Arduino workshop to be held at AAPT meeting

This is the first time an Arduino workshop it to be held at a physics national conference:

American Association of Physics Teachers summer conference in Omaha, NE, July 30, 2011

Search (ctrl+F) for session W20. I don’t know who is organizing this workshop yet but I’ll be there to gauge interest in using arduino for physics teaching.

I will also give a talk on arduino and building teaching apparatus at the meeting. Lots of slides to make and finishing touch to my uniform circular motion apparatus for a competition.

How to calculate current-limiting resistor for LEDs

Here is typically how you calculate the “current-limiting resistor” for an LED.

You will need to know ahead of time:

1) Supply voltage Vdd, in case of the typical 5V arduino boards, Vdd=5V

2) Typical forward bias voltage of the LED Vfb, read the spec sheet. My personal experience is that more powerful LEDs have higher value of Vfb.
Take this one for example:

A range of 3.0 to 3.6V is given. I would try to use the average of 3.3V. Some other spec sheets give typical values and you will use that value.

3) Current rating i_m. The above LED is said to have 20mA current rating. It’s the maximal sustained current you want it to have. Again some other spec sheets give you current at typical forward bias voltage and you can use that value.

Now to calculate the current-limiting resistor, just do this:

Vdd=Vfb+Vr, where Vr is the voltage dropped on the current-limiting resistor. So Vr=i_m*R. Where i_m is the LED current rating.

So R=(Vdd-Vfb)/i_m

For the above LED, R=(5V-3.3V)/0.02A=85 ohm. That is when you are pushing a little bit. Plus, arduino pins can only supply 20mA and the entire ATMEGA chip can supply 200mA. So to be safe, I would use a 150 ohm resistor. Then the LED will be less bright. If you use the LED for lighting purpose, its brightness is relevant to you. But you use it as an indicator instead of illumination, make it not that bright by using larger resistors.

Typically, if an LED is passing 1mA of current, you can see it in the dark, or straight on in a room but hard to see from sideways. If you run 20mA on an LED, it hurts your eyes to stare at it.

The following is taken in a dark bedroom. I believe only 3 LEDs were on together, passing around 6mA each only 🙂

More update to the new phi_prompt library

Here is a teaser showing off scrolling text areas using phi_prompt library that I will release soon. The text area can be any size and be displayed anywhere (no clipping on the display edge!). You also have the option to sacrifice a whole column of characters to display a DOS-looking scroll bar, tailored to the size of your text area and text length! Here you go mowcius!

The top line is just a fixed text output so it doesn’t scroll.

Phi_morse library is rolled out

Now if you want Morse code functions and can’t do it yourself, you can just download this library and call morse_out(“HELLO”);

Check out the library page:

Phi_morse library page

Phi_super_font available

After the success with Phi_big_font, I’ve created a Phi_super_font to use on 20X4 displays. The font is 20 times the size of standard font on character LCDs. Check out its library page! I’m in the process to add some cosmetic feature but the default options are already good enough to distribute 🙂

The last 6 months

For the last six months or so, the number of visits to my blog has increased drastically as I released article after article on Arduino and projects, a number of software libraries, and quite a few hardware for arduino. Thank you very much for visiting. Please keep tuned as I will add more contents in this coming months.