Classic LED
Metronomes
These two Metronome designs simulate
the classic mechanical, invertedpendulum metronome with its batonlike pointer swinging left-and-right,
producing a click at each extreme. Both
designs use only discrete components
and simple logic chips, making them
easy to understand and build. Plus, they
are both great projects for a beginner.
By Randy Keenan
I
dislike typical ‘modern’ electronic
metronomes that only click and/or
blink once per beat. I came up with
these designs to better simulate the
mechanical metronomes that I know
and like. Both designs light a series of
LEDs, accompanied by a speaker that
produces beat sounds.
The first design uses eight LEDs
and fits in a standard plastic case,
while the second, slightly more
complicated design has 10 LEDs
and is housed in an elegant custommade timber case. So the latter is a
good project for those readers who
have some woodworking experience.
In both cases, the LEDs are arranged
in an arc and light up in sequence,
forward and reverse, to mimic the
swing of an inverted pendulum. A
click at each end of the LED arc further simulates a mechanical metronome. A typical metronome tempo
range is 40-208 beats/minute, a ratio
of 5.2 to one; in these Metronomes,
the range is extended to 36-216 beats/
minute, a ratio of six to one.
Either project is excellent for a beginner; there are no high-frequency signals, high voltages or tricky wiring
involved. Nor is there any need to program a chip with software. However,
some measurements and adjustments
will be needed to calibrate the instruments after construction, given the
expected component tolerances.
Two designs
The slightly simpler 8-LED Metronome uses 74HC-series logic ICs and
can be battery-powered, while the
10-LED Metronome uses CD4000series logic ICs and is intended to be
powered from a plugpack. The two circuits operate similarly: a pulse generator clocks an up/down counter IC at
the rate required for the desired tempo.
Another IC decodes the counter value
to light the LEDs sequentially.
A set/reset flip-flop (SR-FF) switches
the counter direction when either end
LED is lit, giving forward and reverse
LED sequences. The click is produced
by ORing the signals to the end LEDs,
followed by a differentiator to shorten
Fig.1: the 8-LED Metronome is based around three 74HC-series digital logic ICs. The 74HC132 generates pulses at a
selectable frequency. These clock the 74HC191 counter, and its three-bit output drives the eight LEDs via the 74HC137
decoder chip. The remaining three gates in the 74HC132 quad NAND package are used to form a set-reset flip-flop to
reverse the LED chaser’s direction each time it reaches one end, and to generate a pulse from the speaker.
14
Practical Electronics | January | 2023
However, all LEDs should have high
luminous intensity, ideally at least
4000mCd (sometimes called ‘superbright’). This is to reduce power consumption. For the 8-LED Metronome,
that maximises battery life, while in
the 10-LED design, it limits the load on
the CD4028 driving IC to a safe level.
Both Metronomes were made using
5mm oval LEDs: green for the 8-LED
version, and red for the 10-LED version. I used oval LEDs because they
glow in a line rather than a dot, providing a more interesting display. 3mm
and 5mm round LEDs are also suitable.
Tinted and diffused lenses look best.
You can use different LEDs from
those specified, but you might need
to adjust some resistor values.
The LED metronome comes in two versions; one with eight LEDs and
another with 10 LEDs (shown in this photo). The case to house it can be as
simple as a small timber frame with a clear plastic panel at the front.
the pulse and a one-transistor amplifier to drive a small loudspeaker.
The block diagrams of the two
Metronomes are shown in Figs.1
and Fig.2. In the 8-LED Metronome,
the tempo pulse is generated by a
Schmitt-trigger NAND gate (part of
a 74HC132). This clocks four-bit up/
down counter IC2 (7HC191). Only
three of the four binary outputs are
used to drive 3-to-8-line decoder IC3
(74HC137) that lights the LEDs in
sequence (eight is the 137’s limit). The
SR-FF is made from two more NAND
gates in the 74HC132.
In the 10-LED Metronome, the pulse
is generated by a CMOS version of the
ubiquitous 555 timer. It clocks four-bit
up/down counter IC3 (CD4029) which
drives decoder IC4 (a CD4028) with 10
outputs. The SR-FF logic is again provided by two gates from IC1, although
it is a different logic chip.
Both versions enable the LEDs at
each end of the arc to flash brighter.
You could also use either of these
circuits anywhere an LED ‘chaser’
is needed.
LED options
There are many options for the LEDs
in these Metronomes. The parts lists
indicate the suggested LEDs, but other
sizes, shapes and colours can be substituted. The two end LEDs could even
be different from the middle LEDs.
The 8-LED version
The LEDs should be of the same type
and matched; but if using different
LEDs at the ends, use matched LEDs
for those two and separately matched
LEDs for the rest. The minimalist circuit
is shown in Fig.3 and works as follows.
Schmitt-trigger quad NAND gate
IC1d is configured as a pulse generator. The pulse frequency, and thus the
metronome tempo, is determined by
potentiometer VR1, resistors R1 and
R2 and capacitors C1-C3.
For an explanation of the operation
of a Schmitt-trigger pulse generator,
see the adjacent panel. Its pulses clock
IC2, an up/down binary counter. IC2’s
outputs are fed to decoder IC3 to light
the eight LEDs in sequence.
The outputs of IC3 and the inputs
of IC1a, IC1b and IC1c use negative
(active-low) logic because IC1 is a
NAND gate rather than a NOR gate
(see the panel on SR-FFs).
The alternative would have been
to use the 74HC7002 Schmitt-trigger
NOR gate with a 74HC237 decoder
and positive logic, but the 74HC7002
is less common and more expensive
than the 74HC132.
