Strobotics

I’m still alive…… Its been awhile between blog entries, unfortunately this is the busiest time of the year for me at work, the few months after the Australian financial year, projects are kicking off due to funding etc.  There is however light at the end of the tunnel and I’m hoping to get back to geeking very soon

I thought I’d let you know that a) I’m still alive, and b) what’s been happening on the development front.

In between work projects I’ve been porting the Contiki operating system to the widget as the underlying event framework for WidgetMesh.

I now have the Hello-world application up and running on the Atmega168 @10Mhz, no radio drivers or anything else for that matter just yet.  For those interested it’s been checked into SVN.  I’m learning a lot about Contiki and how it all fits in together (i.e. I know that I don’t know much at all about it)

On the hardware front the next iteration of the Widgetboard is idle, though pretty well stable as far as the design goes (might be a couple of small tweaks before boards get made).  I just need time and resources to get the next round of PCBs ordered and tested. 

Noteworthy design changes include:

• Standard smd or PTH crystal footprint, allows the use of a standard HC49 crystal of your choice.
• Lots of changes to the board supply side of things. Board voltage supply input range from 0.7V to 16VDC (probably beyond to 20VDC but just to be on the safe side…)  The core components are still powered  @ 3V
• Minor changes based from user feedback of the current board.
• A couple of optional onboard sensors to utilise the normally unused A6 and A7 analogue only ports, these can be disabled via onboard solder jumpers if these ports need to be used for something else, but has a temperature and light sensor, so the widget board can be used as a basic standalone wireless sensor.

I’ve been meaning to run off another batch of WidgetBoards and have been waiting for some processors, a month ago 40 Atmega328P processors arrived after a 5 week backorder, according to my vendor these were hard to get, hopefully supply issues will settle down after production catches up with the the initial demand for Atmega328P. 

All future widget boards will now have this processor as the default in the next batch of boards I make.

Sample parts I’ve been ordering for various future projects have been steadily rocking up and sitting in my ever growing inbox, the RFM22 arrived some time ago and I’ve not yet had time to play with them either   sigh

One of the tasks I want to use the widget boards for is a Wireless Sensor Network around the house for measuring environmental values.  The sensors that I want to live outdoors will need to be self sufficient in terms of power, so I wanted to see if I could charge the batteries via solar, of course this can be done, but again I want to do it as cheaply and as simply as possible, I also want the widget board to monitor the solar voltage and report it back.

I already had a 3.6v 0.2w cell that I picked up some time ago from Sure Electronics for about $10 for 5.  The spec for this cell is max voltage (under load) is 3.6v and max current is 60mA, more than enough to trickle charge x2 AA NIMH batteries. 

The Batteries should not require much of a top up as the node will be sleeping most of the time, from the power saving discussion @ JEELabs, by adding the solar, I should be able to just leave the node without having to change batteries (replacing failed batteries aside).

To measure the voltage produced by the solar cell I need a voltage divider that would produce 1.1v max @ 3.6v.

Why 1.1v I hear you ask?  The reason for the A/D full scale input voltage of 1.1V is that I’ll be using the ATMega168 internal bandgap reference as the AREF source for the onboard A/D and not the default AREF. 

There are a couple for reasons for this.  The internal bandap reference itself is a 1.1v voltage reference, because the battery voltage may change over time (remember I’m not using any onboard voltage regulators) so as the VCC / battery voltage changes, so would my A/D voltage readings unless I use a reference voltage.   Now as the reference is 1.1v this means that my input must not go above 1.1v else I will go over scale and get an incorrect reading.

I’m not going to bore you with the math so a quick calculation using an online voltage divider calculator (http://www.daycounter.com/Calculators/Voltage-Divider-Calculator.phtml)

Input Voltage = 3.6v

Output Voltage = 1.0v  (remember I don’t want to go over 1.1V so I’ve given myself some headroom)

Gave the values for R1 = 26K and R2 = 10K

Luckily I had both these values, however for the 26K I only had  one in 5% tolerance, the 10K I had as 1% (I like to use low tolerance resistors for voltage dividers so my error is minimised),upon testing the 26K resistor actually read 26.4K on my meter, so after plugging these real values in my actual FS Vout is 0.989v, pretty close to 1.0V.

A blocking diode is also required to prevent any damage to the Solar cell by the batteries, see the circuit and photo below.

(Click for larger image)

(Click for larger image and notes)

Arduino Sketch to read the voltage produced by the solar cell, next step is to find a waterproof housing and add some environmental sensors. 

Another benefit of taking voltage readings of the solar cell is I now have a light sensor.

int raw_solar = 0;        // Raw Solar FS ~ 1.0v

int raw_bandgap = 0;      // Intenal bandgap reference

float volt_solar = 0.0;

void setup(){

  Serial.begin(57600);

  analogReference(INTERNAL);  // select internal reference for AREF

}

void loop(){

  raw_solar = analogRead(0);                         // read solar voltage raw values

  volt_solar = (raw_solar * 1.1 * 3.617) / 1024;     // Scale ADC input.  (voltage divider r1= 26.4k  r2 = 10k  vi=vo/(r2/(r1+r2)) = vo/.2777 =  vo * 3.617

  Serial.print(volt_solar);                          // display raw solar voltage reading

  Serial.println("v");  

  delay(1000);                    

}