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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 uses a 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 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. 24 Silicon Chip Australia's electronics magazine siliconchip.com.au 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 a LED ‘chaser’ is needed. LED options 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 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 the pulse and a one-transistor amplifier to drive a small loudspeaker. The block diagrams of the two Metronomes are shown in Figs.1 & 2. In the 8-LED design, the tempo pulse is generated by a Schmitt-trigger NAND gate (part of a 74HC132). This clocks fourbit 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 design, the pulse is generated by a CMOS version of the ubiquitous 555 timer. It clocks fourbit 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, 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. 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 & 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 8-LED version The LEDs should be of the same type and matched; 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 it works as follows. Schmitt-trigger quad NAND gate IC1d is configured as a pulse generator. 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. siliconchip.com.au Australia's electronics magazine January 2022  25 The pulse frequency, and thus the metronome tempo, is determined by potentiometer VR1, resistors R1 & 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. 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. 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. 26 Silicon Chip Australia's electronics magazine siliconchip.com.au 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, a plot of the output voltage versus the input voltage. As a result, the negative-feedback RC circuit shown in Fig.b 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, 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 & 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 siliconchip.com.au Fig.a & 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 & 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 & output waveforms for a Schmitt-trigger inverter used as an oscillator. 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 Australia's electronics magazine 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). January 2022  27 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. As PCB-mounting potentiometers with built-in switches are rare, 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 version 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 & 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 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 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 their values tweaked; that is why some parts do not have an associated value. Whichever version you are building, 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 Fig.4: the 10-LED Metronome uses a more complicated LED brightness control scheme with PNP transistors Q1 & 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. 28 Silicon Chip Australia's electronics magazine siliconchip.com.au 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 & 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. siliconchip.com.au Australia's electronics magazine January 2022  29 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). 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 orientated 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 & VR3) and one 100kW (VR4); as well as two PNP transistors (Q1 & Q2) and one NPN transistor (Q3). Fit them now, being careful not to get the different 30 Silicon Chip 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; 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. 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 & 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. Australia's electronics magazine siliconchip.com.au 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. 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 is to connect several in series, along with a current-limiting resistor, then power the entire string from the bench supply and slowly wind its voltage up. That way, you can make a direct comparison over a range of brightnesses. The construction 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. 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 siliconchip.com.au 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 orientate 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 Australia's electronics magazine 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 bit by pinching them with pliers. Solder the three pots now, after re-checking they have the correct values. Now, paying attention to their orientations (see the A & 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 will probably not be seated on the PCB but spaced away from it by several millimetres. January 2022  31 Fig.8 is the tempo dial; this can be downloaded from the Silicon Chip website. 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 below). The 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 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 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 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%. 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 from our website as a PDF. 32 Silicon Chip Australia's electronics magazine siliconchip.com.au 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 & 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 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 siliconchip.com.au 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 CON1 pin 3 CON1 pin 1 CON1 pin 2 Australia's electronics magazine January 2022  33 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 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 33). 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 accomodate 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 34 Silicon Chip Parts List – 8-/10-LED Metronome 8-LED Metronome 1 double-sided PCB coded 23111211, 71 x 98mm 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 1 12V DC 100mA+ plugpack Australia's electronics magazine siliconchip.com.au Table 2: resistor colour codes 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 & 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 siliconchip.com.au Australia's electronics magazine 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. 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 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 (even a very basic one as found in many DMMs) or scope is helpful for adjusting the tempos. January 2022  35 120 120 50mm 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. 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 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 36 Silicon Chip 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. 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 & 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, experiment with the combined value of C4-6. 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 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. Australia's electronics magazine 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. SC siliconchip.com.au