Fig.2: the 10-LED Metronome uses a 555 timer IC instead of a logic-gate-based oscillator as the pulse generator. The remaining
logic ICs are from the 4000-series. A 4029 acts as the up/down counter, while a 4028 is the 4-to-10 decoder that drives the
LEDs. Two of the gates of the 4001 quad NOR IC form the set-reset flip-flop, and the other two gates form the click pulse.
Practical Electronics | January | 2023
15
When IC3 is counting up, it lights
each LED in sequence, from LED0 to
LED7. When LED7 lights, the low level
at pin 7 of IC3 (Y7) is also applied to
the SR-FF formed by IC1a and IC1b.
This causes pin 6 of IC1b (Q) to go
high, causing IC2 to reverse its direction and count down. Each LED is
now lit in sequence in the opposite
direction. When the first LED, LED0,
is lit, the SR-FF is reset, IC2 reverses
and counts up, and the cycle repeats.
VR2 controls the overall LED
brightness. The circuit is designed to
make LED0 and LED7 brighter than
the others. The relative brightnesses
of the end-vs-middle LEDs is maintained as VR2 is adjusted by LED8
or a zener diode, ZD2. Whenever
current is applied to LED1-LED6,
LED8/ZD2 is in series with that LED,
reducing the voltage across the current-limiting resistors and thus the
LED current.
VR2 could be changed from 500kW
to 1MW to extend the brightness range
down to very dim levels. If you want
VR2 to turn the LEDs completely off
at minimum, fit R3 (300kW), but note
that this creates a large ‘dead zone’ in
the lower range of VR2.
LED8 may be the same type as LED0
through LED7, but for more brightness contrast between the end and
middle LEDs, use a type with higher
forward voltage such as blue or white,
or use a zener diode of approximately
4.7V. If you want all LEDs to have
equal brightness, fit a wire in place
of LED8/ZD2.
Click sounds
When either end LED is lit, the low
level at Y0/Y7 is also sent to IC1c,
which behaves as a NOR gate when
operated in negative logic mode (a low
at either input causes a high output).
Its output is fed to the Click Loudness
control (VR3) and then to a simple
transistor amplifier.
However, the pulse from IC1c is
too long and would cause a click at
the end of the pulse as well as at its
beginning, and the current would be
high during the pulse on-period. To
avoid this, the pulse passes through
C4 and/or C5 to yield a short pulse at
each end of the original pulse, a positive one at the beginning and a negative one at the end.
Diode D1 shunts the negative pulse
so that only the positive pulse is
applied to the base of transistor Q1.
Power supply
This design is powered by a battery
of four AAA cells. 74HC ICs are used
rather than 74HCT or 74LS because
the 74HC series allows a slightly
higher supply voltage, up to 6V. Fresh
standard alkaline AAA cells supply
marginally more than 6V, so the voltage rail for the ICs is limited using
a 47W resistor and a 6V zener diode
(ZD1). Alkaline, dry cell, rechargeable NiMH or Li-ion AAA cells can
be used.
The 4700μF and 470μF bulk bypass
capacitors, in combination with the
47W series resistor, reduce the supply
voltage pull-down by the click-pulse
current through the speaker.
PCB-mounting potentiometers with
built-in switches are rare, so a separate
power switch is used. A regulated 6V
DC plugpack could be used instead of
a battery. Make sure you verify it is
regulated as otherwise, its output voltage would be too high for the circuit.
The 10-LED version
The circuit for this version is shown
in Fig.4. It is similar to the 8-LED
design but positive logic is used
throughout. The pulses are generated by CMOS 555 timer IC2. It clocks
IC3, a four-bit up/down counter. IC3’s
outputs are decoded to 10 individual
outputs by IC4, lighting the 10 LEDs
in sequence.
When an end LED (LED0 or LED9)
is lit, the SR-FF formed by IC2a and
IC2c is set or reset, thus switching IC3
into up or down mode, reversing the
LED sequence.
VR5 controls the LED brightnesses.
Instead of the technique used for the
8-LED design to make the end LEDs
brighter, this version uses a current
mirror comprising Q1 and Q2 with
trimpot VR4, control potentiometer
8-LED ‘Classic’ Metronome
Fig.3: this 8-LED Metronome circuit shows more details than Fig.1. VR2 allows you to set the LED brightness while LED8
or ZD2 reduces the brightness of the middle six LEDs compared to the outer two. Extra capacitors C2-C3 and resistor
R2 allow you to adjust the frequency range to match the beats-per-minute (bpm) range shown on the dial. Additional
capacitor C5 is provided to change how the clicks sound.
16
Practical Electronics | January | 2023
Using a Schmitt-trigger gate as a pulse generator
Gate IC1d of the 74HC132 Schmitttrigger input NAND chip generates the
pulses that clock the counter (IC2).
So what is a Schmitt-trigger gate, and
why are we using one?
An ‘ordinary’ non-Schmitt-trigger
gate or inverter is effectively a highgain but mostly linear amplifier. As
a result, the output transition from
high-to-low or low-to-high takes place
over a narrow input voltage range, as
shown in Fig.a, which is a plot of output voltage versus input voltage.
Consider, the negative-feedback
RC circuit shown in Fig.b. It will typically reach equilibrium at some point
(E), with the output ‘stuck’ at an intermediate voltage. If the input were to
increase in voltage, as indicated by
the arrow, the output would respond
by decreasing and would restore the
circuit to point E with a time constant
determined by R and C. The reverse
is true if the input voltage decreases.
You can test this yourself on a breadboard if you have a spare 74HC00
NAND gate chip. Just remember to
connect all the unused inputs to one
supply rail or the other. A DVM will
show that the voltage at pin 3 is stable.
