Article written

  • on 28.04.2005
  • at 01:29 AM
  • by Peter

Motion-Based iPod Remote Control 0

The goal of the motion-based iPod remote control was to design and build a remote control for the iPod that responds to a user’s motion. An accelerometer is used to detect the motion of a user. A microprocessor then interprets these movements and sends the appropriate commands to the iPod via its remote control port.

Introduction

During the cold winter months in Wisconsin, avid iPod users find difficulty in comfortably controlling their MP3 player. In order to manipulate the touch-based iPod controls or Apple’s standard remote control, pockets must be opened and gloves or mittens removed, losing precious heat to the cold. One possible solution would be to use general body motion to control the iPod as opposed to touch. This would require no dexterous tasks, only general movements

This same concept would also make controlling an iPod easier for active people (eg. riding a bike – you can keep your eyes on the road and don’t have to use fingers) and those who lack dexterity and can’t use their fingers to manipulate the iPod controls (eg. arthritis sufferers).
Summary

The goal of the motion-based iPod remote control was to design and build a remote control for the iPod that responds to a user’s motion. An accelerometer is used to detect the motion of a user. A microprocessor then interprets these movements and sends the appropriate commands to the iPod via its remote control port.

Hardware Components

  • iPod: If you’re here, you probably know what one is. Check them out here if not: www.apple.com/ipod. This project is tailored to the 3rd generation iPod (and should be compatible with 4th, U2, photo, mini revs too), but can be adapted to work with any generation.
  • Accelerometer: A Memsic MXD2125 accelerometer was used to sense the user’s motion. It is a two dimensional accelerometer and senses acceleration in the range of -2g to 2g. It uses pulse width modulation (PWM) to output acceleration measurements. It measures both static and dynamic acceleration. Internally, it heats up a bubble of air and uses an array of thermocouples to detect its position and thus acceleration.
  • Microcontroller: An ATmega8 microcontroller was chosen. It is an 8-bit AVR from Atmel. It polls the accelerometer for data, interprets the readings, and sends commands to the iPod. The ATmega8 was a bit overkill but had all the necessary features (Interrupts on IO pins, onboard UART, internal oscillator, 16-bit timer, etc) and also enough flash and memory to expand the remote in the future. An ATtiny2313 was also used during the development process.
  • RS232 Line Level Converter: A Maxim MAX3233E RS232 line-level converter was used during development to translate the serial output of the microcontroller to RS232 voltage levels needed by a PC’s serial port. Internal charge pumps are used in the MAX3233 which reduces necessary external circuitry.

iPod Remote Port

The 3rd generation iPod has a remote connector on its top next to the headphone jack.

The remote connector has 4 pins- VCC, GND, TX data, and RX data. The remote connector supplies power to the remote control as well as receives its commands. For pinouts, check here.

ipod top

Top of ipod 3rd Generation iPod viewed from above.

iPod Remote Protocol

In order for the ATmega8 to send commands to the iPod, it had to mimic the standard iPod remote control’s output. This reverse engineering of the iPod remote protocol can be done by hooking up an oscilloscope to the remote control Apple sells and monitoring its output when different buttons are pushed. Fortunately, this step had already been mostly done by someone else. The iPod remote uses a 7-byte standard 8N1 19200 baud serial protocol. The commands contain a checksum and you’ll also notice that the “command byte” (byte 6) using bitmapping so that multiple commands can be sent at once. The commands are defined as:

  • Play/Pause: 0xFF 0×55 0×03 0×02 0×00 0×01 0xFA
  • Volume UP: 0xFF 0×55 0×03 0×02 0×00 0×02 0xF9
  • Volume DOWN: 0xFF 0×55 0×03 0×02 0×00 0×04 0xF7
  • Skip Ahead: 0xFF 0×55 0×03 0×02 0×00 0×08 0xF3
  • Skip Back: 0xFF 0×55 0×03 0×02 0×00 0×10 0xEB
  • Button Depress: 0xFF 0×55 0×03 0×02 0×00 0×00 0xFB

Initial Prototype

An initial prototype was created using a breadboard. It utilized an ATtiny2313 rather than the final ATmega8. Although the ATtiny2313 could have been used in the final implementation, its 2Kbyte flash limited its possibilities for growth. The accelerometer was soldered to a Memsic MXEB-002 eval board then connected to the breadboard via 4 wires. The accelerometer eval board (sans accelerometer) can be seen in the upper left of the image along with the rest of the prototype setup.

breadboard
Prototype setup.

