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The Styloclone Project by Phil Prosser The Styloclone musical instrument This reinvention and homage to a musical classic is an excellent project for starters. It’s also lots of fun for the musically inclined, especially those interested in old-school instruments. The whole project fits on one easy-to-build circuit board that can be mounted in a case or built as a free-standing project! T his project is simple enough to be ideal for people learning electronics, but useful enough to be fun for all ages. A Stylophone is a very simple musical instrument that can play a single note at a time, driven by a stylus (or pen). A unique feature of a Stylophone is that it uses tracks on the PCB to form the instrument's keys. The original Dubreq Stylophone was released in 1968. While it has never become as popular as, say, the electric guitar, it definitely made a mark on popular music! Notable uses of a Stylophone are at the start of David Bowie’s “Space Oddity” and throughout the Tornados’ instrumental, “Telstar”. So, while simple, the distinctive sound of this instrument has a real place in music. The name “Styloclone” indicates Practical Electronics | June | 2025 it is not a real Stylophone; it is more an homage to the original instrument, drawing inspiration from it. Our version draws a lot from the 1970s design of the Stylophone, and keeps the essentials such as the PCB tracks forming the keyboard. As an engineer, my immediate reaction to this project was to ‘gold plate’ it, allowing it to do things that no sensible person would want. However, that would miss the essence of the Stylophone. We could have based it on a microcontroller, allowing all manner of clever stuff, including fancy waveforms and effects. Alternatively, we could ‘keep the purity’. Ultimately, we decided to use the KISS principle (“keep it simple, stupid”) and, if people enjoy the old-school goodness of our take on a Stylophone, we could develop a new-­f angled version later. Our Styloclone comprises a PCB, a stylus and an optional case, as shown in the photos. The original Stylophone circuit has two main parts. The first is a simple oscillator in which the stylus changes the RC time constant to play the notes. The secondary vibrato oscillator causes the note frequency to vary slightly but rapidly, making the sound more interesting. You may recognise the concept of vibrato as it is applied to many instruments, including the human voice. Early Stylophones used a unijunction transistor for the note oscillator but those are rare these days. Later versions used a 555 timer IC, which 3 Constructional Project Fig.1: the Styloclone circuit uses just two ICs and one transistor. IC1 is the main oscillator that produces a note when the stylus wired to CON3 touches one of the keypads, shorting a point in the resistor string to ground. The vibrato oscillator is based on transistor Q1; it varies the voltage at pin 5 of IC1, modulating the frequency at around 7Hz. VR1 is for calibration, VR2 for tuning and VR3 for volume control. remains super common today, so we have also used one. The original approach is a masterclass in squeezing as much as possible from the minimum number of parts. We kept the essence and added a few new parts to make a modern, buildable project. That includes a simple output amplifier, allowing us to use a standard 8W speaker. The original circuit used a high-impedance speaker, which is now difficult to obtain. We are using an LM386 amplifier, which is hardly dragging the Stylophone into the 21st century, as that part has been around since the mid4 1970s. The LM386 itself has some fame in the musical domain as a very common IC in guitar practice amplifiers and distortion boxes, so it is a fitting choice. You can hear some audio clips of the prototype Styloclone at siliconchip. au/Shop/6/432 but remember that I am more of an engineer than a musician, so don’t expect the Brandenburg Symphony! Still, they should give you an idea of the tone it produces if built as described here. It’s possible to make some simple modifications