In contrast, the Schmitt-trigger
version of the gate ensures oscillation due to its built-in hysteresis and
associated positive feedback. This is
illustrated in Fig.c, which is an equivalent plot to Fig.a but for a gate with
Schmitt-trigger inputs.
Once the input voltage increases
above the upper-threshold input voltage (VT+, point U), the output immediately ‘snaps’ to a low level (point
V). It remains there until the input
decreases below the lower-threshold
input voltage (VT−, point L) and the
output ‘snaps’ high (point W).
This can be demonstrated by breadboarding the circuit shown in Fig.d.
Begin with the pot at extreme clockwise (pins 1 and 2 at +6V) and apply
power. The LED should remain off.
Slowly decrease the input voltage via
the pot until the LED goes on; note
the input voltage. Now increase the
input voltage gradually until the LED
goes off. There should be a couple of
volts difference; this is the hysteresis
spread (VT+ − VT−).
You have made one clockwise
trip around the hysteresis rectangle,
as indicated by the arrows in Fig.c.
Because of this, if you substitute
a Schmitt-trigger 74HC132 for the
74HC00 in Fig.b, you will find that it
oscillates, generating a square wave
at the output. The input exhibits an
exponential pseudo-triangle wave of
amplitude equal to the hysteresis
spread, as shown in Fig.e.
One crucial point to consider is
how the rate of oscillation will vary
with supply voltage (especially in
a battery-powered circuit). As it
turns out, the deceased capacitor
charging current is somewhat compensated by the decrease in hysteresis spread (as it is somewhat proportional to the IC’s supply voltage).
Thus, the pulse rate only changes by
a few percent from 6V to 5.5V (an
8% change in voltage).
VR5 and some fixed resistors. VR4
adjusts the brightness of the middle
LEDs relative to the brightness of the
two end LEDs. See the adjacent panel
for an explanation of how this works.
Click generation and circuit variations are the same as for the 8-LED
Metronome design. The higher supply voltage of this version provides a
louder click.
Construction
Fig.5 is the PCB layout diagram for
the 8-LED version, while Fig.6 is for
the 10-LED version. Most components
mount on the boards. A few might need
Practical Electronics | January | 2023
Fig.a and Fig.b: the transfer function of a standard NOR
gate. The output is low when the input is high and vice
versa, but if the input voltage is intermediate, the output
voltage can be anywhere in between.
Fig.c and Fig.d: a Schmitt-trigger inverter has
hysteresis, so once its input voltage is high
enough, the output snaps low and stays low
until the input voltage drops significantly.
Similarly, when the input voltage drops and
the output goes high, it remains high until the
input voltage increases significantly.
Fig.e: the input
and output
waveforms for a
Schmitt-trigger
inverter used as
an oscillator.
17
their values tweaked; that is why some
parts do not have an associated value.
PCBs for each version are available
from the PE PCB Service.
Whichever version you build, the
construction process is initially similar.
Start by fitting all the resistors with fixed
values given, using a DMM to check the
values before soldering them in place.
Follow with the diode(s), ensuring
their cathode stripes face as shown
in the appropriate overlay diagram
and that you don’t get the different
types mixed up. Remember that for
the 8-LED version, you either fit zener
diode ZD2 or LED8, not both. If using
LED8, push it down onto the board
with the longer lead to the pad marked
A and then solder it in place.
The ICs are next. They can be soldered directly to the PCB or plugged in
via sockets; it’s up to you. Either way,
make sure the pin 1 notches/dots face
as shown and don’t get the two different 16-pin ICs mixed up. Note that
IC3 on the 8-LED board (74HC137) is
oriented opposite to the other two ICs.
Fit the capacitors next, starting with
the smaller non-polarised types and
then moving onto the electrolytic capacitors, which must be oriented with their
longer positive leads placed towards
the + symbols. The 1000μF capacitor
on the 10-LED board is laid over as
shown before soldering and trimming
its leads. As with the resistors, leave off
any that don’t have values indicated as
those pads are for tuning later.
There are no trimpots or discrete
transistors on the 8-LED board. However, on the 10-LED board there are
three trimpots: two 5kW (VR1 and
VR3) and one 100kW (VR4); as well
as two PNP transistors (Q1 and Q2)
and one NPN transistor (Q3). Fit them
now, being careful not to get the different types of transistors mixed up.
If your Q3 transistor is taller than
the others, bend its leads so that it is
laid over on its side before soldering
to ensure sufficient clearance for the
front panel later.
Note that the PCB has footprints to
accommodate different types of transistors from those specified. However,
assuming you are using the BC558 suggested in the parts list, they are placed
as shown in Fig.6.
Both boards use a 3-way terminal
block for power, although you can solder wires to its pads instead. If fitting
it, do that next, with the wires entering the front of the board and passing
around to the rear via the notch on the
edge of the board.
Continue by selecting the LEDs you
are going to use. You might wish to
order extras so that you can pick out
a matched set from the larger number. Note that it’s common for their
brightnesses to look similar when
fairly bright, but at very low currents
(say around 30μA), they can vary considerably when dim. Try to select the
ones which match best for the middle LEDs.
If you have a bench supply, one
good way to compare the brightness
10-LED Deluxe Metronome
Fig.4: the 10-LED Metronome uses a more complicated LED brightness control scheme with PNP transistors Q1 and Q2
forming a current mirror, so the brightnesses of the middle eight and outer two LEDs track over a wide adjustment range.
LED10 lights up the beats-per-minute adjustment dial. Besides these differences, and using a CMOS 555 timer as the pulse
generator, the circuit is quite similar to the 8-LED version.
18
Practical Electronics | January | 2023
How a current mirror works
A current mirror circuit is used to match two or possibly
more currents under varying conditions. Fig.f shows a basic
example; similar NPN bipolar transistors Q1 and Q2 have
their bases tied together and set at a control voltage, Vb.
