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Temperature and humidity

This two measurements are obtained thanks to the DHT22 sensor, manufactured by Aosong Electronics, a Chinese corporation. They are quite cheap (you can obtain it for around $12USD) and are very precise. The output is not in analogic form but in digital form, meaning that the signal is in the either ones or zeros, thus requiring some extra "intelligence" to interpret the results.

The necessary libraries are already implemented to use along with Arduino and can be found in GitHub. They were initially developed by Ben Adams in 2011 and implements a non-standard single wire protocol, necessary to establish a successful connection between Arduino and the sensor.

As for the values range it can obtain:

0-100% humidity readings with 2-5% accuracy

-40 to 80°C temperature readings ±0.5°C accuracy

Light and noise

To quantify light and noise levels we used a LDR (light dependant resitor) and a microphone. The market is highly saturated with this kind of components, but for the sake of this project we chose the following ones:

LDR: Excelitas Tech VT935G
Microphone: Pro Signal ABM-705-RC

The range of values they can obtain is infinite, since they are analog sensors.

CO - Carbon monoxide

Carbon monoxide levels are measured through the MiCS 5525 sensor, a sensor made by e2v. This time the output is not digital but analogic. That means the sensing resistance decreases when CO is around. The main structure of this sensor consists on a heating resistor and the previously mentioned sensing layer.

CO levels are inversely proportional to Rs/R0, as it can be seen on the datasheet. Rs is varies in an inversely proportional way to CO levels, and R0 is a fixed sensing resistance whill shall be measured under controlled ambient conditions (given that it slightly varies from sensor to sensor). Since in the first iterations of the project we didn't dispose of such conditions, we currently upload Rs to the Internet. Later on, we will update via firmware the value of R0, thus allowing to measure actual CO levels.

To measure CO levels a small procedure must be followed. Before reading, the sensor must be heated for 5 seconds, then let it cold for 9 seconds and finally perform a reading. Actual CO values can vary from 1 to 1000 ppm.

NO2 - Nitrogen dioxide

Nitrogen dioxide is measured buy another e2v sensor. In detail we are talking about the MiCS 2710, a small, low power and precise NO2 sensor.

As NO2 levels arise, internal resistance of the sensor increases, as depicted below. More precisely, NO2 levels are calculated in terms of Rs (varying resistance) and R0 (fixed). This time, Rs/R0 is directly proportional to NO2 levels. And again, the information uploaded to the internet is just the value of Rs since R0 couldn't be measured when the first ships shipped.

To actually gather a value of Rs, the sensor must be heated until it reaches 220ºC. This is done by heating during 30s then perform a read. NO2 values can oscillate between 0.05 and 5 ppm.

Solar Panel

Any solar panel can be soldered to the board while it matches at least our minimum requirements, that is:
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  • tiefpunkt's SCK
  • M, Germany
  • indoor
  • Smart Citizen Kit v1.0
  • Last Update: 23 seconds ago (2014-07-11 23:57:09 UTC)