to change the tone, some of which will be mentioned later. Circuit details The resulting circuit is shown in Fig.1. There are two oscillators, one for vibrato and the second to generate the notes. The vibrato oscillator is built around BC549 transistor Q1, with 100nF capacitors and 68kW resistors forming a feedback network. The result is a very simple phaseshift oscillator. Early Stylophones used a 10MW pullup resistor on the base of Q1 to bias this amplifier, which is effective but subject to significant variation. We have used a slightly more complex arrangement that ensures a defined setpoint for the Practical Electronics | June | 2025 The Styloclone Scopes 1 & 2: the left scope shows the 1µF capacitor being charged and discharged by the 555 timer (cyan) and the output voltage being delivered to the speaker for a 440Hz A note (yellow). The control voltage (mauve) is about 5.3VDC in this case, although there is about 50mV AC superimposed on it from the output. The right scope shows the same signals as in Scope 1 but with a faster timebase and with vibrato enabled, visible as a periodic shifting of the waveform. vibrato oscillator. It should work for any high-gain NPN transistor similar to the BC549. The vibrato can be switched on and off using S2, which shorts the collector of Q1 to ground. This is a brutal but effective way of stopping this oscillator. We will discuss how the vibrato works as we describe the main oscillator. The main oscillator in the original Stylophone used a programmable unijunction transistor in the main oscillator, although early updates replaced that with the NE555 timer IC, which came out in about 1972. The 555 is set up as an astable multi­vibrator, which is a fancy way of saying that it oscillates continuously. For now, let’s look at it with the vibrato switched off and the tuning potentiometer at its midpoint. Note that the 555 will work just fine without the tuning potentiometer but without the ability to adjust the tuning. In this case, with the pen not touching any of the keyboard pads, both the trigger and threshold inputs are pulled to ground via trimpot VR1 and the long string of series resistors that ultimately connects to 0V. Referring to Fig.2, the output of Comparator C goes low, while Comparator T's goes high. The RS flip-flop is reset, so OUT goes low. The output buffer inverts this, so the 555 output goes high. It remains in this state while no note is selected. When the stylus touches the keyboard, it connects the 555 output to part of the resistor string that defines each note. The 1µF timing capacitor charges via these resistors, with the charge rate determined by the resistance in the string (ie, the note selected). Practical Electronics | June | 2025 It continues to charge until the voltage on the Threshold pin exceeds the Control voltage and the Comparator C output goes high. The RS flip-flop is reset, and the OUT pin goes high, so the 555 output pin goes low. The 1µF capacitor starts to discharge via the resistor string. Once the voltage goes below the Comparator T positive input reference voltage, the Comparator T output goes high again, setting the RS flip flop. This drives the output high, and the whole process repeats. The resulting oscillation is demonstrated in Scope 1. There are a few tweaks to the operation of the 555 IC in a Stylophone. The first is that the control voltage (CV) pin is connected to the wiper of a potentiometer. This varies the control volt- age, which changes the voltage over which the 1µF capacitor must charge over the oscillation cycle, allowing the Styloclone to be tuned. We have selected resistors for each note that are in tune with middle A at 440Hz, as long as the 1µF capacitor is reasonably accurate. The tuning works well, but this is a very simple circuit, so if you set the tuning pot very high or low, you will find the octave is a bit off. We have selected values for the keyboard resistor string that give very close to in-tune notes. The second tweak is that the vibrato oscillator is capacitively coupled to the 555's control voltage input. This adds an