Articles, tutorials & how-tos on electronics, Arduino, 3d printing, and more. Here’s the latest articles. Enjoy!

Open Loggage – open source data logger for packages

Every time I fly and have to drop off my luggage at the check-in counter, I wonder what goes inside it during the trip.

  • Is it stored in a cold location? At high altitudes, external plane temp can reach -30F. Is my stuff exposed to that extreme weather?
  • Does it gets opened when I’m not around? As far as I know only customs and security have permission to open the luggage; no one else.
  • What about being dropped? Throw a few feet is probably expected, but how about a 3-meter free fall? Not cool!

To answer those question I decided to create a small device that I could place in my luggage and record some environmental data around it. The idea for OpenLoggage was born!

I have decided to make it an open-source hardware and software project. For several reasons:

  • get the community to collaborate on the application software
  • have fun in this project!


Check out these slides I put together and presented at an Arduino Minnesota meeting in March 2014. It shows a bit more about the project:



Proto1 – Credit Card size

This was the first OpenLoggage board. It’s credit card sized.

It was used to test out all the subsystems. After all is tested, I created proto board 2 where all has been shrunk down.

Here’s what it looks like.



Proto 2 – Gum Stick size


As you may imagine by the name of this board, the Gum Stick board has the size of a standard stick of chewing gum: 2-7/8 inches in length, 7/8 inch in width.

The GS board is the second board I made. The first board was a credit card size and was done that big so that I could quickly test out components, cut traces, etc.

Funny fact: there’s a  Military Standard for chewing gum sticks.


Board Features:

  • Microcontroller: Atmel’s ATMEGA328 (same as in the Arduino Uno)
  • MicroSD card socket
  • Coin cell battery holder
  • Li-ion charging circuit
  • Sensors: temperature, light (LDR)
  • Size: standard chewing gum stick (xx , yy )
  • two unused I/O pins for expansion
  • Very low energy design!!!  (see slides above)

Customization Features:

  • Jumpers for selecting power source (USB, Li-Ion or Coin Cell)

Optional Headers:

I added these headers as an alternative of populating the components directly in the b

  • Header for Arduino Mini (? double check) in case you do not want to hand solder the SOIC chip on the board.
  • Header for RFduino (p/n xxxxyyyy). Easily add Bluetooth 4.0 (BLE) to your board.
  • Header for Accelerometer board (p/n GY-45 with Freescale MMA8451)
  • Header for RTC clock (p/n xxxxyyyy)

Circuit Board Layout




  • Download Gerber files
  • Eagle Cad schematic & layout (coming soon)
  • Sample Sketches & Library (to be ported from Proto1 board)


What’s Next

Plans for OpenLoggage:

(this section is my to-do list for this project. As time allows, I’ll work on the items below. If you’re interested in OpenLoggage, then check back soon)

  • Publish Finish BOM & order parts  (done)
  • Design: Finalize schematic (done)
  • release code, schematics, and BOM
  • add a forum for users to discuss features, how-to, etc
  • Get a better logo. Temporary logo below:



DTMF Decoder IC

If you are looking for a dtmf decoder, your best option is to use a dtmf decoder ic instead of trying to write up software.

Using software in your microcontrdtmf decoder icoller-based project (Arduino or other) to decode dtmf signals to me is like reinventing the wheel.  For sure the software approach is a “free”solution, since you’re not paying for a chip, but you’ll pay the price in development time and possibly performance. Robust dtmf decoding in software is not impossible but is for sure challenging and time consuming.

the easy way: dtmf decoder ic

When it comes to selecting a dtmf decoder ic to do the job, there are two that I recommend. I have been using both for some time now, in my MICO and DTMF boards, and they work great. Here they are:

  • Holtek 9170D link : this ic operates in 2.8 to 5v; and is easy to find for sale
  • Zarlink MT8840DE: this only operates at 5v and is a bit harder to find for sale from authorized distributors. mt8870 datasheet coming soon

Other than the operating voltage I conclude that both IC are functionally identical.

Side note: One interesting fact about these dtmf decoder chips. There seem to be several companies that make the exact same chip: Holtek, Mittel, Zarlink, to name a few. I’m not sure what’s the deal on why so many companies make the exact same die. It may have to do with company acquisitions over time… if I ever find out I’ll update this post.

Save time with dtmf decoder board

dtmf shield for arduinoIf your application require a dtmf decoder ic and you want to save time, feel free to grab the DTMF Shield for Arduino from This development board will help you to cut some days off your project development.

The dtmf shield is very easy to use. You just plug it into your Arduino, load the sample sketch and off you go.

did I mention this shield can also encode dtmf? that’s right, this shield is also a dtmf generator!




Phone Charger Current Comparison

I bought a spare phone charger on eBay several months ago, and I always wondered if that charger was a genuine product or a counterfeit. Looking at it there’s no way of telling. The eBay purchased charger has all the labels just like the original.

I then figured that a way to confirm the charger is genuine is to verify it can charge at the same current level as the charger that came in the box with the phone. And that’s what I set out to do in this experiment!

I’ll do a side-by-side comparison of the charging current of each charger, and I’ll plot the results for easy comparison. In order to do that I’ll use an Arduino Uno board to collect the current and will plot the acquired data in an Excel spreadsheet so that it is very easy to see how each charger compare against the other!


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