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This sensor uses the sound generator in
the RCX to make a 15kHz audio tone, which is almost
ultrasonic. The tone is received with circuitry similar
to my Sound sensor. The output of a crystal microphone
MIC is amplified and then only the very high frequencies
are further amplified (see plot). This signal is
enveloped detected with a diode D1 and capacitor C1. The
voltage on the capacitor will equal the average volume
of high frequency sound the microphone is picking up at
any moment. Motion of objects near to the RCX and
microphone cause the volume level to vary as the signal
path and distance between them changes. The variation in
level is further amplified and fed to the RCX. The RCX
sees a level of about 31 when there is no motion, and
peaks to over 50 when there is rapid significant motion.
The circuit diagram is below. An LM324
quad Opamp is used for all stages, and the diodes are
all 1N4148s.
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Circuits
This is a plot of the frequency response
of the High Frequency Amplifier stage. The gain has been
set to peak at about 15kHz so that interference from
other sounds is minimized.
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Circuits
Here is a photo of my finished sensor. I
built it on a piece of PC board and mounted it inside of
a LEGO #5391 9V Battery Box. The microphone is attached
to the end with double face foam tape.
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Circuits
Here is how I tested the circuit. The
RCX has a Visual Basic program that produces a 15kHz
tone and watches the input for levels over 33. If is
sees one, it turns on an output which just has a light
hooked up to it. The operating range depends on many
mechanical factors. It can pickup motion of large
objects 8" away while small objects may need to be only
a few inches between the RCX and the microphone.
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Circuits |