AC component to the control voltage and modulates the charge/discharge range required for the 1µF capacitor, as seen in Scope 2. Fig.2: the 555 timer generates two reference voltages at 1/3 and 2/3 of its supply voltage, which are fed to the inputs of two comparators. The outputs of those comparators control a flip-flop, which in turn controls the output voltage. The discharge transistor switches on when the output is low. 5 Constructional Project The desktop version of our Styloclone doesn’t need a case, keeping it simple and pure! It uses four standoffs in the corners for feet. Tuning calculations Getting the notes right took a lot of work. The standard formula for oscillation frequency for a 555 timer is f = 1.44 ÷ (C × [Ra + 2 × Rb]). With the tweaks to the circuit, such as not using the discharge circuit, we found we needed to use f = 0.5823 ÷ (R × C). This is because we are not charging from Vcc and discharging to ground but instead using the 555 output as the charge/discharge source. Even then, there was non-linearity across the scale; we were able to use this formula to get close to the right resistances, but then we had to handtune the values. We were conscious that this might introduce variation in behaviour for chips from different suppliers, which may have differing high and low output voltages. To test this theory, we drove around town and bought five different chips from various suppliers and batches. We verified that the resistances we chose worked for all of these with only minor variations. The fact that Stylophones have been made this way for years should have told us that we were jumping at shadows. Our choice of a 1µF timing capacitor in the oscillator defined the resistances required for each note. Some rather 6 odd resistances are required. Table 1 shows each note's ‘ideal’ frequencies and incremental resistances. Because the resistance is in a string, we have worked out the best-fit values from the E24 resistance range. As expected, there are minor errors. While these errors are not huge, they indicate there is no benefit in being overly anxious about achieving the exact modelled resistances. So, you can use 1% E24 resistors of the specified values; there is no need for more precision than that. We considered having one potentiometer per note, but even the cheapest trimpots would have cost more than 10 times that of simple resistors. It would have also made tuning very complicated! If you choose to fine-tune your Styloclone by varying the resistor values, remember to set the tuning control to your ‘zero point’ and keep it there while you select new resistors. You must start with the highest note and then work down the scale. All the tuning resistors add up, so if you go back and change a higher note, you need to retune all the lower notes. We have lined all these resistors up alongside each note on the board. Make sure you check each value as you go and don’t put any in the wrong spot, or the tuning will end up all wonky. The original Sytlophone used a highimpedance speaker. We have added an amplifier and optional line output in case you want to record a hit song with your Styloclone. The circuit around the LM386 is bog standard, and the only part warranting comment is the 1µF capacitor from its pin 3 to ground that rolls off the high-frequency response. The resulting filter has a pretty brutal corner frequency of around 190Hz. The resistance of the RC circuit is formed by the 1kW resistor in parallel with the 4.7kW resistor from the volume control plus the volume control’s resistance. This filter tones down the harshness of the square wave output a lot. If you want a brighter sound, reducing this 1µF capacitor will give you that. We have used a simple 57mm speaker for this device; they are cheap and rugged. This speaker can produce plenty of output, but if more sound is required, you can certainly plug it into your Marshall Stack via the mono 3.5mm jack. The stylus We have used yet another Biro (ballpoint pen) case as the stylus handle in this project. This seems to be something of a tradition in the making! As the