Piezos Pt 2 – Contact microphones

The first plan I had was to use piezos as contact microphones.  This would enable me to amplify small string and percussion instruments, and, with additional circuitry, create new ambient electronic instruments.

The first step would be to wire the elements to cables and jacks.  It’s best I had read, to use shielded wire to do this, especially where the cables are long, so I thought it would be easier to buy and reuse cables with jacks already attached.  These are generally only about £1.50 – £2.00, so I bought some 2m leads with mono 3.5mm and 6.35mm jacks, and cut them in half.

I soldered the stranded shield to the outside of the disc and the core to the centre, using lengths of different diameter shrink tubing to strengthen the connections and make sure the cut wires were kept apart.  The already-attached leads were colour-coded black and red to indicate their function.

At this point I tested the discs to make sure they were picking up sounds before moving on to the next stage.


The next thing I did – again, a procedure suggested by Nic Collins – was to seal and further strengthen the piezo discs with rubberized paint.

There don’t seem to be many of these on the market these days, and although – compared to the other components in these projects – it was rather expensive (having had to be imported from the US), I opted for the one Nic Collins used: Plasti-Dip.

There is a cheaper Ronseal product, which is advertised as a liquid rubber for sealing flat roofs, but I wasn’t sure if it would be the right consistency.  Ronseal has a very annoying website which doesn’t allow you to search for products by name, and I couldn’t find it there, but it’s called Isoflex , and you can get more details by searching suppliers’ websites.

The Plasti-Dip looked rather thick, but the piezo discs are very thin and it wasn’t at all difficult to dip the ends in and cover them to a level just above the shrink tubing – the tin, as can be seen in the picture, is tall and thin, presumably for this reason.

The Plasti-Dip did the job perfectly, and I’ll be looking for other uses for it in projects and around the house, as hardly any of it was used up!  I bought the 400ml tin, as, at around £18 it seemed better value than the small tin at about £14 for 250ml, but even 250 ml would be a lifetime’s supply at the present rate.

By the way, although there are many colours available, I chose black because of its lack of visibility, in case the microphones would be used in situations where they needed to be discreet – e.g. in the presence of wildlife.  This would potentially be important in one particular future project.

Nic Collins recommends putting a small piece of insulating tape over the solder connections on the backs of the discs to give them extra protection against breaking off – which would be a great shame after all the soldering and dipping – but I forgot to do this, so I hope the dipping is enough to keep them together.

After dipping them I hung them up to dry overnight and dipped them again the following day to improve the seal

After two dips they looked fine.  I was worried that too much paint would dull the sound too much: although there would be a number of applications where I would use the piezos without dipping – or with a single dip – Nic Collins certainly mentions two dips and says that three would cause dampening of the sound pickup.  I was thinking that these ‘stand alone’ piezos might be used in a context where they needed to be waterproof, and one dip didn’t seem to seal them enough, so I left it there.

The following picture shows two sizes of piezo mic, 18mm diameter on the right, 27mm in the centre.  The one on the left has two elements connected in parallel.  I thought this might be useful to record oddly-shaped objects or objects with a large surface area.


I had read in more than one place that, although piezos can be used as contact mics just by plugging them into an amplifier or recorder, they work much better if the signal is run though a buffer circuit first.  The buffer needn’t necessarily amplify, but the low-frequency response would in any case be significantly improved.

A very useful series of articles starting here or here gives much greater detail on this.  I adapted the low noise preamp from that site, here or here:

Features of the circuit: the two diodes protect the opamp inputs from damage by restricting the maximum voltage that they can receive from the piezos.  The TL072, which I used – or the lower noise pin-for-pin replacement, the NE 5532 – is a dual opamp, so two of these circuits can be built using the same chip.   (The TL074 is a quad version, from which four  circuits can be made).  The pin numbers in brackets are those used by the second circuit built around the TL072: the second circuit is identical, and the only place where the two circuits meet is at the point marked A, the half supply voltage point created by the voltage divider – these two 100k resistors don’t need to be repeated for the second circuit.

The majority of piezo buffer circuits I found seem to use FET transistors, but these are quite expensive in comparison to the TL072, TL074 or NE5532.  I also read in one of the articles referred to above that ‘the manufacturers of FETs don’t control their parameters well . . . The gate-source voltage needed to bias the transistor into the linear region can vary between 0.25V and 8V, which leaves a good 7.75V down to a hopeless 0.4V for the transistor and load if used with a typical NiCad 8.4V PP3.  You’ll have to get more FETs than you need and throw out the dogs . . . Design manuals get all sniffy about that sort of thing because selecting FETs obviously adds to the cost if you are mass producing something. That’s not the case here, and there’s just no way to cope with a manufacturing tolerance which can throw more than 90% of the battery voltage away in variations in manufacture without screening the bad ‘uns.’

