Improving your
Arduino-based
Theremin
By BAO SMITH
Last month we had a short article on building an Arduino-based
digital Theremin which may have left some a bit wanting. This
month we show how to add a second sensor onto the Theremin
which is used to control volume.
Y
ou can’t really call something
a Theremin if all it does is alter
pitch. So, we decided to improve on
the Theremin kit from Jaycar by adding a second ultrasonic sensor which
is used to alter volume.
This extra HC-SR04 ultrasonic sensor is cheap – it can be bought from
Jaycar for $7.50 (Cat. XC4442).
Adding the second sensor
The second sensor is aimed perpendicular relative to the first and moving
your hand closer to it increases the volume, decreasing it if you move away.
While the physical change to this kit
is very simple, there is much more that
needs to be altered on the software side
to provide the volume-altering effect.
Because of the lack of space around
the DIGITAL pins due to the pitch-controlling sensor being located there, we
opted to plug the second sensor into
the ANALOG pins.
80 Silicon Chip
The second ultrasonic sensor is fitted so that VCC goes to ANALOG pin 2, while
Trig goes to pin 3. Note that the amplifier power lead has been bent slightly so
that there is better spacing between parts.
siliconchip.com.au
Conveniently, the ANALOG pins on
the Arduino Uno can be used as digital pins, however, when manipulating
them, the pin number needs to be prefixed with 'A', ergo A2 corresponds to
ANALOG pin 2 on the board.
We have placed the addition sensor with VCC on ANALOG pin 2, Trig
on pin 3, Echo on pin 4 and GND on
pin 5. We also slightly bent the 2-pin
male header that the amplifier power
supply connection was attached to so
the lead does not come into contact
with the sensor.
As detailed in last month’s article,
the pin locations of the new sensor can
be altered (if necessary) by changing
what is defined in the software. But it’s
easiest to use the same pins we have.
Then all that needs to be done is
upload the new software to the board.
The new software will still work with
just one sensor, as shown last month,
and can be downloaded for free from
our website www.siliconchip.com.au
Software
Once again, the software details are
left to an interested reader. Instead, we
will just go over some of the more important points. At the top of the Ultrasonic_Theremin.ino file there is a new
macro called VOL_SENSOR which is
set to 1 by default.
When set to 1, the software will act
as if both sensors are attached, and
thus attempts to request data from both
sensors. If set to 0 the software functions as if only the pitch-controlling
sensor is attached and thus only polls
one sensor.
The amplifier’s audio signal level is
determined by the value of the 8-bit
OCR2B register, which can range between 0 to 255 inclusive.
Now that we have the additional
sensor, a second distance measurement is computed (simultaneously
with the first, to avoid slowing down
the feedback loop). This distance
measurement is then used to scale the
sinewave value written to the OCR2B
register, effectively attenuating the
sound level depending on how far
your hand is from the new sensor.
By default, the software uses the
same MAX_DIST setting for both sensors to set their maximum detection
range. If for some reason you wanted
to use a different value for each sensor, you would need to modify the
software.
The trickiest part of modifying the
software to handle two sensors was
the code to measure the distance for
each simultaneously. This involves
sending simultaneous trigger pulses,
then waiting for both echo pulses to
be received while separately timing
the start and end of each echo, so that
we can later subtract them and calculate the distance measured. We recommend that interested readers take
a close look at this part of the source
code to see how we did it.
Of course, one of the great things
about Arduino is that you can download our software and easily experi-
Projects with
SIZZLE!
Two high-voltage projects
which use the same PCB:
High Energy Electronic
Ignition for Cars
Use to replace failed ignition module or
to upgrade a mechanical ignition system
Published in Nov/Dec 2012
(siliconchip.com.au/project/
high-energy+igniton)
Jacob's Ladder
A spectacular (and noisy! )
display of crackling,
menacing sparks as they
mysteriously climb the
“ladder”
Published in
Nov/Dec 2013
Parts available from PCB, IGBT
SILICON CHIP On-Line Shop: programmed PIC,
siliconchip.com.au/shop
Look for all the details at
siliconchip.com.au/project/jacob
ment with making changes to see what
effect they have.
More Arduino projects
If you’re interested in building other Arduino projects, check out Jaycar’s guides at: www.jaycar.com.au/
arduino
SC
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January 2017 81
