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Recently, I've been playing with various sensors. I've been interested in measuring air temperature, humidity and pressure. (Didn't get a CO2 and dust sensor yet.)


I thought I could post just a simple idea how to test a range of a humidity sensor. For measuring humidity, I tried SHT21, SHT31 and HTS221 sensors.


The pictures below contain a reference design with SHT21.


  • Very low relative humidity

What is needed: a small plastic bag with working sealing, silica gel (often supplied with shoes).




(It is possible to get even smaller relative humidity, but I didn't want to wait longer than a few minutes.) I also tried to let the bag opened and the silica gel has almost no effect. This means packaging anything with silica gel inside is useful only if the whole package is airtight.


  • Very high relative humidity

What is needed: a bathroom (the smaller and less ventilated, the better results) and someone having a shower / a bath.




(I didn't want to let the water condensate on the sensor, so I stopped at this value.)


SHT21 and SHT31 typically output +/- 2 % RH difference when placed next to each other, they can get as close as +/- 0,5 % RH in home environment after a while (nothing is moving around and there is almost none air movement), From an engineering point of view, humidity accuracy is typically +/-2 % RH for this sensor, which means the 2 decimal digits are only for reference. (Typical temperature accuracy is +/- 0,3 centigrade, so the same applies here.) However, seeing the data interpreted to the LSB gives a nice overview about the stability of output values and the noise present.




Note 2: I also tried 2 pieces of HTS221 sensor and was surprised by the results. One gave the output of 72 % RH , while SHT21/31 showed 59 % (which is about 22 % error). At higher humidity, the difference was even bigger: HTS221 gave output of 117 % RH, while SHT21/31 showed 91 % (which is about 28 % error). The other sensor gave output of 66 % RH while SHT21/31 showed 60 % (which is about 10 % error). Both sensors should have been calibrated and places on ST's reference designs, so I asked a question on ST community hoping to get an explanation of this difference by an ST engineer. I will post some update if I get any response. (I don't want to start a duplicate discussion here.)


I noticed there is a new event combining Microchip's  PICs and click boards from MikroElektronika.…

50 project in 50 days, reminds me of 100 projects in 100 days with PSoc devices (almost 4 years ago).


There is a chance to win some prizes, good luck


In case you've never heard of it, there are still a few days to pick a Humble Book Bundle, stuffed with Arduino and Raspberry Pi books and magazines by Make.


It is a "pay what you want" model (with certain price levels to get all the books) and you decide where the money goes - the publisher, a charity of your choice and/or the Humble Bundle team.


Note 1 : only 5 days remaining.

Note 2: this is not the first (and probably even last) books electronics-related bundle they made.

While browsing for construction ideas, I stumbled upon a construction by guys from MikroElectronika:

So I put out an old metal construction kit I inherited from my father and built a "car" chassis. You can see the result under the original project, in Replications section.


What I like about this construction (I mean the DIY variant, not buying the original kit):

  • easy to build (ca 1 - 2 hours including build the chassis)
  • cheap electronics - ca $10 for the parts, not including power source - my power bank was a giveaway (previously I used 4x NiMH AA battery and a battery holder)
  • cheap chassis - could be made from metal, wood, plastic, maybe even hard paper
  • cheap remote control - I assume almost everyone has a cell phone / tablet with WiFi and HTML5 capable browser
  • easy to expand - remote control software, micro-controller firmware, the hardware - more wheels, sensors, etc.
  • doesn't rely on 3rd party cloud services, registrations, ...


(More images are on the original project page.)


What do you think?