Thus, their emitters will be at equal voltages, approximately
0.6V lower than Vb.
If the emitter resistors, R1 and R2, are of equal resistance, they will conduct equal currents of approximately
Ic = (Vb – 0.6V) ÷ R.
Assuming a sufficiently high current gain (>≈100) for the
transistors, and thus negligible base currents, the collector current of each BJT would be the same as its emitter
current; in other words, the currents through LOAD 1 and
LOAD 2 would be matched. If the base voltage (Vb) is varied,
the emitter and collector currents will vary, but will remain
matched between the two transistors.
Likewise, if one or both loads vary in resistance – within
limits – their currents will still be equal and given by the
equation above.
For the brightness control of the 10-LED Metronome, we
want the current to the middle LEDs to be a fraction of the
current to the end LEDs, and to be the same fraction over
a wide range of currents. If we used the above scheme, the
circuit would be something like Fig.g, with R being a fraction of VR + R, ie, unequal emitter resistors.
However, there is a problem with this: since each group of
LEDs is alternately turned off, that load resistance becomes
extremely high. As a result, the transistor in the off leg of
the circuit has no collector current, and the base current
becomes large because the base-emitter junction is a forward-biased diode. This reduces the base voltage, and thus
the collector current of the other transistor.
For example, Fig.g shows one of the middle LEDs on,
while the end LEDs are both off, resulting in high base current through their transistor (Q2).
To avoid this, I devised a different scheme for the 10-LED
Metronome, shown in Fig.h. This works because the two
loads are, in practice, nearly constant and equal, each consisting of one LED at a time.
The current-mirror circuit is turned on its head, using PNP
transistors rather than NPN. Each group of LEDs is made
part of an emitter circuit, in series with a resistor that will
determine its current and relative brightness.
When lit in sequence, each middle LED is in series with
R1 + VR4, which is made larger than R2 in series with each
end LED. The collector current of Q1 will be a fraction of
that of Q2 (R2 ÷ [R1 + VR4]). This fraction — the ratio of currents — will be maintained over a range of Vb as controlled
by VR5, and thus the brightnesses of the eight middle LEDs
will be a fraction of the brightnesses of the two end LEDs
over a wide range.
This situation will break down if Vb is above 12V − Vled −
0.6V or about 10V. This is avoided by padding the ends of
VR5 with fixed resistors. Trimpot VR4 allows the resistance
ratio R2 ÷ [R1 + VR4] to be set as desired, thus setting the
brightness difference.
Fig.f: a basic current mirror circuit. Since Q1 and Q2
are similar transistors, and thanks to the negative
feedback provided by the emitter resistors, varying their
base voltages using the potentiometer results in closely
matched currents through the two independent loads.
Fig.g: different emitter resistor values can be used to
make the load currents different, but they keep similar
current ratios when the base voltage is varied.
Fig.h: the circuit shown in Fig.g can suffer from excessive base current problems when the loads can be switched on
and off independently. This circuit solves that by swapping the NPN transistors for PNP and keeping the currentsetting resistor connections at the transistor emitters.
is to connect several in series, along
with an appropriate current-limiting
resistor, then power the entire string
from the bench supply and slowly
wind its voltage up. That way, you can
Practical Electronics | January | 2023
make a direct comparison of several
candidate LEDs over a useful range
of brightnesses.
The construction approach now
diverges for the two versions.
Finishing the 8-LED Metronome
Measure the resistance across VR1’s
track (from one end pin to the other) and
divide the reading by five. This is the
value you should aim for with R1 + R2.
19
Fig.5: most of the
8-LED Metronome
components mount
on the PCB, as shown
here. Assembly is
straightforward
but be careful
to orientate the
ICs, LEDs and
diodes as shown.
Also, don’t get the
potentiometers
mixed up as they all
have different values
(check the codes
printed on their
bodies).
We’ve specified two 10kW resistors in
the parts list because VR1 should be
close to 100kW.
If VR1 does not measure close to
100kW, vary the values of one or both of
those 10kW resistors (eg, changing one
to 9.1kW or 11kW) to get their total as
close as possible to 20% of VR1’s value.
The 8-LED version fits in a Serpac
131-BK plastic enclosure, but other
enclosures could be used instead. If
using the 131-BK, use the side with
the best appearance as the upper end.
After selecting the LEDs, drill the LED
and potentiometer holes in the front
half of the enclosure. Fig.7 may be used
as a drilling template.
You can print the guide onto card
stock, punch out the mounting holes
to 5mm and temporarily glue the
guide to the inside of the front half
of the enclosure.
If using the recommended oval
LEDs, you will need to carefully elongate the holes after drilling. Note that
the illuminated line from an oval LED
is perpendicular to the larger dimension of the LED body. Decide which
orientation you want and orient the
LEDs and holes accordingly. When
drilling or adjusting the LED holes,
check that the LEDs fit into the holes
snugly but do not require excessive
insertion force.
Fit the three pots to the PCB without
soldering them, and attach the PCB to
the front of the enclosure. I removed
the small protruding bits on the front
of each pot. To allow space for the components on the PCB, you might need
spacers on the screws. Check the shaft
20
lengths and shorten them as needed
for your knobs.
The shaft-gripping sections of the
knobs that I used were recessed by
several mm. So for pots VR2 and
VR3, I sanded down the backs of the
knobs to about 12.5mm total height
to enable the knobs to grip the shorter
pot shafts adequately.
If the knobs are not tight enough,
the plastic shafts of the potentiometers can be deformed a little by pinching them with pliers. Solder the three
pots now, after re-checking they have
the correct values.