So I decided to stick with the opamps – the most recent batch of TL072’s I bought were between 4p and 5p each; the NE5532’s were more expensive at about 20p.

So, as soon as the microphone assemblies were ready, I made up one of these circuits with a TL072 and tested it out.

I plugged it into the line input of my MacBook, and it seemed to produce little background noise.  I attached two of the piezos which I had prepared as above, and clipped them to a plate, recording the sound in the Audacity app as I tapped the plate with a pen. As a comparison, I turned off the buffer and plugged the piezos directly into the computer.

The following short sound file illustrates the difference the buffer makes.  The output was a little louder with the buffer than without so I adjusted the recordings to be more or less the same volume.  First heard are the taps with the piezos plugged directly into the computer; then the taps using the buffer.  The difference is quite striking.

The buffer does amplify the sound, but I’ve tried to minimise this in the recording: nevertheless, there is a noticeable increase in the lower frequency response when the buffer is used.  The sound is much fuller, so, although the small expense and time involved in making buffers does add to the cost and the effort of piezo projects, I think it’s probably worthwhile for the improvement in the quality of the results.

In the following posts I’ll describe some particular projects in which I used the piezo elements as microphones, mostly with the buffers.


Piezos Pt 1 – General

Piezos, Piezo Sensors, Piezo Transducers, or Piezo Elements are small, cheap components that can be useful in several different ways to the electronic musician.  I had some ideas for ways I’d like to use them, and this series will describe some projects in which piezo elements were employed.

A transducer is a device which changes energy from one form to another – for example, tape heads and record pickup cartridges are transducers as they change magnetic signals on the tape or movement in the grooves of a record into electrical signals; microphones and speakers are transducers because in the first case they change movement in the air into electrical signals , or in the other they change electrical signals into movement in the air.

Piezo elements are transducers because they can transform physical movement into electrical signals like a pickup cartridge, or electrical signals into movement in the air like a speaker.  They do this not by sensing magnetic fields, like a tape head or guitar pickup, but by the movement of crystals, and this is what a piezo element has inside it.

Piezos work especially well when attached to something which will vibrate and produce electrical signals which can be amplified, or can amplify the vibration of signals fed into it.

In everyday life, they’re usually found in mobile phones, in buzzers or in place of speakers in smaller children’s toys.  This gives a clue as to the different ways in which they can be employed in electronic music circuits: as microphones, as speakers, or as triggers for switches.

Much of the information below was gleaned from Nic Collins’ book Handmade Electronic Music, and his series of videos on YouTube called Hack of the Month Club.

First of all, this is what piezo elements look like if you buy them from a components supplier, or take one out of a phone or musical toy:

or sometimes like this, if they come in the form of a sounder or buzzer – in this case, the element can be carefully removed from the plastic surround.

They vary in diameter from 10mm to 50mm.  I have used some of the smaller ones, but most of the ones I have are 18 or  20mm, and 27 or 35mm.

When I buy them, I prefer the ones with the leads already soldered on.  This saves a job – and it’s said to be quite tricky to do the soldering effectively – and they’re still very cheap.  The last batch I bought worked out at about 12p each for the 35mm diameter ones, and just 6p each for the 18mm ones.

Perhaps the first piece of music to use transducers in its realisation was Cartridge Music by John Cage, composed and first performed in 1960. As described on the webite of the John Cage Trust: ‘The word ‘Cartridge’ in the title refers to the cartridge of phonographic pick-ups, into the aperture of which is fitted a needle. In Cartridge Music, the performer is instructed to insert all manner of unspecified small objects into the cartridge; prior performances have involved such items as pipe cleaners, matches, feathers, wires, etc. Furniture may be used as well, amplified via contact microphones. All sounds are to be amplified and are controlled by the performer(s).’

Another composer who famously used transducers was David Tudor. Tudor – who was closely associated with John Cage – created a piece called Rainforest – originally in the mid 1960’s, but it went through a number of changes during the rest of the decade as Tudor’s techniques and equipment developed. The piece was based on the idea of making objects other than speakers vibrate, picking up the sounds they made with microphones and then filtering and mixing the resultant sounds.