tip, we used a 4mm Posidriv machine screw (Phillips head would also be fine), to which we soldered the stylus lead. We then glued it into the tip of that obligatory Biro case using Araldite epoxy – see the photo below. You might find another way of doing this. For example, you could simply place a small diameter heatshrink tubing around a stiff piece of wire and bend the end back so it isn’t sharp. However, if you use an alternative approach, ensure that the player is insulated from the stylus tip, as otherwise, skin resistance and body The stylus is made from a Biro (ballpoint pen) case, an M4 machine screw and siliconeinsulated wire soldered to the end of the screw. Practical Electronics | June | 2025 The Styloclone capacitance could interfere with its operation. Our goal was to make a conductive stylus tip that did not have sharp edges that would scratch and wear the PCB ‘keys’. For the stylus wire, we used superflexible silicone-insulated wire on the assumption that the lead will be waved around a lot. We don’t want the lead breaking! The wire I used has 95 strands of copper with silicone insulation and is made for this sort of application (well, multimeter leads etc). The large number of thin wires in the wire will make this very tolerant of flexing and maximises the fatigue life. The length of the stylus lead can be tweaked, but 600mm feels about right to us. To attach the wire to the screw, we held the screw in a vise, applied flux to end top of the screw and tinned it. We then soldered the flexible wire to the end of the screw. We have included two 4mm holes in the PCB to secure the stylus lead using a zip/cable tie in front of the stylus connector. This allows you to run the stylus lead through a simple hole you drill in the side of the case without the risk of it being pulled too hard and damaged. We recommend you select a side to suit your right- or left-handed preference. Case options We have produced two slightly different PCB designs. The first, coded 23106241, fits into a Gainta G1183 sloped instrument case and allows you to mount the board to the front panel using the inbuilt mounting holes. It gives a neat finish and delivers a neatly packaged product. The way this board mounts requires all the components to be placed on the back of the board, with the ‘keys’ on the front of the board so they can be presented to the user through a large rectangular hole in the case. However, we recognise that the case costs more than all the electronic components, and it isn’t strictly required. So the other PCB option, coded 23106242, has all the components on the top side of a rectangular board, with holes in the corners to use 10mm Nylon standoffs as feet. This way, you can set it on a flat surface to play it as a bare electronic assembly, or perhaps mount it onto Practical Electronics | June | 2025 a piece of timber or other type of stiff board. You need to decide which version you want to build before starting construction since the circuits are identical but the board layouts are quite different. Case preparation For the case-mounting version, we have put in some effort so that mounting the board is easy. The hardest part is neatly cutting the rectangular hole in the case to access the keys. We’ll describe how to prepare the case before assembling the PCB, as it might be easier to do it first. You can skip this section if you are building the version without the case. The best approach to cutting the large hole is to mark its outline on the case, then drill 6mm holes in each corner 3mm inside the actual corner junction, so the edges of the holes align with the cutout. Next, use a hand saw or rotary tool to cut just inside the lines. You can then file the hole to size. ABS plastic works very easily and does not clog files too severely, so tidying up the hole is a lot easier than you might expect. The drawing for the front panel cutout and drilling is in Fig.3. I used a really sharp knife and ruler to score each line, but you have to be very careful not to slip and cut your fingers while doing that! If you cut like this repeatedly, you