New equipment and waiting for some more

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It’s been a while since the last post. Recently I bought a RTC plug and a BMP085 plug. Didn’t try anything yet but soldering the headers to them. It will not take too long before I make the first sketch or try some of the shelf. I’m waiting to get one or more Raspberry Pi’s but they were sold out in no time :-( It will take another 1-2 months before I’ll get hold of one so this means I’m gonna focus on the JeeNodes for now. The intermediate ‘layer’ has to wait for some months.

 

Send and receive

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The last days I’ve been playing around with my JeeLink and a JeeNode including three sensors. I made a JeePlug with the TMP36, an LDR and a TSOP infrared receiver, which I can plug into my JeeNode. So I don’t have to plug everything in a breadboard and connect the wires, just place the JeePlug on top of my JeeNode in ports 1 and 4. I use port 1 for the temperature (TMP36) on the analog pin and I use the analog pin of port 4 for the LDR. The TSOP is connected to the digital pin of port 4 which leave the digital pin of port 1 empty to be used in the future.

The goal is to create a WSN based on several JeeNode and a Jeelink as central node. The JeeLink/JeeNode have a RF12Demo sketch with which it is possible to easily create a mesh network. The JeeLink is connected to a computer through USB. JeeNodes can communicate wirelessm using the on board RF12 chipset. All you have to do is configure the two units s that they can ‘find’ each other. See POF03 @ JeeLabs for an instruction on how to do that.

After I made the setup of my nodes and got the sketch started I saw data being displayed in my serial monitor. The JeeNode (sensor node) displayed information about the light and temperature on screen and sent the data to the JeeLink. I saw valid messages in the serial monitor of the JeeLink. The data shown by the JeeLink was something like:

OK 33 103 113 175 65

This line contains the measured temperature sent by the JeeNode. I knew that the value measured and sent was related to 21.93 degrees Centigrade. So somewhere in the sequence could be found. When you take a look at the Arduino Reference page about float (floating-point variables) you find that a float is represented by 4 bytes. So 4 of the 5 numbers in the message will together form the floating point number.

I wanted to know what 4 bytes were sent by the JeeNode so I did put in some extra Serial.print statements to display not only the measured temperature of (in this case) 21.93 degrees but I also the seperate bytes. To do this I had to ‘convert’ the float into an array of bytes. The lines below show you how:

float temperature;
byte  *ArrayOfFourBytes;

temperature = 21.93;

ArrayOfFourBytes = (byte*)&temperature;

Serial.println(temperature); // print the temperature

Serial.println(ArrayOfFourBytes[0],BIN); // print the 'first' byte in binary notation
Serial.println(ArrayOfFourBytes[1],BIN); // print the 'first' byte in binary notation
Serial.println(ArrayOfFourBytes[2],BIN); // print the 'first' byte in binary notation
Serial.println(ArrayOfFourBytes[3],BIN); // print the 'first' byte in binary notation

The printed binary numbers were:

1100111
1110001
10101111
1000001

In decimal notation equal to 103, 113, 165 and 65 which are the last 4 numbers of the received message. This still doesn’t look like 21.93 but we’re getting there.

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A float is represented by 4 bytes which are equal to 32 bits. The printed binaries above are not all 8-bits but three of them are 7-bits. This is because the preceding zero’s are left out and you have to place them yourself in front of the 7-bits.
Al together these 32-bits give you three pieces of the puzzle; a sign, a exponent and a mantisse.

The formula for a IEEE 754 float is:  sign * 2exponent * mantissa  , where sign is +1 or -1

There are many of pages that explain more about floating-point numbers and their representation in bits, together with formulas. You can also find websites with converters that let you enter a number in the different representations and show that number in a binary, hexadecimal, float or decimal representation. I used this website.

I entered 21.93 in the converter and got 01000001 10101111 01110000 10100100 (in bytes this is equal to 65 175 112 164). This isn’t equal to 103, 113, 165 and 65; except for the 65 which might be a coincidence or not.
When I entered the original bit sequence with preceding zeros 01100111 01110001 10101111 01000001 I got the float number 1.1413232E24 and not 21.93.

I know that there are more possibilities of ordering and combining bytes to get e.g. a float. You have big-endian and little-endian (see Wikipedia about Endianness). This has a relation with the hardware platform you are using.