‘My piece Rainforest IV‘, Tudor explained, ‘was developed from ideas I had as early as 1965. The basic notion, which is a technical one, was the idea that the loudspeaker should have a voice which was unique and not just an instrument of reproduction, but as an instrument unto itself . . .’

‘. . . I eventually acquired some devices called audio transducers. They were first developed for the US Navy because they needed a device which could sound above and under the water simultaneously . . . I had them in 1968 when MC [choreographer Merce Cunningham] asked me for a dance score and I decided that I would try to do the sounding sculpture on a very small scale. I took these transducers and attached them to very small objects and then programmed them with signals from sound generators. The sound they produced was then picked up by phono cartridges and then sent to a large speaker system.’

‘Several different versions of this piece were produced. In 1973 I made Rainforest IV where the objects that the sounds are sent through are very large so that they have their own presence in space. I mean, they actually sound locally in the space where they are hanging as well as being supplemented by a loudspeaker system. The idea is that if you send sound through materials, the resonant nodes of the materials are released and those can be picked up by contact microphones or phono cartridges and those have a different kind of sound than the object does when you listen to it very close where it’s hanging. It becomes like a reflection and it makes, I thought, quite a harmonious and beautiful atmosphere, because wherever you move in the room, you have reminiscences of something you have heard at some other point in the space.’

(from An Interview with David Tudor by Teddy Hultberg in Dusseldorf, May 17-18, 1988, http://davidtudor.org/Articles/hultberg.html).

A reviewer present at a performance of Rainforest described the appearance and sound of the piece as follows: ‘The entire piece sounds at first like an ethereal insect chorus, but the layers gradually disperse into patterns of jagged counterpoint, which in the performance seemed to harmonize perfectly with the movements of the dancers . . .

‘Most of the sounds are created by sine tones being reverberated through a forest of suspended metal containers, pieces of junk that function as “biased” loudspeakers imparting their own timbral colouration to the sounds which pass through them. These sounds are picked up by contact microphones, fed back into Tudor’s mixing and filtering controls, and then recycled back into the expanding forest of increasingly hybrid noises. The array of metal containers usually fills an entire gallery, and spectators are invited to walk around and put their heads inside the containers.’

(Roger Sutherland, Musicworks, Number 75, Fall 1999, http://moderecords.com/catalog/064tudor.html)

Some modern performances of Cartridge Music will use piezo elements instead of cartridges, although this might be considered cheating. Piezos, on the other hand, are ideal for achieving the kinds of effects employed by Tudor in Rainforest, and the different projects I planned with them will hopefully cover these uses and more.




The Binary Winder

I’d been planning a circuit which required a rapid input of binary numbers, ideally from a microprocessor, and it occurred to me that this could be a useful companion to the Bigfoot sequencer or the Chessboard Keyboard  – both methods I was using for sending binary data to keyboards and other devices.

I figured that this could be done manually by using a 16-way hex switch with no end stop – as they are commonly found – and a winding handle as used for re-stringing a guitar.

I bought a couple of these very cheaply on eBay:

and glued the switch inside the body of the string winder – the part which normally fits over the guitar machine head:

This would be the part of the device which did all the work.  Connecting +V to the common pin of the switch would enable it, as it was turned by the handle, to output the 16 binary numbers from 0000 to 1111.

The actual circuit itself was built inside one of the small transparent plastic boxes – described as ‘jewellery cases’ – which I had previously used for small projects such as the Touch-Radio and various effects devices.

In the above pictures the 16-way switch can be seen on the right-hand side, the 5-pin DIN binary output socket at the bottom, and 4 LEDs at the top to indicate the binary number.

The purpose of the 4 DPDT switches in the middle is to change the order of the 4 bits of the binary number.  This was so that winding the handle wouldn’t just produce an output running up and down the scale, but could be changed to give a bit of variety.


The actual circuitry consisted essentially of passing 9v to the 16-way switch, and the outputs of the 16-way switch to the 4-pin DIN socket via the 4 DPDT switches and a 4050 output buffer.

There was space inside the case for a 9v PP3 battery, so I included a battery clip inside, but also added a 3.5mm socket for external power.

As with many of my devices, I stuck a square of velcro on the back of the box so that a PP3 battery in a holder could be attached.

Space was a little tight inside, but not enough so to cause problems, and after some time the switches were all connected together with the 4050.  I didn’t bother with a circuit board, but just soldered all the connections to the 16-pin i.c. holder which the 4050 was plugged into.

Surprisingly, the case  closed without difficulties, and I was able to test it out with a couple of recent devices with binary inputs, The Telephone and the Carousel Keyboard.