can actually go all the way through the plastic. If you decide to do that, we suggest you wear chainmail (mesh) gloves. We are not making this up; chefs use them to avoid cutting their fingers. They are readily available, not too expensive, and surprisingly flexible. Search for “cut-resistant gloves” or “chef’s gloves” to find them. The remainder of the case preparation is drilling the speaker holes on the top panel, plus the switch and potentiometer holes in the rear panel. We found it kind of fiddly to get the measurements right for the rear panel, which is at an angle to the front panel and has rounded edges. So be Table 1 – Styloclone note ideal frequencies, resistors & actual frequencies Note Ideal Resistor Running total Measured Error B 493.9Hz 68Ω 1179Ω 493Hz -0.9Hz -0.3 to -0.1 A♯ 466.2Hz 68Ω 1247Ω 466Hz -0.2Hz -0.2 to +0.1 A 440.0Hz 75Ω 1322Ω 440Hz 0.0Hz -0.1 to +0.1 G♯ 415.3Hz 82Ω 1404Ω 416Hz 0.7Hz +0.0 to +0.3 G 392.0Hz 82Ω 1486Ω 394Hz 2.0Hz +0.4 to +0.6 F♯ 370.0Hz 91Ω 1577Ω 371Hz 1.0Hz +0.1 to +0.4 F 349.2Hz 91Ω 1668Ω 351Hz 1.8Hz +0.4 to +0.7 E 329.6Hz 100Ω 1768Ω 332Hz 2.4Hz +0.6 to +0.9 D♯ 311.1Hz 120Ω 1888Ω 312Hz 0.9Hz +0.1 to +0.5 D 293.7Hz 120Ω 2008Ω 294Hz 0.3Hz -0.1 to +0.3 C♯ 277.2Hz 120Ω 2128Ω 278Hz 0.8Hz +0.1 to +0.5 C 261.6Hz 120Ω 2248Ω 264Hz 2.4Hz +0.7 to +1.1 B 246.9Hz 130Ω 2378Ω 251Hz 4.1Hz +1.5 to +1.9 A♯ 233.1Hz 150Ω 2528Ω 236Hz 2.9Hz +1.0 to +1.5 A 220.0Hz 150Ω 2678Ω 223Hz 3.0Hz +1.1 to +1.6 G♯ 207.7Hz 180Ω 2858Ω 210Hz 2.3Hz +0.9 to +1.3 G 196.0Hz 180Ω 3038Ω 198Hz 2.0Hz +0.8 to +1.3 F♯ 185.0Hz 200Ω 3238Ω 186Hz 1.0Hz +0.3 to +0.8 F 174.6Hz 200Ω 3438Ω 175Hz 0.4Hz -0.1 to +0.5 E 164.8Hz 200Ω 3638Ω 164Hz -0.8Hz -0.8 to -0.2 D♯ 155.6Hz 200Ω 3838Ω 155Hz -0.6Hz -0.7 to -0.1 D 146.8Hz 220Ω 4058Ω 147Hz 0.2Hz -0.2 to +0.5 C♯ 138.6Hz 220Ω 4278Ω 139Hz 0.4Hz -0.1 to +0.6 130.8Hz 240Ω 4518Ω 131Hz The Error column has an uncertainty of ±0.5Hz. 0.2Hz -0.2 to +0.5 C Error (%) 7 Constructional Project cautious with this; perhaps start with smaller holes than required and be prepared to file them to a final size. Fig.4 shows the drilling details for the rear panel. Regarding labelling, we were on a roll with the retro feel of this project and had just purchased a 3D printer for the young enthusiast. So we got out the 3D modelling software and made a cool label for the project. We reckon it looks pretty good. The STL file is available from siliconchip.au/Shop/11/434 Note that it needs to be printed at a 5% scale, a simple selection in the Cura slicer program. You could probably use super glue to attach the PLA 3D-printed parts to the ABS plastic case, but we read that you can melt ABS plastic in acetone to make “ABS glue”. We picked up some of the swarf from drilling the front panel, put it in a teaspoon of ac- etone and mixed until we had a thick, sticky liquid. We dabbed it onto the back of the PLA labelling and carefully placed it on the front panel, where it stuck perfectly. Get it in the right spot when you put it down and do not move it. PCB assembly Building the Styloclone electronics is pretty straightforward. All parts are through-hole types, and we have used larger pads where possible to facilitate soldering. First, check that you have the correct PCB, either the one coded 23106241 that measures 179 × 123mm for the case-mounting version or the standalone board that’s coded 23106242 and measures 207 × 124.5mm. Figs.5 & 6 are the PCB overlay diagrams for the two versions that show where all the components go. The best place to start is with the resistors. We have had to use several E24 resistors with less-familiar values like 91W, 200W etc. These are used to get the tuning right for each note, so you really need the specified parts. Most local retailers stock these E24 values, and the larger online suppliers like Farnell, Mouser, DigiKey and RS have them too. When fitting these, we recommend measuring each part's resistance as you go since the colour codes can be tricky to read sometimes. If you get a part in the wrong spot, you will find that some of the notes are out of tune. Once all the resistors are in place, add 1N5819 