Next, paying close attention to
their orientations (see the A and K
markings on the PCB), insert the LED
leads into the board without soldering them. If using oval LEDs, they
will need to be twisted slightly to
conform to the arc.
Again, attach the PCB to the front
half of the enclosure and manoeuvre
each LED into its proper hole in the
front of the enclosure. It’s best to have
them protruding slightly. Check the
LEDs’ appearance and adjust as necessary, then solder the LEDs to the
PCB while it is in place. The LEDs
Fig.7: a drilling template
for the front panel of
the 8-LED version.
Eleven holes need to
be drilled: eight for the
LEDs (size and shape
to suit the LEDs you
are using, marked ‘A’)
and three 8mm holes
for the potentiometer
shafts, marked ‘B’.
The dashed circles
show the positions of
mounting posts within
the specified case; do not
drill those.
While we specify 3mm
diameter holes for ‘A’, the
size will depend on what
type of LEDs you are using.
This is available to
download as a PDF from
the January 2023 page of
the PE website (https://
bit.ly/pe-downloads
Practical Electronics | January | 2023
Fig.6: the 10-LED Metronome is slightly more complicated than the 8-LED version. Note the components laid on their sides
and make sure to place the transistors in the positions shown, unless you are substituting those with a different pinout.
will probably not be seated on the
PCB but spaced away from it by several millimetres.
Fig.8 is the tempo dial; this can be
downloaded from the January 2023
page of the PE website (https://bit.ly/
pe-downloads). It is a good idea to print
it on photo paper for a good appearance.
This assumes that VR1 is equivalent
to the type specified in the parts list;
it needs to rotate through a 280° arc.
Align the dial to the tempo pot shaft
and glue it to the front of the enclosure. Fit the knob to the tempo pot
such that its rotation extends equally
beyond the 36 and 216 tempo lines;
this is because pots typically have a
dead zone at each extreme where the
resistance changes very little.
NPN transistor Q1, the 4700μF
capacitor, switch S1, the speaker and
the battery holder are not mounted on
the PCB but attached to the rear half
of the enclosure (see the photo). The
diagram, or you can get dedicated flipflop ICs. In the 10-LED Metronome,
we’re already using NOR gates for other
purposes, so doing it this way avoids
the need for an extra IC.
It works as follows. Imagine that
both the S and R inputs are low. The
circuit can initially be in either state:
set, with Q high and Q low, or reset, vice
versa. Pulsing S high will cause Q to go
low or to stay low, which will cause Q to
go high, which is the set state. Further
pulsing of S will have no effect since Q
holds the upper NOR-gate input high,
assuming that R remains low.
Similarly, pulsing R will cause Q to
go low and thus Q to go high, which
is the reset state. Further pulsing of
R will have no effect, assuming that S
remains low.
For the 8-LED Metronome, the SR-FF
is constructed from two NAND gates
rather than NOR gates. All this means
is that the SR-FF uses negative logic;
in negative logic, NAND gates become
NOR gates, and the SR-FF is set and
reset by negative (low) pulses, specifically, from LED7 (set) and LED0
(reset). The SR-FF Q output is sent to
the 74HC191 counter to change its
counting direction.
The 10-LED Metronome has an SR-FF,
constructed from two NOR gates in the
CD4001. Positive logic is used, and the
SR-FF operates as described above.
Set-reset flip-flops (SR-FF)
Both Metronome designs incorporate a
set/reset flip-flop (SR-FF), a logic circuit
with two states: set and reset. Applying
a high level to the S input while keeping
the R input low puts the flip-flop into
the set state, and it remains there until
reset. Similarly, applying a high level to
the R input while keeping the S input
low puts the SR-FF into the reset state,
staying there until set again.
By today’s naming standards, the
SR-FF is a transparent latch and not a
flip-flop as it has no clock input, but the
traditional term ‘flip-flop’ continues to be
used. Another way of thinking of it is as
a 1-bit memory store or a bistable circuit.
An SR-FF is a simple type of sequential logic circuit, which means that its
output depends on its ‘history’; it has a
memory. Compare this to combinatorial logic in which the outputs depend
only on the value of the inputs; there is
no history or memory involved.
An SR-FF can be made from two
NOR gates, as shown in the adjacent
Practical Electronics | January | 2023
A set-reset flip-flop (SR-FF) made from
two NOR gates. The Q and Q outputs
always have opposite polarity; Q is
brought high when the S input goes
high, while Q goes low when the R
input goes high. Both inputs must not
be high at the same time.
21
50mm
120
120
speaker holes may be in any pattern. I
used a perforated metal sheet, selected
a drill bit of the diameter of its holes,
clamped the sheet to the inside of the
rear half of the enclosure and used it
as a drilling guide.
Attach the slide switch to the panel
using small screws and nuts. The battery holder and speaker are held in
place with clips made from a large,
heavy-duty paper clip.
Q1 and the 4700μF capacitor are
mounted close to the speaker and
wired directly to the speaker terminals to minimise parasitic resistances;
they are not switched by S1, likewise
to reduce parasitic resistance. This can
be important since the supply voltage is relatively low and the speaker
impedance is 8W.
When the Metronome is switched
off, there will be only a minuscule
leakage current through these components. However, if the Metronome
is unused for an extended period, it’s
best to remove the cells.
Solder the emitter lead of transistor
Q1, the negative lead of the capacitor,
and a wire to a solder lug before fitting them to the enclosure. Check the
Fig.9: there are two dials for the 10-LED Metronome to suit the larger 300°
potentiometer (left), or the standard 280° potentiometer (right). Unlike the 8-LED
version, these are printed on transparent film and connected to the rotating pot
shaft. Thus LED10 behind can shine through and illuminate the selected tempo.
wiring of these components carefully:
a mistake can cause excessive current
and damage Q1 or cook the speaker
coil and cone.