In fact, there is a limit to how quickly these devices can respond to the winder – especially when the pitch is lowered, which seems to slow down the instruments’ responses as well.  However, it was very effective indeed in creating an instant sequence more quickly and accurately than it could be played on a keyboard – especially a keyboard with tiny keys like the Carousel.


The Carousel Keyboard

‘Carousel’ was just the brand name of this toy keyboard, it didn’t unfortunately look like a carousel . . .

. . . but having just finished the Animal Band and The Telephone, I wanted, while it was in my mind, to work on another device that could be controlled by the Bigfoot, only this time with a full two octave span, the ability to play a variety of different scales, and be tuned to play these scales in any key.

I opened up the Carousel, and it looked as though it would fit the bill.  The chip on which it was based was securely hidden under a black blob, but there were sufficient additional components to make me think a few simple hacks would be possible.

First of all, I wanted to check the instrument’s ability to be tuned.  I inserted batteries, switched on and began testing the circuit using the traditional wetted finger method – that is, starting the instrument playing one of its demo tunes and applying a wetted fingertip to different resistors on the board.

After a short test, I found the resistor that controlled the instrument’s pitch and timing.  It was a tiny SMD (surface-mounted) component, just 3 or 4 millimetres long, so was easily – though carefully – removed.  Its value – again, printed on the circuit board – was 300k, so after it was removed I replaced it with a 1M potentiometer, to increase the range of notes the instrument could produce.  The spot from which the resistor was taken is arrowed on this photograph, and the two leads going to the potentiometer can be seen:

(Also visible in the background are the wires connected to the PCB tracks required to trigger the notes).

I experimented with the resistance and found that the minimum the device would accept without crashing was about 200k, so I added an extra couple of 100k resistors before the 1M potentiometer, and found that a considerable variation in the pitch was achievable.


Early on in this process I disconnected the internal speaker, which was quite terrible, and added my standard 4mm banana sockets for attaching an external speaker.  The sound was 100% improved, and experimenting became a great deal more pleasurable.

Just beneath the speaker sockets a switch can be seen, which I connected up to allow the internal speaker to be selected, if an external speaker wasn’t available.

I later added an audio out socket, to allow the Carousel Keyboard to be played through the Taurus amplifier.  At first this didn’t work at all – virtually no sound came out, even though it would work with the internal or external speakers.  But I realised the output of the circuit needed a load in place of the speaker, so I put a 10 ohm resistor between the audio out and ground pins on the audio out socket, and after that it worked fine with an external amplifier.


I was saved from having to do the job I had performed on the Animal Band and The Telephone, working out which PCB tracks needed to be connected to produce the different notes: to my surprise, this was printed on the back of the circuit board.

There appeared to be an 8 x 5 matrix, so connections were made to the relevant PCB tracks, ready to be brought out to a new board.


The intention was to be able to control the keyboard with the Bigfoot sequencer, so I added a 5-pin DIN socket and 4050 buffer, as usual, ready to accept the Bigfoot’s 4-bit binary input.  This is in the form A B C D, where A is the last – rightmost – bit in the binary number, and D is the leftmost.  A is sometimes referred to as the LSB (Least Significant Bit) and D the MSB (Most Significant Bit).

The binary number 0010, for example (the number 2) would mean that D was 0, C was 0, B was 1 and A was 0; in practical terms this means that input D was 0v, C was 0v, B was +v and A was 0v.

I needed one of the inputs (D) to be inverted – i.e. if the input was 0v, I needed +v, and if the input was +v, I needed 0v – so with the 4050 was a 40106 inverter, with sections which change 0v to +v and +v to 0v.  There was very little space inside the keyboard case, so the board with the 4050 and 40106 was tucked under the output socket.  (This was before I added the 10 ohm resistor on the socket).

The circuit of the input section was like this:

There are 6 buffers in the 4050 chip; 4 of these are used (marked A B C D) and the inputs of the other two are connected to ground.  Likewise, there are 6 inverters in the 40106; 1 of these is used and the inputs of the other 5 are connected to ground.  Unused inputs on CMOS chips should normally be grounded like this to ensure correct operation.

Normally, I would use a 4067 to cover the two octaves produced by the Bigfoot – as in the Animal Band and The Telephone – but in this case that wouldn’t work.  The 4067 is essentially a 16-way switch with a single pole; in this case there were 4 different ‘poles’ to which connections needed to be made, since the notes are produced by a matrix.

