Meteo node II

I'm reporting some progress on the meteo node today. It worked more or less on first power on - with the exception of the RF24 not working at all, but the problem was easy to discover - swapped pins MISO and SCK! I don't get how could I make that mistake as I was following the table, and I am sure I counted the pins right. Anyway, after fixing that, all worked just fine.
The node still misses the battery measurement support, but I am going to leave it out until I know what kind of battery I am actually going to use. The voltage divider will depend on if I am going to power it from AA cell, LiIon cell or something else. I ordered some samples of step-up converter from Linear, which could be used, and I am considering those MCP1640 from Farnell (they're still the cheapest), but that needs some testing first.
The high voltage capacitors finally arrived from Austria - sheesh, it took more than a week to get them shipped and delivered, and given the S/H price... I just hope they're any good, or else I ended up with 200 pieces of junk. They're surprisingly small for 1KV 10nF capacitors. Anyway, I'll see if I find some time this weekend to build - yes, you guessed - yet another HV power supply for GM tubes. In fact, I am thinking about putting together the not-so-common components and offer it on eBay for some reasonable price, saving people from hassle of getting bits and pieces here and there. I have a load of MPSA42 transistors, 10nF capacitors, 10mH inductors, fuse holders etc etc.
The lead accu I ordered month ago still did not arrive, and eBay returned me the money. I ordered some goldcaps today, and will do further experiments with them. I'll get two 22F/2.3V ones that could be great if I'll manage to make the step up converters working.

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!

Temperature sensors precision

I was always curious how much can I trust various types of temperature sensors. I loved the TI's TMP100, tiny I2C based sensors, and used them a lot before. In one project, I got a feeling that the sensors are actually lying to me, and I became a bit cautios. I later abandoned them in favor of DS1820 style sensors, and the few remaining TMP100's were resting in the box. As I recently got the DHT22 that also has temperature sensor, I felt like I should really take a look at various sensors and compare their readings. The results are interesting.
First thing first - the contestants are:

• TMP100 - I2C based sensor from TI in SOT23-6 package
• DS1820 (and similar ones) - 1-Wire based sensor from Dallas / Maxim
• DHT22 - sort of one-wire protocol, temperature and humidity, based on AM2302

I already played with all the DS1820 (DS18B20, DS1822 etc.) sensors I have, comparing all of them, and I found out that at least at room temperature, they all report more or less the same temperature, with maximal difference between the lowest and highest value around 0.5°C.

As always, I took Arduino, attached LCD display to it and all three sensors, using sample code from Arduino libraries. I did not see any example for TMP100, but Google suggested the Fork Robotics page, and the example described on that page worked just fine. I do not offer the code for download, it's a big mess, and throwing it together took me about ten minutes anyway.

The results are interesting, and reassuring me I can use any kind of sensor without risking the temperatures being too different compared to any other sensor. In other words, I can use the DHT22 for the meteo node, and trust the temperature. There is no need for an additional DS18xx sensor on that node.

I tweaked the code for the central node a little, nothing important. I am considering a change of the packet definition, making just a single structure that will use anonymous structures and unions to describe any kind of sensor packet. I am still not sure if there are any drawback other than the obvious one (prone to mistake). Just to explain what do I mean:

typedef union {
  SensorPayloadTemperature_t Temp;
  SensorPayloadMeteo_t Meteo;
  struct {
    uint8_t PacketType;
    uint8_t BattLevel;
    uint8_t ___pad[2];
    uint16_t SensorId;
  };
} SensorPayload_t;

This allows me to directly refer to PacketType, BattLevel etc. without 'going into' the Temp structure. Arduino supports this, at least the 1.5.8 version.