I can safely use DHT22 for temperature measurement, I started to work on the meteo node. It's not tested (my wife needed some changes on her blog that took me some time to figure out), but it already is getting shape. The white thingy is the DHT22 sensor, the small bluish one is BMP180 module. I found out the module actually had 3.3V LDO so I  had to remove it, that's why you can see some missing components, and the wire on it. I was originally planning on powering the whole thing from 18650 LiIon accu, but I can't find the holders I bought some time ago. CR2032 was the second choice, but that probably won't work for DHT22 as it is specified for 3.3-6V, and according to some into on web, it starts to show erratic behaviour under 3V. I'll try a step up booster and power the thing from one AA battery. I wanted to order some MCP1640 chips, so this is a good time to do so. Farnell has them for nice 41 cents when ordering 10 pieces... just the shipping price is way too high, so I'd better think of some other stuff I need before I make the purchase. Anyway, the CR2032 should do for the moment.
There is a few things that are still missing - the battery voltage measurement, and I should probably switch off the sensors when Arduino is sleeping to conserve power. The DHT22 draws something like 40uA when idle, which is pretty much a lot of power. Now the question is, how to do that? Using a PNP transistor as a high side switch should work, but I am a bit worried of the voltage drop on it. Of course that would not work at all with CR2032 (that's why I did not add this to the node yet). Another option is to power the sensor from the Arduino digital pin. That should be pretty easy, but again, I am not sure about the voltage. The safest would be to use something like 3.5V for Arduino, which should be fine for all other devices, like the NRF24L01, and still give a plenty of headroom for powering the sensors. Time to do some measurement tomorrow!

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