Cut a timber base to suit the enclosure and attach the rear half to it using
screws, giving the enclosure a slight
backward tilt.
Finally, attach the off-PCB parts to
the 3-way terminal block, as shown
in the photos. If you don’t want to use
a terminal block, you can solder the
wires directly to the PCB pads.
Adjustments
The tempo and its range will likely
need adjustment. Eight different
74HC132 ICs showed a spread of
a few percent, with one about 7%
above the average. The tempo may
For the LED Metronome, some of
the components such as the speaker
and battery holder are not mounted
on the PCB, but are instead fitted
onto the rear of the enclosure. This
photo shows the 8-LED Metronome
arrangement.
48
44
40
38
36
54 60 72
88
104
120
160
216
50mm
Fig.8: print this dial artwork for the
8-LED version on photo paper, cut it
out and glue it to the front of the case.
The exact diameter is not critical,
but it should be close to 50mm. This
is available to download as a PDF
from the January 2023 page of the PE
website (https://bit.ly/pe-downloads.
22
Practical Electronics | January | 2023
not correspond to the dial markings
because of this, plus variations in the
timing capacitors and the resistances
of VR1 and R1/R2. Pots can vary by as
much as 20%.
If you are not happy and want to
bring the tempos into agreement, a
frequency meter is very helpful. The
type built into many low-cost DMMs
is adequate.
Measure the pulse frequency at pin
11 of IC1 or pin 14 of IC2. With VR1 set
at the lowest tempo (aligned with the
marking showing 36 beats/minute),
find a capacitor or paralleled capacitors for C1-C3 that give a pulse frequency of 4.2Hz (see Table 1).
If you don’t have a selection of
capacitors to try out (or want to save
time), calculate the percentage error in
the frequency (actual vs expected) and
measure the capacitance across C1-C3.
Multiply the capacitance reading by
the percentage and divide by 100. This
is how much capacitance you need to
add (if it’s too fast) or subtract (if it’s
too slow).
To subtract capacitance, you’ll need
to replace C1 and/or C2 with lower
value capacitors, then re-check and
possibly add a bit more capacitance by
fitting C3 to get the frequency spot-on.
Assuming you selected R1 and
R2 as 1/5th the value of VR1, with
VR1 aligned with 216 beats/min, you
should get a frequency reading of
25.2Hz (see Table 1). If this is significantly off, you might want to adjust
those resistor values, reducing the total
to speed it up or increasing them to
slow it down. Calculating the percentage frequency error relative to 25.2Hz
tells you the percentage by which the
total resistance must change.
CON1
3
2
1
Similar to the 8-LED Metronome, the 10-LED metronome also has components
mounted on the rear panel rather than on the PCB, as can be seen in the photo
below, with the wires emanating from the top of this one.
The final result should have all tempos from 36 to 216 (and thus pulse frequencies in Table 1) agreeing with the
dial markings.
Finally, check the click loudness
and timbre. Check that VR3 varies
the click loudness smoothly from zero
to maximum. If it is too soft at maximum, a transistor with a higher hFE
is needed. If the click does not vary
smoothly, replace the 220W resistor
with a higher value until the loudness
variation is satisfactory.
You can vary the timbre of the click
by adding a capacitor at the position marked C5. Adding capacitance
should give a more ‘mellow’ click.
You can also try the speaker in both
polarities as that can affect the sound.
Troubleshooting
If the LEDs don’t light up or behave
strangely, check the orientation of all
the parts.
Check for solder bridges or poor
joints and that IC pins are not bent.
Check also that there is a pulse at pin
11 of IC1 and pin 14 of IC2.
Finishing the 10-LED Metronome
The mounting holes for VR2 can accept
a 280° pot (matching the others) or a
Table 1 – Pulse frequency (Hz)
versus tempo (beats/minute)
Tempo
(bpm)
36
38
40
44
48
54
60
66
72
80
88
104
120
160
216
8 LEDs
(Hz)
4.20
4.43
4.67
5.13
5.60
6.30
7.00
7.70
8.40
9.33
10.3
12.1
14.0
18.7
25.2
10 LEDs
(Hz)
5.40
5.70
6.00
6.60
7.20
8.10
9.00
9.90
10.8
12.0
13.2
15.6
18.0
24.0
32.4
Practical Electronics | January | 2023
CON1 pin 3
CON1 pin 1
CON1 pin 2
23
larger 300° pot. While the difference is
subtle, the 300° pot allows the tempo
numbers to be spread out slightly
more. Before fitting VR2, measure its
resistance, divide by 5.5 and check that
this is close to 18kW. If not, you might
need to replace the 18kW resistor with
a different value that’s close to this.
If your IC3 is a 4029, fit the solid
red wire link shown in Fig.6. Otherwise, fit the wire link where there is
a dashed red line. You can use a component lead off-cut for either.
Insert LED10 through the PCB from
the back (see the photo on page 23).
Solder its leads on the back of the PCB
to pads ‘K’ and ‘A’.
Rather than a dial applique, as used
in the 8-LED version, the 10-LED Metronome uses a transparent plastic
disc printed black with clear tempo
numbers (Fig.9). Choose the design
appropriate for your VR2 potentiometer type. If the printed disc is not
sufficiently rigid, a clear backing disc
might be needed.
Glue the disc to the back of a plastic
bushing fitted by friction or glue onto
the shaft of VR2. This bushing can be
made from a cut-down knob. LED10
illuminates the tempo numbers of the
disc to show through the plastic panel.