schottky diode D1 (taking care to match its orientation to the overlay) and the 200W trimpot. At this point, it is convenient to mount the 555 and LM386 ICs. Do this before the capacitors, as it will give Fig.3: cutting the large rectangular hole is the fiddliest part of the project. We used a Dremel cutting tool and file for ours. The easiest way is to mark and drill the corner holes for the cutout from the inside, then do the remainder of cutting and drilling from the outside. If you need an inside template, you can print this out mirrored. Regardless, double-check which side you drill the speaker holes on. All dimensions in this diagram are in millimetres. 8 Practical Electronics | June | 2025 The Styloclone Shown at left is a view of the case-mounting version inside the case, from the underside, where the components live. The keyboard is accessible through a cutout on the other side. The finished Styloclone is shown above in its case. This version in a box gives much richer sound than the free standing version. you more room to get them in place. The 555 and LM386 look the same, so you will need to check the part numbers and ensure they are both the right way around. The dot or indent on the chips goes to the top of the board, and the PCB silkscreen has dots in nearby positions to help you. It is then time to install the capacitors. Make sure you use film capacitors for values up to 1µF; either MKT or greencaps will work fine, although greencaps may need their leads bent to fit the pads. Do not use ceramic capacitors, as they have a huge temperature coefficient and large values can even be microphonic. The 1µF capacitor on pin 2 of the 555 is particularly critical; it must be close to 1µF, so use a part with a decent tolerance (5% if possible; failing that, 10%). After that, you can add the electrolytic capacitors. Make sure each one is the right way around, with the longer positive lead on the side with the + symbol. The stripe on the can indicates the negative side, so it should be opposite the + symbol in each case. To make this process easier, all capacitors are orientated in the same direction. Next come the two 5mm screw terminals and the battery clip. We like to make connections to offboard components using terminals as it makes it easy to service, but you could simply solder flying leads to the board if that suits you. Put a dab of neutral-cure silicone sealant under the battery clip before you solder it to the board, as that will keep it secure when the Styloclone is in use. We have added two 4mm holes on either side of the battery, allowing you to ‘zip tie’ it to the board so it can’t come out during transport. Now fit the two switches. We have used PCB-mounting switches from a local supplier. Similar switches are available from Mouser and other larger suppliers; you could run flying leads from the PCB pads to panel-mounted switches in a pinch. Two similar but not identical types of potentiometers are used: a 5kW linear type for the tuning control and a 5kW logarithmic type for the volume control. Both are standard 16mm-size devices. The volume pot will be labelled A5K, where A indicates “Audio Fig.4: the drilling details for the Styloclone’s rear panel. This is the top of the case; the front panel is at the bottom in this drawing. All measurements are referenced to the top of the fixing post on the left. Practical Electronics | June | 2025 9 Constructional Project MINI SPEAKER E A# G# F G D2# C2# B A C2 G2 # F2 # D2 E2 3.3kW D C F# D# C# TUNE F2 G2 A2# A2 B2 VOLUME VR3 5kW log. + 1kW 10kW 100nF REAR OF MINI SPEAKER 100nF 68kW 100nF 68kW 100nF 4.7kW Q1 BC549 + 100mF FINE CON2 TUNE SPEAKER 240W 220W 220W 200W 200W 200W 200W 180W 270kW 180W 130W STYLUS TP2 100W 91W 91 W 82 W 82W 75W 68 W 68 W B2 100nF 1kW 100nF TP1 VR1 200W CON3 120W + 2 .7 W TO STYLUS 470mF 120W + 2 20 m F 1m F 120W 100nF IC1 555 120W IC2 LM386N 1k W 5819 D1 150W BAT1 9V BATTERY HOLDER VR2 5kW lin. 4.7kW 1kW 120kW 47kW 10mF 4.7kW 100nF 1mF S2 150W S1 CON1 VIBRATO POWER OUTPUT FRONT OF BOARD C UNDERSIDE OF BOARD (TRUNCATED) 10 Practical Electronics | June | 2025 The Styloclone Fig.5: here is where to fit all the components for the case-mounting version of the Styloclone. In this version, all parts mount on the bottom while the ‘keys’ are on the top. Take care with the orientations of the diode, ICs and electrolytic capacitors and make sure you don’t mix up all the different-value resistors! 