circuit
ideas
116 OctOber 2010 electroni cs for you www. e f y ma g . c O m
Raj K. GoRKhali
ElEctRonic MEtRonoME
SANI THEO
A
metronome is used by musi-
cians for practice in maintain-
ing a consistent tempo, or
rubato, around a fxed beat. This circuit
produces a regular beat at the rate of
40 to 200 beats per minute. It accentu-
ates every second, third, fourth, ffth,
sixth or eighth beat, which is adjust-
able as per your liking and require-
ment. Every beat is indicated by the
glowing of an LED. The accented beat
is indicated by another LED.
The beat is derived from an astable
multivibrator (IC1) running between
0.67 Hz (40 beats per minute) and 3.47
Hz (208 beats per minute), and a pulse
generator built around NOR gates N1
and N3, resistor R3 and capacitor C2.
The beat covers all the musical tempi
from adagio to presto. The results are
a very short burst of sound, remi-
niscent of the tick of a mechanical
metronome. If you prefer a beep rather
than a tick sound, the pulses should be
lengthened by reducing the value of R3
to, say, 5.6 or 6.8 kilo-ohms.
IC1 drives the pulse generator.
The length of the pulse is about 10 ms,
and it appears at pin 1 of IC3 (NOR
gate N3). At each pulse, the red LED
(LED1) fashes to indicate occurrence
of the beat. The pulse passes through
NAND gates N6 and N7 of IC4. The
pulse output from pin 6 of N7 is fed to
NAND gate N8. The audio signal out-
put generated by another multivibrator
(IC6) is also fed to gate N8. The audio
signal can be adjusted to obtain a note
of suitable pitch.
The output from IC1 also goes
to IC2 (CD4022), which is a divide-
by-eight counter/divider with eight
decoded outputs. Rotary switch S1 al-
lows the counter to be reset every two,
three, four, fve or six counts, or cycle
through eight counts without resetting.
Output Q0 of IC2 drives the sec-
ond pulse generator built around
NOR gates N2 and N4, resistor R4
and capacitor C3. The output is an
accented beat pulse, which is fed
to NAND gates N5 and N9 and the
base of transistor T2. Since C3 has a
higher capacitance than C2, this pulse
is longer (about 40ms) and is used to
mark the accented beat. The result is a
tick sound lasting about 40 ms, which
sounds every second, third, fourth,
ffth, sixth or eighth beat, depending
on the setting of S1. The accent pulse
makes the yellow LED (LED2) fash.
It is important that the base tick
note or beat is not heard on the ac-
cented beat. This is achieved by gates
N5 through N7 of IC4.
The fnal audio signal appears at
pin 3 of IC5 (NAND gate N10). This
signal can be fed to the audio power
amplifer stage. When you supply 6V
DC to the circuit, you can hear the
base or tempo beats and accented beats
from the speaker of your power ampli-
fer. The red LED (LED1) fashes with
the beat and the yellow LED (LED2)
fashes on the accented beat.
Construction and testing is simple.
Assemble the circuit on a breadboard
or general-purpose PCB. Mount all the
components, except S1, and temporar-
ily connect pin 15 of IC2 to ground rail.
IC1 produces an audible tick sound
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circuit
ideas
electroni cs for you OctOber 2010 117 www. e f y ma g . c O m
(tempo beat) at a fxed rate that varies
as VR1 is adjusted. IC6 produces a tone
that varies in pitch from about 250 Hz
(about an octave below middle C) to
about 2 kHz (about two octaves above
middle C) as VR2 is adjusted. The
counter goes through its normal eight-
stage cycle and the yellow LED (LED2)
fashes once for every eight fashes of
the red LED (LED1).
Now connect a loudspeaker to pin
3 of NAND gate N10 through a 10F
capacitor. The circuit should produce a
series of tick sound with a double-tick
sound at every eighth tick sound. If
this works well, remove pin 15 of IC2
from the ground rail and connect to
six-way rotary switch S1. Remove the
speaker and 10F capacitor from pin 3
of N10 and connect pin 3 to an audio
power amplifer. Use presets VR1 and
VR2 such that turning their knobs
clockwise increases the tempo and the
pitch, respectively.
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