Mortens meninger

After a DHT22 arrived a couple of days ago, I have had quite a few sensors running on my desktop:

• DHT11 humidity and temperature sensor
• DHT22 humidity and temperature sensor
• BMP085 air pressure and temperature sensor
• DS18B20 temperature sensor

DS18B20 in foreground. DHT11 (blue) at the back, DHT22 (white) in the middle and BMP085 (red PCB) at the right.

All the sensors are sampled at approximately 15 minutes intervals. The DHT11, DHT22 and BMP085 are all connected to the same controller and are sampled at the same cycle, The DS18B20 is connected to another system which may be sampling at other moments, although at the same frequency. (more information on the hard- and software for sampling can be found here)

Temperature
The DS18B20 is set up to 0.5 degree resolution. According to the documentation, this is consistent with the accuracy, although it may report values with a much higher resolution. The DHT11 is reporting temperatures with a 1 degree resolution the BMP085 and DHT22 both returns temperatures with a 0.1 °C resolultion. The temperatures from the last approx 36 hours seems to fit reasonably well.

Temperatures measured with different sensors

Relative humidity
When it comes to relative humidity, only the DHT22 and DHT11 can be compared. The DHT11 is reporting humitities with a 1% resolution and the DHT22 with a 0.1% resolution, so some differences are expected. But it turns out that the DHT11 seems to be a much poorer detector than the DHT22. It kind of gets the right humidity, but is really inaccurate.

Relative humidity logged with a DHT11 and a DHT22.

Using just one detector of each kind, it is of cource difficult to give any general conclusions. By breathing on it, I can get the DHT11 to show a much higher relative humidity. It may be that one of my detectors is defective- but so far, it seems really to be worth it to shell out the extra to get a DHT22 rather than the DHT11.

The BMP085 is a barometric and temperature sensor. I’ve got one on a breakout board from geeetech. Using this and an arduino, it is possible to build an electronic barometer.

It can be connected to an arduino uno or nano:

BMP – Arduino
VCC – 3.3V
GND – GND
SCL – A5
SDA – A4

The sketch is originally written by Jim Lindblom at SparkFun Electronics, I updated it slightly to be able to use a more up to date wire library. BMP085.

This will report the temperature and barometric pressure (in Pascal) on the serial port. It is also possible to combine this with a DHT11 (or DHT21 / DHT22) sensor to also get a humidity reading. See https://github.com/sickel/DHT_BMP085. The DHT has to be connected to +5 and gnd, signal to port D2 on the arduino. The DHT library can be downloaded from arduino.cc.

Arduino nano, DHT11 temperature and humidity sensor (blue – at top),BMP085 temperature and pressure sensor (red – at bottom).

This work has been done in Arduino 1.0.1 on a debian system.

I have set up a system logging temperatures (and later hopefully also other parameters) to a postgres database. The data are now read out and presented as an svg-graph. (this may not work in some older browsers, i.e. msie< 9 - it is possible to use svgweb, but I have presently not implemented this)

Temperature graph. The number in the lower left corner is number of data points in the graph.

The source code for the system is available at my github account. The version presented here, is at tag passive_plot – in the last version it is possible to read out data, clicking in the chart. Later it will also be shown how to put more charts on one web page.

Why not just use cosm? Well, in fact I am uploading my data to cosm (in fact the intermediate json format used in this project is compatible with cosm), but on one side, I like to have control and know that I know where and in which format my data are, I also do not like the ways it is possible to present data on cosm (e.g. if a long time series is presented,points are thrown away, which may be strongly misleading if a point with a (local) minimum or maxima is removed – that happens quite often)

Testing a infrared reflective sensor module.

I set it up with an arduino board, running the following sketch:

int sensorPin = 10;
int value = 0;
int prevval =0;
int count=0;
void setup() {
  pinMode(sensorPin,INPUT);
  Serial.begin(9600);
}
void loop() {
 value = digitalRead(sensorPin);
 if(value==0 && prevval==1){ // something is dected
   count++;
 }
 prevval=value;
 Serial.print(count);
 Serial.print("   ");
 Serial.println(value, DEC);
 delay(50);
}

To run this, connect dout on the sensor to digital pin 10 on the arduino, connect + on the sensor to 5V and – to ground.
When the infrared light is reflected back to the detector, the output goes low. The sensitivity can be adjusted using the trim pot, turning it counter-clockwise lowers the sensitivity, i.e. what passes in front must come closer or be more reflective. In addition to watching the serial monitor for changes, there is a led that lights up when something is detected.

Adjustments of the sensor

The sensor module is marked “auto-ctrl.com” and “LOGO-sensor Var1.5”.

I am setting up a quick and dirty couchdb database to store data from an arduino temperature server running on http://192.168.0.177. (only visible on my local network)

All the code below is run under bash 4.2 on the debian system where the database resides.

The db is called arduino and is created by

curl -X PUT http://127.0.0.1:5984/arduino

Pulling out the data:

GET http://192.168.0.177/json

Storing it in the database:

curl -X POST http://localhost:5984/arduino -H 'Content-Type: application/json' -d <jsonstring>

This can be linked together:

GET http://192.168.0.177/json | curl -X POST http://localhost:5984/arduino -H Content-Type: application/json' -d @-

Note the last @- that makes curl read from standard input.

What we miss now is the timestamp. We can use sed to insert a timestamp in the json string:

sed -e "s/\"mil/\"time\":\"$dte\",\"mil/"

Will insert the string “time”: and the content of the variable $dte just before the “millis” in the json.

export dte=`date +"%F %T %z"`
GET 192.168.0.177/json | sed -e "s/\"mil/\"time\":\"$dte\",\"mil/" | curl -X POST http://localhost:5984/arduino -H 'Content-Type: application/json' -d @-

And to make it run each 2nd minute put it all into a file:

#!/bin/bash
while true; do
export dte=`date +"%F %T %z"`
GET 192.168.0.177/json | sed -e "s/\"mil/\"time\":\"$dte\",\"mil/" | curl -X POST http://localhost:5984/arduino -H 'Content-Type: application/json' -d @-
sleep 120
done

and run this with a nohup

To keep an eye on what goes in:

id=`tail -n1 nohup.out | awk -F'[,:\"]' '{ print $9 }'`;curl -X GET http://localhost:5984/arduino/$id

In the longer run, I am pretty sure these data should be stored in postgres – but for the first run testing, couchdb works fine. (Although, I could have rewritten the server to return sql-strings and run them through psql.)