240W C C# D# E MINI SPEAKER 200W 200W F# 200W 270kW 200W F 3.3kW 180W 4.7kW 150W + B GND 100nF 130W FINE TUNE 120W 120W VR1 200W CON3 STYLUS 1mF IC1 555 120W 100W 10mF TP1 OUTPUT 1m F IC2 LM386N 220mF VR3 5kW log. + 470mF 1kW E2 VOLUME 100nF 100nF D2# S2 VIBRATO 100nF 120W D2 1kW 68kW C2 C2# 100nF 68kW VR2 5kW lin. 1kW A# 150W 4.7kW A Q1 100nF BC549 120kW 47kW 100mF 4.7kW TUNING SPEAKER 1kW G# 180W 10kW G + 100nF CON1 91W F2 82W G2 G2# 82W 100nF BAT1 TP2 9V BATTERY HOLDER + 75W A2 D1 A2# 5819 68 W POWER 68W S1 ► B2 91W – F2# 2.7W ► Practical Electronics | June | 2025 220W D Testing your Styloclone Once all the parts are in, insert a fresh battery and measure the voltage across it. It should be near 9V. If the voltage is low, look for parts getting hot; if none are, pull the battery out and check that it is fresh. With the voltage rail all good, set the volume and tuning controls to midrange and touch the stylus to one of the key pads. You should hear a tone. Run up and down the keys and you should get a reasonable set of notes. If you get nothing: 220W + Taper”, while the tuning pot is linear and will be labelled “B5K”. Do not get these mixed up. If you intend to plug the Styloclone into an amplifier or recorder, mount the 3.5mm socket now. If you will never use it, you can save yourself a bit of money and leave it off. We used a thin bead of neutral-cure silicone sealant around the hole in the PCB to attach the speaker. After applying the sealant, gently push the speaker into place. An alternative is to use 5-minute Araldite (or another epoxy glue), which works a treat and is pretty permanent. The speaker must go in so that the cone is visible from the same side as the tin-plated ‘keys’. If building the board designed to mount in the case, the speaker will be inserted from the opposite side of the board to the majority of the components, so the magnet comes through to the same side as the components and the cone can present through the front panel. If you are building the standalone board, the speaker is inserted from the same side as the components, and the magnet will be on the underside of the board. Once you glue the speaker in, take a break and ensure your glue cures. Once that silicone cures, the speaker will never fall out, but until then, it will fall out and make a mess of everything near it. Don't ask me how I know! Solder wires to the speaker terminal and screw them into the speaker header; it doesn’t matter which way around they go. Also connect a wire to the stylus connector for testing. Depending on whether you are left- or right-handed, drill a 3mm hole for your stylus wire to go through in one side of the case. This should be on the top half of the case, about halfway down the length. Fig.6: for the standalone (non-case) version of the Stylophone, all parts mount on the top side, which also has the keyboard. All the same parts are used in both versions. Again, watch the orientations of the diode, ICs and electrolytic capacitors and make sure you don’t mix up all the different-value resistors. 11 Constructional Project ‰ Check the voltage on pin 8 of IC1, the 555 timer. It should be more than 8V. If not, then something is wrong with the power supply. Is it on? Is diode D1 the right way around? ‰ Is IC1 indeed a 555 and is it the right way around? ‰ Check that pin 5 of IC1 is between 4V and 6V. The tuning pot sets this, so try adjusting that. It does not need to be exact, but it should not be pegged to one of the rails. ‰ Check for an AC voltage on pin 3 of IC1. This will be a square wave. ‰ If there is a voltage there, trace through the 10µF capacitor to the clockwise terminal of volume control VR3, then to its wiper and on to pin 3 of IC2, the LM386. Try turning the volume up if you lose the signal