Schematic/PCB Creation

The prototype circuit was drawn up by hand and was relatively straight-forward. The next step was to create an electronic version of the schematic and its corresponding PCB. PowerLogic and PowerPCB were used to create the schematic and PCB, respectively. To reduce the final size of the remote itself, the MAX3233E RS232 converter and programming circuitry was placed on an external “debug board” which would connects via a flat flex cable to the “remote board” which has the accelerometer and microcontroller.

The schematic entry was straight-forward, however the PCB layout was not quite as simple. Decals had to be created for non-standard symbols, design rules checked against the PCB fabricator rules, etc. This may be easy for someone with extensive experience, but was a new experience for me and took a good deal of time. PowerLogic and PowerPCB are designed to work together and thus when the schematic would change, the change could be “forwarded” to PowerPCB with little hassle. The process took a while, but was a good experience all-in-all. The final remote has 1 red power LED and 3 general purpose LEDs connected to some of the ATmega8′s spare IO pins. The packages were LLC8 for the accelerometer, TQFP32 for the ATmega8, and 603s for the discrete components.

debug setup

Debug setup Debug board (left) <-> Remote board (right).

debug sch
Debug board schematic Debug board schematic.

remote sch
Remote schematic Remote board schematic.

Programming/Serial Debug

As mentioned above, the “debug board” contained the circuitry necessary to program the ATmega8 via a PC parallel port as well as convert its serial I/O to standard RS232 serial port voltage levels. The “debug board” connects to the parallel and serial port of a PC via a RJ45 jack which split into two connectors. RJ45 headers were used on all ends of the cable and the serial and parallel connector had female RJ45 connectors on them. Only 7 signals were needed, so standard ethernet cable and RJ45 headers worked great. I’ll draw up a wiring diagram for the Programming/Serial cable one of these days (it can be derived from the schematic if necessary).

To connect the remote control to the iPod, a connector was fashioned using an old ISA card and some 2-row header pins. Berg pin connectors were crimped onto the ends of the wire then insulated with heat shrink. Credit goes to the the iPod Linux project for the idea. Check out putting Linux on your iPod while you’re at it.

An sp12 programmer was used to program the ATmega8. It is a software/hardware combo that has programming software under both Linux and Windows. It uses a standard parallel port. The sp12 hardware is located on the “debug board”.

serial programming cable
serial debug and programming cable Programming/Serial debug cable + headers.

isa connector
ipod remote connector based on old ISA card ISA connector-based remote connector.

ipod remote port
remote connector plugging into the ipod Connector + iPod.

Result

The final remote control was roughly the size of a quarter, slightly smaller than the standard Apple-supplied remote:)
finished remote Final remote.

finished remote

Software Components

  • Development Environment: avr-gcc was used to compile code for the ATmega8. avr-gcc allows a host computer (x86 in my case) to cross-compile for an AVR target using the standard GNU tools like make, objcopy, ld, etc. avr-gcc is supported under both Windows and Linux(which I’d recommend:). All code for the project was written in C.
  • AVR Programmer: As mentioned above, a sp12 programmer was used to download the compiled files to the ATmega8 via a PC parallel port. The software to program it is available for both Windows and Linux.

Reading Accelerations

The MXD2125 accelerometer has two outputs – one for X accelerations, one for Y. These accelerations are represented by the duty cycle of a square wave. A 50% duty cycle is equivalent of 0g. Duty cycle’s above 50% are positive accelerations, duty cycles below 50% are negative accelerations. A formula provided by Memsic converts the duty cycle reading into acceleration. To read the accelerations, the INT0 and INT1 pins of the ATmega8 were connected to the MXD2125 outputs. These pins allow triggering of external interrupts. Additionally, they can be configured to trigger on positive or negative edges, or logic level changes. This allows the ATmega8 to read the duty cycle using the following process:

  • Interrupt on a positive edge, record the time
  • Reconfigure the pin to trigger on a negative edge
  • Interrupt on negative edge, record time
  • Disable interrupts on the pin
  • Repeat the process on the other input, thus reading both accelerations

These times in addition to the constant period of the MXD2125 output can then be used to calculate the duty cycles, and thus the accelerations.

duty calculation

Interrupt-based duty cycle calculation.

