Category Archives: Uncategorized

Datasheets can lie!

So, when working on the permanent version of my weather station setup, I have installed at Atmega 1284p as the MCU, and have a DHT22 and WIZ820io ethernet module installed so far…

So, after getting a web server running on the board, which was reporting the temperature and humdity, it was time to power it standalone for the night… easy… just plug in the 5v power supply… or so I thought…

Plug in the power supply… and nothing happens. The WIZ820io lights just flashed for a moment and nothing else happened.

Plugged in the FTDI programming cable again and plugged in back into the computer… and it worked again! Strange. So I plugged in a external USB power supply, and powered it via the FTDI cable… and it also worked. So I left it overnight, and it was still happily running the next morning.

From the WIZ820io datasheet

Upon checking the datasheet… it reports that the WIZ820io module is 5v tolerant (with an absolute maximum of 5.5v). No problem… the power supply is outputting 5.3v, so is still within spec… or is it? So, I put it on a adjustable power supply, and run it at 4.8v… runs without any issues (and so it should… FTDI cable on the computer was outputting 4.7v) Anyway, ramped up to 4.9v… still ok… 5.0v… and the lights on the WIZ820io go out…

So it would appear that the 5.5v (max) tolerant WIZ820io didn’t like getting any logic signals above 5.0v! Since I didn’t want to use a fancy power supply… I just added a 1N4004 diode in line, and the resulting 0.7v voltage drop fixed it up perfectly. So the 5.3v power supply is now running my raspberry Pi and weather station as intended now!

Work in progress weather station / datalogger

Arduinoos blog -> Relays (Part 1)

The Arduinoos blog has been doing some nice blog posts lately about the Arduino – PWM, using SD cards, etc. This latest one is about using relays with the Arduino… food for thought!

Relays (Part 1):
Relays may look outdated in the world of microprocessors: they are big, greedy (in mA) and slow. However, some of their properties make them unavoidable still, when comes the question of: galvanic isolation and requirement for fully floating contacts.

Driving relays is no big deal as long as we care about the fit of the caracteristics from the driver and the relay. The simplest design consist in hooking one of the coil end to VCC (actually 5V with Arduino) and to drive the relay by switching the other end to ground. This will be acceptable as long as the relay draws less than 40mA, which is the upper current limit for Arduino digital ports. In other words, the resistance of the coil should not be more than (5 / 0.040) = 125 Ohms.

You will not see this design much because it fails to protect the driving port from the reverse voltage generated while opening the coil circuit. Fortunately, the solution if simple and cheap and it consists in adding a diode in parallel with the coil, so that the reverse voltage will be absorbed by this diode. A general purpose diode will suffice.

When the relay draws more than 40mA, it is necessary to use an interface between Arduino and the relay. The simplest way to achieve that is to use a general purpose NPN transistor, which Ice fits the current drawn by the relay.

Arduino digital port can drive up to 40mA per port, but the summ of these currents must not exceed 200mA. What shall we do now if we need to drive say 6 reed relays without transistors? Here are a couple of solutions. They are based of the fact that the current drawn by the relay is not linear and follows a transient curve. At switching time, the current must be at the maximum, but the need for sustaining the contact activated is lower in the steady state. At this precise time, it is not necessary to apply the full voltage to the coil.

If we are rich in digital ports, this is an option for driving the relay in eco-mode:

Both digital ports are set to 0 for swtiching the relay, and the closest port from the relay is set to input mode after a short delay (delay = time taken for the transient to happen)

An other option makes use of a capacitor. Before turning the coil on, the capcacitor is fully charged. At switching time, the capacitor shall be equivalent to a short circuit and let the whole current travel through the coil; it will then discharge and be transparent so that the current flows through the resistor which is set in parallel with the capacitor.

Mobile blogging

Whilst blogging on the go is not as easy as it is with other blogs such as WordPress through the use of a custom application or mobile-optimised website, Blogger is not without options. If you want, you can post text and images via you phone using SMS/MMS, or you can send emails to a special address – as I'm now doing (once you've setup your mobile device, of course!).

SKills that are learnt through use of blogs, wikis and podcasting…

After reading this blog post on, it was interesting to reflect on the poster images included in the blog entry, as they summed up the key message of the post. I have linked to three of the images that relate to the current theme of this blog (wikis, blogs and podcasting), and there were another two, one on video conferencing and the other about digital storytelling.



These images by Silvia Rosenthal Tolisano are licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

Word walls… the Web 2.0 way!

This is just one of the sample of what you can do with Wordle. In addition to creating your own, there are a lot of Wordles that other people have created which are freely accessible and search-able on the Wordle website. In a nutshell, if you hadn’t gathered already, Worldle lets you create colourful word walls. Why is this useful? You could use it to analyse a students writing (or they could do it themselves) to see what words they use repeatedly (and, the, then, etc). You could use it to identify/hight the key topics in a passage of text, or to identify the recurring theme in a online class discussion. These are just a few of the things you could do with this great free service!

Wordle: Period G

Children of Lesbian Parents Grow up Just Fine

After the first of three intensive Childwise / Classroom Management for the Contemporary Consumer (CMCC) sessions, it was interesting to go back to this news item, as one of the topics that was raised in the last session was same-sex relationships. Based on studies such as this, and feedback from programs such as Childwise, it doesn’t matter if there are both male and female partners in a relationship, only that they are seen to be working well together, providing a safe and secure environment for the child.

Study: Kids of Lesbian Parents Grow up Just Fine – CBS News: “Children raised by same-sex, female couples grow up to be psychologically well adjusted, experience academic achievement and exhibit healthy social behaviors, according to recent study.”

First post

This is my first post using the WordPress based blog. Will be interesting to see how this blog evolves over time, or if it will continue to evolve at all!

Google, Google, Google!

Been listening to the first episodes of This Week in Google (TWiG) and there are some really features and tips mentioned. No wonder it managed reach number one on the iTunes store on the first episode.

For example, a lot of the talk is about Google Wave, and how it will revolutionise how news is presented, and people’s expectations of the web. Can’t wait till I get my invite! Also, there was a nice tip about using filters and the labs canned response feature to create automated responses to emails from specified people, or with specific keywords or subjects. Cool!

Talk about HTML5 was promising, which will bring with it the capability to do stuff like play video and audio without needing plug-ins such as QuickTime or Flash, which I will appreciate, as my web browser will crash less!

And what about the google search sandbox for the  update to the google indexing engine… don’t see much difference in the results, but I do like the code name – ‘caffeine’!

I like the sound of the new ‘social’ features, such as following friends and other gmail contacts, and the send to feature, allowing you to send items to other sites or services, allowing you to, for example, post the article to your own site, or create a pdf from it.  And the ‘mark items as read after x days’ feature is well overdue, and very much appreciated.

It’s Friday!

Well, it is the end of the week, and I’ve taught my last classes for this week. I am glad to say that the feedback from my primary mentor teacher has been steadily improving, and I’m getting more comfortable with the classes. This is the last day of my block prac, and I’m reverting back to one-day-a-week visits, and will be coordinating with my mentor teachers in order to work out what I’ll be teaching on those days.


Just tried a free online serivce called MyPodcast, and it’s great. They host your podcast on their website for free, and it’s pretty customisable. To make things even easier, you can download their client software, which lets you record the podcast using a very simple and easy to use interface. So, stay tuned for podcasts in the future…