at the wiper of VR3. ‰ Check pin 6 of IC2 for 8-9V DC. If this is not present, track back to the battery again. ‰ Check pin 5 of IC2 for an AC voltage. If this is present, is the output capacitor the right way around, and is the speaker wired up properly? Are its terminals shorted? If the notes are all wrong: ‰ Check that 200W trimpot VR1 on the board is set to around 110W. You should measure close to 1110W across test points TP1 and TP2, which are just below the battery (this measures VR1 and a 1kW series resistor). ‰ Check that the correct values have been used for the row of resistors near the keys. ‰ Set the tuning potentiometer, VR2, to about 2/3 scale. Turn the Styloclone on and measure the voltage at pin 5 of the 555 timer. This should be about 5.3V. If not, adjust the trimpot and see if it can be set to about 5.3V. Check that you have not swapped the linear and log pots. ‰ Touch the stylus to the high B. You should get a reasonably high note at around 494Hz. If this is way off, check the value of the 1µF timing capacitor. If this note is wrong, every other note will also be wrong. ‰ Assuming the high B is OK, run along the notes going down the scale. If you find a wrong note, check the associated resistor and correct the problem. Repeat until they are all correct. ‰ Remember that all lower notes are built on the preceding notes, so you should only fix a resistor associ12 Parts List – Styloclone 1 134 × 189 × 55mm sloping ABS desktop instrument case [Gainta G1183B(UL), available from TME] OR 4 M3 × 10mm tapped Nylon spacers 1 double-sided PCB coded 23106241 (case version), 179 × 123mm OR 1 double-sided PCB coded 23106242 (standalone version), 207 × 124.5mm 1 57mm (2¼-inch) diameter 8Ω 700mW+ loudspeaker (SPK1) 2 PCB-mount right-angle miniature SPDT toggle switches (S1, S2) 1 200Ω top-adjust mini trimpot (VR1) 1 5kΩ 16mm single-gang linear (B5K) potentiometer (VR2) 1 5kΩ 16mm single-gang logarithmic (A5K) potentiometer (VR3) 1 PCB-mount 9V battery holder (BAT1) 1 PCB-mount 3.5mm SPST 3-pin mono jack socket (CON1; optional) [SCHURTER 4832.222, Farnell 152206] 2 2-way 5mm/5.08 miniature PCB-mounting terminal blocks (CON2, CON3) 4 M3 × 6mm panhead machine screws 4 M3 shakeproof (star) washers 1 ballpoint pen case 1 short M4 panhead machine screw 1 9V battery 2 short 2.5mm- or 3.5mm-wide cable ties (‘zip ties’) 1 60cm length of white silicone-insulated hookup wire (outside diameter ~2.5mm) Semiconductors 1 555 timer IC, DIP-8 (IC1) 1 LM368N mono amplifier IC, DIP-8 (IC2) 1 BC549 30V 100mA NPN transistor, TO-92 (Q1) 1 1N5819 40V 1A schottky diode (D1) Capacitors (16V electrolytic unless noted) 1 470μF 1 10μF 50V electrolytic 1 220μF 2 1μF ±5% 63V/100V MKT 1 100μF 8 100nF 63V/100V MKT Resistors (all 1/4W 1% axial unless noted) 1 270kΩ 4 1kΩ 4 120Ω 1 120kΩ 1 240Ω 1 100Ω 2 68kΩ 2 220Ω 2 91Ω 1 47kΩ 4 200Ω 2 82Ω 1 10kΩ 2 180Ω 1 75Ω 3 4.7kΩ 2 150Ω 2 68Ω 1 3.3kΩ 1 130Ω 1 2.7Ω (5% OK) ated with a wrong note if all the notes above it are correct. Switch on the vibrato using switch S2 and check that you get a warbling effect. Now you should have a working Styloclone. We used VR1 and VR2 to tune the upper A on our unit to 440Hz. At this frequency, the resistors we selected have pretty much the whole range of notes in tune (just as importantly, you’ll be in tune with a concert grand piano). The tuning process is: 1. Using a DMM, adjust trimpot VR1 to achieve 1110W between TP1 and TP2. 2. Adjust tuning potentiometer VR2 to get 440Hz at pin 3 of IC1 when the upper A is played. If you have a frequency meter, probe the speaker output. 3. Check that the other notes are in tune. If they are not, use a DMM to check the associated resistor values. How long will the battery last? That depends on how loud you play it. When switched on but not playing a note, ours drew 8.5mA. A typical 9V battery would idle for about 50 hours before going flat. At moderate volumes, the current draw increases to about 60mA, which means it should provide about 3-6 hours of playing time. You should now be set to go and PE create your masterpiece! Practical Electronics | June | 2025