So I tried the sequence: 01000001 10101111 01110001 01100111 (which takes the ‘last’ byte first, then the third, second and first byte). The converter now gave me the floating-point number  21.930372 which is not exactly 21.93 but close enough to continue.

The explanation for the difference between 21.93 and 21.930372 can be found on the Arduino Reference page about Serial.print. Where the second example shows: Serial.print(1.23456) gives “1.23”, in other words the float is rounded or capped to two decimals.

This difference also explains the difference in bit sequence returned by the converter.

Knowing this I can continue sending, receiving and translating data to build my own WSN solution with central reporting functionality.

 

Synology and Java

For now it like to investigate the use of a Synology (DS-107+) NAS together with Java. This might be a good way to connect a JeeNode/Arduino to the internet. In the future I like to use a Raspberry Pi to connect to the internet, together with other functions of course.
Important is that I can run Java code in combination with the serial communication capabilities of the rxtxSerial library. I found a really nice post on how to install Java on a Synology, because I really don’t want to loose the steps described I’ll cite the text of the post below:


1. install ipkg
http://forum.synology.com/wiki/index.php/How_to_Install_Bootstrap

2. install jamvm

To start with you need to install the following packages with ipkg:

ipkg install classpath
ipkg install jamvm
ipkg install jikes
ipkg install zlib
ipkg install file

Once these have installed then you are almost ready to go.  Being a java developer, I was used to using javac and java to compile and execute programs, so jikes and jamvm didn’t sit well with me!  To get around this I created a symbolic link and a shell script in /opt/bin
These are as follows:
/opt/bin/java

ln -s /opt/bin/jamvm /opt/bin/java

/opt/bin/javac

/opt/bin/jikes -classpath /opt/share/jamvm/classes.zip:/opt/share/classpath/glibj.zip $*

Remember to change the permissions on the shells scripts to add the execute flag!

I found out that there is also a  rxtxSerial ipk package which can be installed on the Synology. To do this just enter ipkg install  rxtx_2.1.7r2-1_arm.ipk on the command-line (you have to be logged in as root).

The cited post referred to this page.
Another page about Java and Synology is found here.

I’m wondering what is possible having all this installed (and what I need to install further). The code I tested uses URLConnection related classes and is not working yet … will be continued.

How to communicate using URLConnection in Java

Reading data with serial communication between Arduino and PC is one step. After the PC has received the data I want it to send the data (raw or after some calculations) to a website. The website will store the data and show the data in some nice graphs and tables.
The other way around I want to send some commands from the website to the PC where the Java app has to send it to the Arduino which on its turn will perform some actions. I will write another post on this subject some other time.

I found a nice page at Oracle describing how to use the Java URLConnection class.

Depending on the functionality of your website script/app you could start very simple with the code below.


import java.io.*;
import java.net.*;

public class Reverse {
    public static void main(String[] args) throws Exception {

	URL url = new URL("http://www.your-domain.nl/store_value.php?type=KAKU&value=1234567");
	URLConnection connection = url.openConnection();
	connection.setDoOutput(true);

	OutputStreamWriter out = new OutputStreamWriter(
                              connection.getOutputStream());
	out.close();

	BufferedReader in = new BufferedReader(
				new InputStreamReader(
				connection.getInputStream()));
	in.close();
    }
}

All we do here is opening an URL to a PHP page and provide it with two variables. These variables can be extracted from the URL by the PHP script. The snippet below shows a piece of PHP code to extract the variables from the URL. After you got the variable values they can be stored in a database, written to a file, etc..

<?php

$type = $_GET['type'];
$value = $_GET['value'];

// store it in a database or write it to a file or ...

print "TYPE = ".$type." -- VALUE =".$value."</BR>"; 

?>

I know it is very basic and not secure in the dangerous open network world (internet), but as a starting point for private network environments it might be helpful.

 

 

Is there light? LDR sensor input

You can imagine it is helpful to know when it gets dark in or around the house. Knowing this you could turn on the lights automatically.  There are different sensors you can use to measure light intensity. At the moment I have some LDR (light dependent resistor)  elements to do some tests.

An LDR has a  high resistance when no light is sensed, the resistance will decrease when the sensor is illuminated (see also).

A basic test scenario is to connect an LDR together with a 10K Ohm resistor to your Arduino.

