Saturday, September 14, 2013

Determine a magnet polarity

Knowing the polarity of a magnet is important in order to use the same polarity when more than one magnet are used in a motor for example.
Here some notes about how to determine the polarity of a magnet.

There are many different ways to determine the polarity of a magnet :

  • using a compass or equivalent
  • suspending the magnet freely and looking at the final direction (it is assumed to know where the Earth North pole is)
  • comparing it with a known magnet
The most important thing is to know something about the standard.
In USA the "North" pole of a magnet, defined by the National Bureau of Standards (NBS) convention,is based on the following: 

"The North Pole of a magnet is that pole which is attracted to the geographic North Pole. 
Therefore, the North Pole of a magnet will repel the north seeking pole of a magnetic compass."

The magnets

For the experiment we'll be using round shape magnets, easier to be glued to a rotor.
Something like the ones in the picture.

On a disc, the poles are present on the surface.

Marking the poles

In order to figure out what pole is on a surface, the fastest method is to use a compass or an equivalent electronic one.
Pointing the disc surface toward the compass, will force one of the two side of the row toward the disc.
The North pole of the magnet will attract the South pole of the compass.
The South pole of the magnet will attract the North pole of the compass.

Since all the smartphones have a magnetic sensor (Hall effect sensor) it is relatively easy to build an application capable to identify a pole.
On the market there are already many different applications that do so.
For the purpose to identify what is the pole of a magnet we choose the Magnetic Pole Detector.
With this application is easy to identify the pole of a magnet.

It is enough to place the magnet with one surface toward the smartphone and push a button to see what is the pole.

The pictures show a measurement.
The face of the magnet oriented toward the phone, is the South pole of the magnet.
The opposite side is the North pole.

There are of course also many specific tools for this purpose if you down own or want to use a smartphone.

Here few examples :

Saturday, September 7, 2013

Alphaline mod. 18281

Since I have a nice bricked Alphaline MP4 player, and nobody willing or able to service it,  why not take it apart and see what is inside ?

Open the player

The shell of the player is in plastic. No screws, only small plastic hooks.
With a very small screwdriver, start to gently force close to the bottom angles of the player, where the USB connector and phone connectors are.
Then very gently start to work all around the border of the player.
A certain point it will be possible to user the fingers to gently separate the shells.

The inside

After the shells are separated, the PCB and the battery appears (the gray "thingy" on the right).
3 big chipsets are visible, plus the microphone and the pinhole camera.
Also the micro SD card reader, the USB connector and the headphone jack are visible,

The three chipset are so marked :
  • ATJ2259C - ZC55LBB B7A
    The microcontroller - So far I was unable to retrieve datasheet or even basic information about this controller.
    It is a Chinese processor and in the typical Chinese way, no information are released.
    Pity. We should really stop to buy stuff produced in China.
  • Infineon HYB39S25616odt
    HYB39S256160DT-7 - 256-MBit Synchronous DRAM
  • PF369 - AR - 1134
    Probably LCD/touchscreen manager but like the processor I was unable to find information.

Wednesday, September 4, 2013

Smartphone audio cable adapter

Unfortunately doesn't seems in the smartphone and tablet world, to exist a single standard for the audio connectors (see article about the smartphone audio jack).

On the market do exists cable converter of any kind, however they are often unnecessarily costly.
If you need an audio jack adapter between the main two standard in the smartphone world, OMTP and CTIA, here what I did.

Shopping list

Quite easy ... we need three things :
  • a 4 wire cable
  • a TRRS male jack
  • a TRRS female jack
I solved buying the needed material from Sparkfun.
I ordered a TRRS male jack with 18" of cable (CAB-11580) for $1.95 and a TRRS jack breakout (BOB-11570) for $3.95.
The TRRS jack breakout

It is possible maybe to find just a TRRS female for less, but the breakout is more flexible since it can be used also to provide a test platform (hook up oscilloscope, ecc.).
For about 9$ (the material plus shipment) is possible to have all the necessary to build the adapter.
A ready made cable converter can be found usually around 11$ plus shipment, but  the advantage to build it is that can be modified for other type of connections.



Well, is really quite straightforward.
It is simply matter to connect the Tip and first Ring directly, and then swap  the other two.
The TRRS jack with the cable has this pinout :

  • Tip - red wire
  • Ring - white wire
  • Ring - green wire
  • Sleeve - black wire
On the PCB are marked the positions, so it is straightforward to connect the wires.