LED10’s brightness is determined by
the value of resistor R2, specified as
10kW; if you aren’t happy with the
brightness, lower the value of R2 to
make it brighter or increase it to make
it dimmer.
For the 10-LED version, the LEDs do
not protrude through the front panel,
but show through, so holes for the
LEDs are not needed. The holes for
the two lowest potentiometers (VR5/6)
can be 8mm, but VR2 will require a
larger hole to accommodate the bushing holding the tempo dial.
You will need to cut a thin panel on
which to mount the PCB with 15mm
threaded standoffs. This panel will fit
into the back of the timber frame. This
panel also carries the speaker, power
jack, power switch, 4700µF electrolytic and two 22W resistors (see photo).
Use the correct power socket to match
your plugpack output.
The speaker is held in place by screws
and nuts, with washers that have been
bent down on one side. You can use hot
melt glue or silicone sealant to secure
the large electrolytic capacitor.
This panel, and thus the PCB, is
secured to the frame by small wood
screws that attach to four approximately 8 x 8 x 12mm pieces of timber
glued to the inside corners of the frame.
There’s nothing extraordinary about
the case; I made mine in the same manner as a picture frame, with four 45°
mitred timber pieces glued together.
24
Parts List – 8-/10-LED Metronome
8-LED Metronome
1 double-sided PCB coded 23111211, 71 x 98mm (from the PE PCB Service)
1 3-way terminal block (CON1)
1 Serpac 131-BK plastic instrument case, 111 x 82.5 x 38mm
[Mouser 635-131-B, Digi-Key SR131B-ND]
1 timber base, 75 x 90 x 12.5mm (DIY)
4 AAA cells, preferably NiMH rechargeables (BAT1)
1 4xAAA battery holder (BAT1) [Keystone Electronics 2482;
Mouser 534-2482, Digi-Key 36-2482-ND]
1 8W loudspeaker, 36mm diameter (SPK1) [DB Unlimited SM360608-1;
Mouser 497-SM360608-1, Digi-Key 2104-SM260608-1-ND]
1 100kW linear 9mm/10mm vertical potentiometer (VR1)
[Mouser 652-PTV09A4025UB104]
1 500kW linear 9mm/10mm vertical potentiometer (VR2)
[Mouser 652-PTV09A4020UB504]
1 5kW linear 9mm/10mm vertical potentiometer (VR3) [Mouser
652-PTV09A4030UB502, Digi-Key PTV09A4030UB502-ND]
1 SPST or SPDT slide switch (S1) [Alpha SS60012F-0102-4V-NB; Mouser
SS60012F-0102-4V-NB]
1 14-pin DIL IC socket (optional; for IC1)
2 16-pin DIL IC sockets (optional; for IC2 and IC3)
3 knobs to suit VR1-VR3
4 adhesive rubber feet
2 small machine screws and nuts (for mounting slide switch)
1 large, heavy-duty paper clip
8 No.4 x 6mm self-tapping screws
2 small, short (~10mm) panhead wood screws (for mounting case to base)
1 solder lug with ~3.25mm diameter hole
various lengths and colours of light-duty hookup wire
Semiconductors
1 74HC132 quad 2-input Schmitt-trigger NAND gate, DIP-14 (IC1)
1 74HC191 presettable 4-bit binary up/down counter, DIP-16 (IC2)
1 74HC137 or 74HC138 3-to-8 line decoder, DIP-16 (IC3)
1 30V 1A NPN transistor, TO-92 (Q1)
[KSD471ACYTA, KSC2328AYTA or ZTX690B]
8 ‘superbright’ LEDs, round or oval (LED0-LED7)
[Broadcom HLMP-HM74-34CDD (green, oval), Kingbright WP7083ZGD/G
(green, 5mm), Jameco 2169846 (green, 3mm)]
1 6.0V 500mW zener diode (ZD1) [1N5233 or equivalent]
1 blue/white LED or 4.7V zener diode (LED8/ZD2) [1N5231]
1 150mA schottky diode; eg, BAT46/BAT48/BAT85 (D1)
[Jaycar ZR1141, Altronics Z0044, Mouser 511-BAT46]
Capacitors
1 4700μF 6.3V electrolytic [Mouser 667-EEU-FS0J472]
1 470μF 6.3V low-profile electrolytic [Mouser 232-63AX470MEFC8X75]
4 1μF 50V multi-layer ceramic
1 100nF 50V ceramic
1 220pF 50V ceramic
Resistors (1% 1/4W, 1/8W or 1/16W small body metal film unless otherwise stated)
1 300kW (optional)
2 10kW
While we recommend using 1% resistors, you can
1 2.2kW
use 5% resistors if desired. It might need more
2 220W
adjustments to get the tempo range correct.
1 47W
10-LED Metronome
1 double-sided PCB coded 23111212, 108 x 89mm (from the PE PCB Service)
1 12V DC 100mA+ plugpack
If you don’t like doing woodwork,
you could probably find a plastic box
with a clear lid that’s large enough to
house the PCB and other components,
and drill holes in the lid for the pots.