Together with the code snippet below you can experiment with more or less light. I don’t know whether the printed values are close to reality but you can give it a try.

/* Photocell simple testing sketch.

Connect one end of the photocell to 5V, the other end to Analog 0.
Then connect one end of a 10K resistor from Analog 0 to ground

For more information see www.ladyada.net/learn/sensors/cds.html
Modified by M.A. de Pablo. October 18, 2009.
Thanks to Grumpy_Mike for equations improvement.
*/


int photocellPin0 = 0;     // the cell and 10K pulldown are connected to a0
int photocellReading0;     // the analog reading from the analog resistor divider
float Res0=10.0;		  // Resistance in the circuit of sensor 0 (KOhms)
// depending of the Resistance used, you could measure better at dark or at bright conditions.
// you could use a double circuit (using other LDR connected to analog pin 1) to have fun testing the sensors.
// Change the value of Res0 depending of what you use in the circuit

void setup(void) {
  // We'll send debugging information via the Serial monitor
  Serial.begin(9600);  
}


void loop(void) {
  photocellReading0 = analogRead(photocellPin0);   // Read the analogue pin
  float Vout0=photocellReading0*0.0048828125;	// calculate the voltage
  int lux0=500/(Res0*((5-Vout0)/Vout0));	     // calculate the Lux
  Serial.print("Luminosidad 0: ");		     // Print the measurement (in Lux units) in the screen
  Serial.print(lux0);
  Serial.print(" Lux\t");
  Serial.print("Voltage: ");			     // Print the calculated voltage returned to pin 0
  Serial.print(Vout0);
  Serial.print(" Volts\t");
  Serial.print("Output: ");
  Serial.print(photocellReading0);		   // Print the measured level at pin 0
  Serial.print("Ligth conditions: ");		// Print an approach to ligth conditions
  if (photocellReading0 < 10) {
    Serial.println(" - Dark");
  } else if (photocellReading0 < 200) {
    Serial.println(" - Dim");
  } else if (photocellReading0 < 500) {
    Serial.println(" - Light");
  } else if (photocellReading0 < 800) {
    Serial.println(" - Bright");
  } else {
    Serial.println(" - Very bright");
  }
  delay(1000);
} 

Ladyada has a page with information about photocells, light, lux and Arduino. This page has a great sample scheme including a code example.

TMP36 – Temperature sensor

I picked the TMP36 (datasheet) to measure temperature. It’s a cheap and easy to use sensor with a broad range of temperatures it can measure. Several websites describe how to use it in combination with an Arduino, so it is easy to test it. In this post I use the information of several sources to describe how to use the TMP36. At the bottom I give the links to all sources.

The TMP36 can be used with voltages between 2.7V and 5.5V. The analog output lays between approx. 0V (ground) to 1.75V.

There are two formulas to calculate the voltages at the input pins,
one if you use 5V:

Voltage at pin in milliVolts = (reading from ADC) * (5000/1024)
This formula converts the number 0-1023 from the ADC into 0-5000mV (= 5V)

And one to use with  3.3V:

Voltage at pin in milliVolts = (reading from ADC) * (3300/1024)
This formula converts the number 0-1023 from the ADC into 0-3300mV (= 3.3V)

To convert the milliVolts to Centigrade use the formula:

Centigrade temperature = [(analog voltage in mV) – 500] / 10

Below is a code snippet that will ‘read’ the temperature in Centigrade.

/*  
*  Sample sketch to measure the temperature using a TMP36
*  The TMP36 is connected to 5V and analog pin 0
*/

// The Arduino pin the TMP36 signal pin is connected to
int tempPin = 0;    

void setup()
{
  Serial.begin(9600);  
}

void loop()
{
      //  Get the value from the sensor
      int readvalue = analogRead(tempPin);       
      float voltage = readvalue * 5.0;                     
      voltage = voltage / 1024.0;                    
      float centigrade = (voltage - 0.5) * 100 ;
      Serial.print(centigrade);
      delay(5000);
}

Ladyada.net descibes the sensor in this page.
Oomlout describes an example on its page.
Tronixstuff has also a description of how to use it.

To use the TMP36 with a JeeNode you can connect it:
TMP36 V+ (pin1) to JeeNode (+)
TMP36 Vout (pin2) to JeeNode Analog (A)
TMP36 GND (pin3) to JeeNode GND (G)