Audio Detector for Starting Robots
The following circuit can be used to detect those
annoying piezo whistles that one can get from Radio Shack. For the
Trinity Home Fire Fighting Contest, additional points accrue when the
robot is started by a sound similar to a fire alarm. The actual whistle
is specified as a particular Radio Shack piezo element, but the
following circuit should be tunable across a wide range of pitches.
PDF version of the
schematic
The Circuit Description
Gain Stage: The input is a standard surplus
electrect microphone. Although I have not tried it, I suspect any
condenser computer microphone will work just fine. The microphone
is powered by the 4.7k load resistor. The resulting signal is
coupled to the input stage via C4 and amplified by roughly 100x.
The gain is set by the ratio of R2/R5.
The second stage is a band pass filter. The
center frequency of the filter is set by C7, C5 and R1. The Q (or
narrowness) of the filter is set by R9. With R9=560 the filter is
pretty particular.
The third stage is simply more gain, about 30x and the
forth stage is a peak detector. The diode makes a rectifier that
charges C9 to the peak value of the signal from the second gain stage.
R14 is then chosen to determine the decay of the peak detector.
With R14=10k the capacitor discharges in a couple milliseconds.
There is nothing critical in the circuit except for
the band pass components, C5, C7, R1 and R9. Most +5v quad op amps
should work fine and if you use an op amp that works rail-to-rail,
modify R6 and R7 to bias the amps more in the center of the voltage
range.
Tuning
To tune the audio detector, hook an oscilloscope, or
DVM to the output of the circuit. With your signal on, adjust R1
to the center of the peak reading. If your signal is really loud
it will be hard to determine the center frequency. In that case,
move your signal across the room or tape the piezo element so it isn't
so loud.
Use
The detector will detect random noises or whistles or
hand claps, but, the output will not be held high for very long.
So, to determine when a valid signal has been detected, sample the
output and verify that it has been active for more than 2-300ms.
My Fire Fighting robot, Dilbert
uses the detector as a "Push On, Push Off" switch; so the
output is sampled every 100ms (10hz) and five consecutive active samples
are required before changing the state of the internal switch.
PCB Artwork
If you have the EAGLE
cad package you can use the following files to generate your
artwork:
EAGLE Schematic File
EAGLE Board File
If not, the following images are the top and bottom
view of the PCB. The bottom view is mirrored so it is correct when
placed upon the bottom of the blank PCB. The images were intended
to be printed out at 300dpi - that is why they are so big on your
screen. So, copy them down, load them into your favorite image
processor and print them out at 300dpi.. The board is ~1.2" x
1.5" - so it should be pretty tiny.
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Last update 15-Mar-2003
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