Interpreting Accelerations

This was definitely the hardest part of the remote. The accelerometer detects both static and dynamic acceleration in 2 dimensions. Therefore, it can detect movement in a plane, or, using simple geometry, the angle of tilt of the sensor. The angle of tilt is measuring what portion of Earth’s gravity is acting horizontally on the sensor rather than acceleration caused by movement.

Since everyone moves at different rates while gravity is constant, it is much easier to accurately detect tilts. Thus the angle of tilt of the accelerometer was most often used in the algorithm to interpret commands. A state machine was then implemented in software which would interpret the user’s motions into commands. To prevent the remote from generating random commands while running, waving, etc, the remote must be put into a “command mode” by holding the remote flat for approximately 1 second before it begins issuing commands. Once the remote begins accepting commands, multiple commands can be issued until the remote is held still for approximately 1 more second.

The movements could be more complex, but for the purposes of a prototype, the above worked fine and demonstrated the remote’s functionality. In the movie section below, you can see a different detection algorithm that responds to circular motions to control volume.

tilt calculation

Tilt calculation.

state machine
Command recognition stat machine.

Source Code

To be released…

Result

The final result was a success. The Motion-Based iPod remote control met the goals set out at the beginning of the semester. It was a lot of fun to design and it I got two independent study credits out of it to boot:) It won first prize for individuals at the University of Wisconsin Engineering Expo this fall as well. The final implementation was programmed to be worn on the user’s wrist, but could be reprogrammed to be worn on a headphone, in a pocket, etc. I’m currently pursuing the possibility of getting the remote or some of its technology patented via the Wisconsin Alumni Research Foundation.

Future Possibilities

  • More complex motion detection
  • Wireless
  • 3D accelerometer
  • TV/DVD/VCR/etc remote
  • Audio feedback

Movies

tilt_control_mov.jpg

This movie demonstrates the Play, Volume Down, Volume UP, Fast Forward, and Pause commands.

close_up_mov.jpg

This movie is a close-up demonstration of a Pause, Play, Volume Down, Volume Up, Fast Forward, and Rewind commands. Notice the small amber LED on the remote. When lit, it signals that it is ready to accept commands. Another amber LED blinks when a command it sent giving visual feedback to the user.

wax_on_volume_mov.jpg

This movie demonstrates an alternate detection algorithm to detect volume up and down commands. The remote is configured to interpret clockwise circular motions as “Volume Up” and counter-clockwise circular motions as “Volume Down”. It is meant to allow the user to mimic the scroll wheel finger motion with their arm. More creative patterns could be developed. Mr. Miyagi would be proud…

volume_mov_PREV.jpg

In the movie, the remote is shook (like it would be in a real world situation if it were strapped to your wrist) No commands are sent while it is being moved since it is in a “Wait” state. After it is held flat for 1 second, an LED lights up signaling it is ready to accept a command. Each time the remote is tilted upwards, the volume on the iPod increases and a LED on the remote flashes showing the user that a command has been sent. Tilting the remote downward would decrease the volume similarly. The remote is held flat for 1 second to put it back into the “Wait” mode.

pause_play_mov_PREV.jpg

In the movie, the remote is shook (like it would be in a real world situation if it were strapped to your wrist) No commands are sent while it is being moved since it is in a “Wait” state. After it is held flat for 1 second, an LED lights up signaling it is ready to accept a command. When the remote is moved quickly left to right, a Pause command is sent to the iPod and a LED on the remote flashes showing the user that a command has been sent. The above process is repeated to issue the Play command.

ff_rew_mov_PREV.jpg

In the movie, the remote is shook (like it would be in a real world situation if it were strapped to your wrist) No commands are sent while it is being moved since it is in a “Wait” state. After it is held flat for 1 second, an LED lights up signaling it is ready to accept a command. Each time the remote is tilted to the right, the iPod skips the the next track and a LED on the remote flashes showing the user that a command has been sent. Each time the remote is tilted to the left, the iPod skips to the previous track and a LED on the remote flashes showing the user that a command has been sent. In the movie, the remote skips ahead two songs, then skips back one song.

Datasheets/Links

Questions, Comments?

I probably didn’t do the best explaining certain aspects of the project, so if you have questions or comments, email me @ petertyser @ nospam (dot ) gmail (dot) com.

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