E00cH02.PDF

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series 01
002
Electronic Ear for Lego RCX Module
H. Steeman
One interface that is missing in the Lego MindStorms sys-
tem is an electronic ear. We don’t mean that the RCX should
respond to spoken commands (which requires a large
amount of electronics and software), but it would be handy
if it could respond to basic sounds (or sound levels). The cir-
cuit presented here allows the RCX to sense different sound
levels. The sound is picked up by a crystal microphone,
which is an inexpensive component that is available in
every electronics shop. The signal from the microphone is
converted into a variable quasi- resistance value. The RCX,
in turn, can use this value to determine if a particular sound
level has been exceeded. If the trigger threshold is set to the
right level, the RCX will then react to a previously set sound
level. The RCX input must be configured as a light sensor
input for this function.
The operation of the circuit is simple. IC1, which is wired as a
non-inverting amplifier, amplifies the microphone signal by a
factor of 100. The output signal from the opamp is rectified by
D1 and smoothed by C1. Resistor R2 allows the capacitor to
discharge. The resulting DC voltage drives IC2, which acts as
a buffer. The output of this opamp is connected to the sensor
input of the RCX via a 1-k
20
Elektor Electronics
12/2000
resistor (R1).
Just as with the analogue input adapter described elsewhere
Ω
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SMALL CIRCUITS COLLECTION
in this issue, the RCX sees a
variable resistance value at
the sensor input, and it con-
verts this into a measurement
value between 0 and 100. In
the idle state, when no sound
is sensed, the measurement
value lies between 90 and 100.
The louder the sensed sound,
the lower the measurement
value. You can use the light-
sensor routine of the Lego
software to set the responses
to various sound levels. If you
use a threshold value of
around 85, then a level under
85 will be sensed as a sound
signal, while a level above 85
will be sensed as silence. If you clap your hands near the sen-
sor, the circuit will detect this. If you use these ‘observations’
to increment a counter, it is even possible to measure the num-
8
6x 1N4148
IC1
R4
100k
D5
D3
D1
4
2
R1
1k
RCX
1
6
IC1a
D7
7
3
IC1b
D6
D4
D2
5
1N4148
IC1 = LM324
MIC1
R5
R3
R2
C1
C2
100n
68µ
16V
004102 - 11
ber of sound pulses within a defined interval, and then to carry
out some action based on the result.
(004102-1)
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