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Hardware Description
Interactive Audio-Controlled Ferrofluid Sculpture
Odin Ringsred
WWU
5/9/13
Introduction:
The interactive ferrofluid sculpture consists of seven different sculpture pieces that can
be manipulated with audio and preset patterns. Each sculpture piece is an electromagnet of
adjustable strength, which interacts with the magnetic liquid known as ferrofluid. When using
audio as an input, each of the seven different sculpture pieces represent a frequency band from
63Hz to 16kHz, which are used as a graphic equalizer to visualize the frequency spectrum of the
audio signal by the intensity and shape of the changeable ferrofluid sculpture pieces. Audio
inputs include a standard phone jack for a mp3 or CD player, ¼” instrument jack for guitar, and
there’s an on-board microphone for capturing the surrounding ambient sound in the room.
There will be a keypad and an LCD as a user interface to switch between modes and control the
input channels. The preset digital patterns will be selectable via the menu on the LCD, and
provide a simply and captivating centerpiece and visualizer.
Hardware Overview:
The sculpture is made up of mostly analog based circuits for handling all of the signal
leveling and adjustment. There are preamps and line leveling circuits for the audio inputs,
noiseless audio switches, analog mixer, seven-band graphic equalizer, audio speaker amp,
electromagnet driving circuitry, and digital interface ICs such as the DAC and the demultiplexor.
I chose mostly analog components to reduce the MCU requirements, and in addition to the
circuitry mentioned above, there will be an LCD, pushbutton keypad, and LED indicators show
the current status of channels and the like.
Power:
All of the components in this design, including the MCU, operate off of the ±5V or ±12V
rails available from the 380W Antec computer power supply. I chose to use a computer power
supply because the electromagnets and the high-power amplifiers draw a significant amount of
current and dissipate a considerable amount of energy, which I was sure the Antec Supply was
able to handle and provide. It also has an off/on power switch and cooling fan, which also
eliminates the need for the additional cost of adding them separately.
Switches:
For switching in between audio channels I chose to use the MAX4622 (dual SPDT) and
MAX4623 (dual DPST) audio switches. They each have internal DC blocking to prevent DC
voltage pops when switching and are break-before-make switches. This is an important feature
of the switches because small clicks and pops can become a real annoyance when amplified.
Using these switches also allows the entire system to be controlled by the MCU and a few I/0
pins. The MAX4623 switches are used to turn on and off the different audio channels including
the stereo phone, instrument, and ambient microphone channels and each channels indicator
LED. The MAX4622 switches are used to switch between the digital and audio channels (or
modes) for driving the electromagnets, and also to bypass the mixer section. One of the two
switches on the mixer bypass MAX4622 is also used to as a mute to the speaker.
Input Circuits:
There are three audio inputs total, two of which are user inputs, and one which is
internal to the device. Since the ¼” instrument and stereo phone jacks are used directly by the
operator, I have used the impedance bridging concept to avoid impedance matching circuitry
for the variable output impedance found in electrical instruments. Zener diodes are used to
protect both of the user controlled inputs from accidental high DC voltage inputs that could
damage the other circuitry. The stereo phone and instrument inputs have a high pass filter with
a cutoff frequency of 1 or 2HZ to block any DC signals from distorting the audio signal, and then
go through a unity gain op-amp buffer for impedance matching. After each level is filtered and
brought to a level around 100mV, each signal then goes through and amplifier with a variable
gain from 1 to 30, to bring the expected signal up to professional line level (4dBu). After the
channel levels are adjusted they are combined by a summing amplifier and then have
frequencies outside the audible range (20kHz) filtered out by the amplifiers low-pass filter. The
output of the summing amplifier has two zener diodes to prevent large audio signals from
overdriving the mixer stage.
All op-amps used in the audio filtering and leveling are NE5532N dual op-amps with a
low noise (CMRR) that makes them ideal for audio applications. Because each eight pin DIP has
two op amps, it will save on cost and board space.
Mixer Circuit:
The mixer (or graphic equalizer) circuit is a made up of seven different narrow-band
filters, each with their own frequency band centered at the frequencies 63Hz, 160Hz, 400Hz,
1kHz, 2.5khz, 6.25kHz, and 16kHz. Each of these frequencies corresponds to a different
sculpture piece, and with the potentiometers, one may adjust the gain of each band
individually. The outputs of each of these filters are then summed up and the signal’s
magnitude is attenuated 100 times by the summing amplifier, which also provides a gain of 10
for overall volume control. The attenuation just mentioned is necessary to bring the signal level
down to ~100 - 400mv for input into the TDA7297SA audio/speaker amplifier and the MSGEQ7
seven-band graphic equalizer IC used in the electromagnet driving section. There is one
MAX4622 dual audio switch that controls the audio signal bypass route, and also the mute
function of the audio amplifier. The mixer bypass switch is pretty self-explanatory, and the
audio amplifier is wired to have the switch turn on and off the power to the amp causing it to
“mute” the speaker and also turn on the mute indicator LED. There is also a bypass mixer LED
that is controlled with the remaining part of the MAX4623 audio switch used to control the
microphone channel.
Electromagnet Driving Circuitry:
The driving circuitry for the electromagnets is controlled by either a digital or an analog
audio channel, using a MAX4623 audio switch. Both channels use a multiplexed output which is
controlled by the STROBE pin of the MCU, acting as the heartbeat of the driving section.
The analog audio channel uses the output of the mixer circuit as the input to the sevenband equalizer (MSGEQ7), which then outputs a multiplexed DC value relative to the intensities
of the seven frequency bands mentioned previously. Each of the DC values in the multiplexed
output are then sampled one at a time by LF398N sample and hold ICs, which then hold that
sampled DC value at their output until the next sampling period. The LF398Ns are each enabled
one at a time by a SN74F138 3-8 demultiplexor IC, which is itself controlled by the digital
electromagnet selection pins of the MCU. The outputs of the LF398Ns are used as inputs to the
L272 high power op-amps, amplifying the output current relative to the input voltage. Because
the electromagnets are wound in 23 gauge wire, the max amount of current to safely transmit
power continuously over time is 729mV. The L272 was chosen because it can sufficiently drive
the electromagnets with enough current while also guaranteeing that the windings won’t get
too hot, because of its max output current rating of 700mV. Since the electromagnets will be
driven with DC voltage, they will be made out of soft iron so that they demagnetize quickly and
won’t get hot from the DV voltage.
The digital channel uses a DAC0808 digital-to-analog converter that has an output
voltage of 0 to Vref (5V) based on the 8-bit parallel digital inputs from the MCU. The DC output
is multiplexed at the same rate as the audio channel with the MCU’s STROBE pin, and each DC
value is routed to the specific electromagnet the same way as with the audio channel.
Microcontroller:
The MCU used in this design is the MC9S12DP512, which was chosen for familiarity and
developmental ease. The main functions of the MC9S12DP512 are to control the switches and
channel routing, provide a digital channel for controlling the sculptures, control the driving
circuitry, and provide user interface with the LCD and keypad. The MCU is powered from the 5V
rail, and has the reset connected to trigger when the supply voltage drops too far for normal
operation. Of the numerous GPIO pins used on the MCU, 11 are used by the LCD, 6 for the user
menu pushbuttons, 8 to control the digital-to-analog converter, and 9 more to control switches,
enable pins, and the timing of the drive circuitry. Other than this, the MCU only needs a 16MHz
crystal and a PLL circuit for clocking the system.