A Fantasy on the Stylophone Theme

I decided to make an electronic musical toy as a Christmas gift for my little son. I browsed the Web looking for inspiration and found the Stylophone, a device on which I based my own design. In fact, I replaced short keys of the Stylophone with long ones, thus creating a kind of writing pad. Indeed, you can write characters and even words on this pad, and every character would have its own ‘sound portrait’. I think it would be useful for children who learn to write, making the learning process amusing.

This article was also published today (21 February 2019) in Nuts and Volts, a magazine for amateurs of practical electronics. http://nutsvolts.texterity.com/nutsvolts/201901/MobilePagedReplica.action?pm=2&folio=32#pg32

The circuit is basically an astable oscillator built with an

IC 555; you can find a description of how this circuit works, for example, on www.electronics-tutorials.com . The frequency of the oscillations depends on the values of R1, R2 and C1, and is calculated as:

(1) f = 1.44/(R1 +2*R2)*C1

Therefore, if you want to change the frequency, you should change either R or C. When playing a Stylophone you change R2 to change the frequency of the sound. I transformed the formula (1) to separate R2:

(2) R2 = 1/2* {1.44/(f*C1) - R1}

The range of my device includes 12 notes – from C6 (chosen at will) to C5#/D5b; the reason of this is purely geometrical – I used an available wooden box (198 x 98 x 31mm) as the enclosure for the device, and available aluminium stripes 7 mm wide; so, only 12 keys fitted in the width of the box.

C#5/Db5 554.37

D5 587.33

D#5/Eb5 622.25

E5 659.25

F5 698.46

F#5/Gb5 739.99

G5 783.99

G#5/Ab5 830.61

A5 880.00

A#5/Bb5 932.33

B5 987.77

C6 1046.50

A complete table could be found here: http://pages.mtu.edu/~suits/notefreqs.html

Let’s take R1 = 10 kΩ and C1 = 100 nF, then R2 for the
frequency of C6 (1046.50 Hz) calculated with the formula (2) is 1876 ohm (rounded to the whole number). The values for other frequencies can be calculated in the same way; the lower the frequency, the bigger the value of R2. Let’s add a series of resistors (R3, R4, etc.) to R2; then, as you touch the point ‘Key1’ with the stylus, it’s (R2 + R3) that are connected to the circuit; when you touch the point ‘Key2’, you connect (R2 + R3 +R4), and so on. Thus, the value of R3 is calculated as:

(3) R3 = 1/2*{1.44/(f(B5)*C1) - R1} - R2,

where f(B5) - is the frecuency corresponding to the note B5

The other values are calculated in the same way, they are indicated in the bill of materials. If you need to calculate new values, you can use an online calculator, for example, one from this site: www.ohmslawcalculator.com. The values of the resistors not being standard, it’s necessary to combine a required value from standard ones; however, you might replace permanent resistors with trimmers and establish the required values using an ohmmeter.

The circuit is mounted on a perforated plate, connections are made with flexible wires. I suggest to position the components on the plate exactly as they are positioned in the circuit diagram.

IC1 = NE555

R1 = 10 kΩ, R2 = 1876Ω, R3 = 411Ω, R4 = 438Ω, R5 = 456Ω, R6 = 482Ω, R7 = 520Ω, R8 = 546Ω, R9 = 570Ω, R10 = 626Ω, R11 = 650Ω, R12 = 690Ω, R13 = 730Ω; all resistors have power rating of 0.125W

C1 = 100 nF, ceramic; C2 = 10 mF x 10V, electrolytic

LS1 – a speaker with impedance of 8 ohm.

SW1 – a miniature toggle slide switch

B1 = 4 x 1.5 V batteries type AA

You see the outside and inside of the device on the pictures; you are free to choose you own way to position the components in the box.

I made them from an aluminium strip of the cross section 7x1mm. A thin layer of aluminum oxide that is formed on the surface of the keys protects them from further oxidation but does not prevent electrical contact between keys and the stylus. Pictures show a drawing of a key and also explain how to attach wires to the keys and fix the keys to the box. It’s important that lateral sides of the keys have no chamfers, otherwise the stylus would not move smoothly on the surface.

I made the stylus from a ball point pen that finished its service. The contact that touches the keys is, in fact, a pin of an electrical plug; I soldered a flexible wire to it, put the pin into the pen and filled the space around the pin with a transparent resin. There’s a requirement: the end of the pin should be half round and have a smooth surface, it’s necessary to avoid scratching the keys.

You’ll surely need an ohmmeter, if you use trimmers to establish required values of R3, R4, etc.; if you wish to get exact notes, you could use a kamerton to tune the device. A soldering iron and a wire-cutter will be needed to assemble the circuit; a small hacksaw, a drill and a file – to make the keys. However, your choice of other tools depends mostly on the enclosure that you’ll make for your device; I don’t exclude that somebody would 3D-print it.

This video shows you how to create a 'sound portrait' of a character.