Checkout and adjustment
Before applying power, carefully check
the wiring to the off-board components;
a mistake here can cause excessive current and damage Q3 or cook the speaker
Practical Electronics | January | 2023
1 3-way terminal block (CON1)
1 chassis-mount barrel socket to suit plugpack (CON2)
1 8W loudspeaker, 50mm diameter (SPK1)
[DB Unlimited SM500208-1; Mouser 497-SM500208-1]
2 5kW top-adjust mini trimpots (VR1, VR3)
1 100kW 280° linear 9mm/10mm vertical potentiometer (VR2)
[Mouser 652-PTV09A4025UB104] OR
1 100kW 300° linear 9mm/10mm vertical potentiometer (VR2)
[Mouser 652-PDB12-M4251-104BF] (see text)
1 100kW top-adjust mini trimpot (VR4)
1 20kW linear 9mm/10mm vertical potentiometer (VR5)
[Mouser 652-PTV09A-4030UB203, Digi-Key PTV09A-4030U-B203-ND]
1 5kW linear 9mm/10mm vertical potentiometer (VR6)
[Mouser 652-PTV09A4030UB502, Digi-Key PTV09A4030UB502-ND]
1 SPDT slide switch (S1) [Alpha SS60012F-0102-4V-NB;
Mouser SS60012F-0102-4V-NB]
1 14-pin low-profile DIL IC socket (optional; for IC1)
1 8-pin low-profile DIL IC socket (optional; for IC2)
2 16-pin low-profile DIL IC sockets (optional; for IC3 and IC4)
1 timber base, 75 x 150 x 12mm (DIY)
1 timber frame, 110 x 130 x 40mm (DIY)
1 red-tinted transparent acrylic panel, 100 x 125 x 2.5-3mm (or to fit frame)
3 knobs to suit VR2, VR5 and VR6
1 clear, printable plastic and plastic backing for tempo dial, bushing or cutdown knob for mounting onto VR2
4 M3-tapped 15mm spacers (for mounting PCB to panel)
8 M3 x 6mm panhead machine screws (for mounting PCB to panel)
2 small machine screws and nuts (for mounting slide switch)
2 M3 x 10mm panhead machine screws, flat washers and nuts (for mounting
speaker)
1 large, heavy-duty paper clip
6 small, short (~10mm) panhead wood screws
various lengths and colours of light-duty hookup wire
1 55x55mm square of speaker cloth
Semiconductors
1 CD4001BE quad 2-input NOR gate, DIP-14 (IC1)
1 TLC555IP or LMC555CN CMOS timer, DIP-8 (IC2)
1 CD4029BE, CD4510BE or CD4516BE 4-bit binary up/down counter, DIP-16
(IC3)
1 CD4028BE 4-to-10 binary decoder, DIP-16 (IC4)
2 BC558 30V 100mA PNP transistors, TO-92 (Q1, Q2)
1 30V 2A NPN transistor, TO-92 (Q3) [KSC2328AYTA or ZTX690B]
1 150mA schottky diode; eg, BAT46/BAT48/BAT85 (D1)
[Jaycar ZR1141, Altronics Z0044, Mouser 511-BAT46]
10 ‘superbright’ LEDs, round or oval (LED0-LED9)
[Cree C566D-RFE-CV0X0BB1 (red, oval) recommended]
1 5mm ‘superbright’ red LED (LED10) [Kingbright WP7113SRD/J4
recommended]
Capacitors
1 4700μF 16V electrolytic, 13mm diameter [Mouser 232-16PK4700MEFC125X]
1 1000μF 16V electrolytic, 8mm diameter [Mouser 232-16ZLH1000MEFC8X2]
5 1μF 50V multi-layer ceramic
2 100nF 50V ceramic
Resistors (1% 1/4W, 1/8W or 1/16W small body metal film unless otherwise stated)
1 22kW
1 18kW
2 10kW
1 4.7kW
1 3.3kW
2 390W ½W
4 220W
2 22W
coil or cone. Compare your wiring to
that shown in our photos.
Because of variations in components, the tempo will likely need to be
brought into line. A frequency meter
Practical Electronics | January | 2023
(even a very basic one as found in
many DMMs) or scope is helpful for
adjusting the tempos.
Turn VR2 to the slowest tempo (36
beats/min) and measure the pulse
frequency at pin 3 of IC2 or pin 15 of
IC3. Adjust the control voltage (pin 5)
of the timer (IC2) via trimpot VR1 to
get a frequency of 5.4Hz (see Table 1).
If adjusting trimpot VR1 cannot bring
the frequency to 5.4Hz, you need to
add another capacitor in parallel with
C1 and C2 (at position C3) to slow it
down, or reduce the value of C1 and/
or C2 to speed it up.
Once this frequency is correct, set
the tempo to 216 beats/min and adjust
trimpot VR3 to get 32.4Hz. If VR3 cannot bring the frequency to 32.4Hz,
change the value of its 18kW series
resistor, then repeat the adjustments
for the slowest and fastest tempos.
Finally, adjust trimpot VR4 to
the desired difference in brightness
between the two end LEDs and the
middle LEDs.
Click timbre and loudness can also
be modified for the 10-LED version.
Adjust the value of resistor R1 for
a smooth variation in loudness, as
described for the 8-LED version.
To change the timbre of the click,
you need to experiment with the
combined value of C4-C6. A larger
capacitance should produce a more
mellow click. The speaker can also
affect the tone, so try the speaker in
both polarities if you aren’t satisfied
with the initial result.
Troubleshooting
If the Metronome is not working, check
the orientation of IC2 and associated
parts. Also, check that the IC pins are
all inserted correctly; they sometimes
get bent and don’t go into the socket or
PCB. Check if there is a pulse at pin 3
of IC2 and pin 15 of IC3.
If the LED sequence is only in one
direction, it is likely that the SR-FF is
not working or not receiving the S and
R pulses from IC4.
Operation
The operation of either version is
straightforward. Turn the Metronome
on and adjust the Click Loudness, LED
Brightness and Tempo as desired.
The supply current for the 8-LED
Metronome is about 2-4mA, depending on the LED brightness, click loudness and tempo. AAA cells typically
are rated at about 900mAh. Thus,
assuming it is used for about half an
hour a day, alkaline or rechargeable
cells should power the 8-LED version
for about a month.
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
25