Using four 4067’s would be perfectly possible, but unnecessarily expensive – each one costs between 30 and 50p, and is physically quite large, being in 24 pin wide format.  As there would only be 4 or 5 connections to each pole, it would be more effective in terms of cost and space in this circuit to use 4051’s.  The 4051 is an 8 pole switch which works in more or less exactly the same way as the 4067, but is physically smaller  – and costs less than 15p!

An important difference between the 4051 and the 4067 – related to the number of outputs – is that the 4067 requires a 4-bit binary input (16 numbers, from 0000 to 1111), but the 4051 only a 3-bit (8 numbers from 000 to 111).  This means that a different way must be found with the 4051’s to ensure that outputs for the first 8 numbers are separate from the outputs for the second 8 numbers.

This can be done by using the Enable/Inhibit pins of the 4051’s.  Each 4051 – just like the 4067, in fact – has an Enable/Inhibit pin: if this pin is at 0v, the chip will work, and convert its binary inputs into individual outputs; if the pin is at +v, it won’t work.

So, the first 3 inputs from the Bigfoot binary input socket, A B & C, are passed on to the 4051’s in the next part of the circuit, but the 4th input, D, is not.  Instead, the 4th digit is used to turn pairs of the 4051’s off and on via their Enable/Inhibit pins.

4051’s 1 & 2 output the lower 8 numbers (0000 to 0111), so as long as the 4th, leftmost, digit is ‘0’, these two 4051’s are enabled.  0v at the Enable/Inhibit pin achieves this.

4051’s 3 & 4 output the higher 8 numbers (1000 to 1111), so if the 4th digit is ‘1’, an inverted signal from the 40106 sends 0v to enable these two.

The circuit to convert the binary input to separate outputs looks like this. (The 40106 gate is repeated from the diagram of the input circuit):

The lower 8 notes are divided between the top two 4051’s in the diagram, which work together with no overlap in notes, and the higher 8 notes are divided similarly between the bottom two, according to which common pin they must connect to.  The pin connections are named in the diagram as they appear printed on the keyboard’s main PCB: the 4 common pins are connected to tracks named BP10, BP11, BP12 and BP13.

The reason there are many more than 16 output pins shown is connected with the principle of the Bigfoot sequencer.  The idea is that the sequencer outputs the notes of a scale – do, re, mi, fa, so, la, ti, do – but the exact scale – major, minor, melodic, harmonic, etc. – is determined by switches on the receiving instrument.  Normally there would be 5 double pole switches, but due to the configuration of the pins in the Carousel keyboard, one of the switches (SW2) needs to have 3 poles.


As with other of my designs like this, there wasn’t a lot of circuitry as such – just a lot of interconnection between the chips.  After soldering the dozens of wires needed to link the 4051’s with the switches and the Carousel keyboard’s PCB, the inside of the instrument looked like this:

There was just enough room for the new circuit board with the four 4051’s on it.

The Chessboard Keyboard proved very useful in checking that everything was working properly – one wrong note revealed a connection error on one of the switches! – and the 4 LEDs were a good double check that the binary input was being interpreted correctly.


Although the pitch control potentiometer worked well, I decided there was a need to be more precise about the pitch, which would effectively set the key the instrument would be playing in when controlled by Bigfoot.  So, as I had done earlier with The Telephone  – referred to above – I added a switch to change between the potentiometer and a 12-way switch.

Between each of the output pins of the switch, I inserted a 100k trimmer – with an extra 100k trimmer before pin 1 – so that the pitch of the instrument could be set to any one of the 12 steps in the octave.

In The Telephone I used ordinary single-turn trimmers, but I though it would be a good test to see if multi-turn trimmers would be as good – that is, as accurate in establishing the pitch, remaining in pitch, and not taking up too much space in the cramped enclosure.

The type I chose looked like this:

Buying 20 at a time enabled me to get them at a reasonable price – about 8p each, although this was probably twice as much the single-turn trimmers I had used in The Telephone.  They were also much more than twice as big.

However, I soldered them all in place and set about adjusting the pitches.

In this instance, I didn’t really find a big advantage in using the multi-turn trimmers.  I was tuning the pitches by ear – maybe I was able to be more accurate than with the single-turn trimmers, maybe not.  It took longer to tune each note, of course, because of the number of extra turns required.

I would have been glad if the potentiometer/trimmer arrangement had been a bit smaller, but I found space on the right-hand side of the keyboard to fit it in with the potentiometer and the other switch.  This picture shows 1 – the 12-way switch with trimmers, 2- the 1M potentiometer, and 3 – the SPDT switch which allows either the 12-way switch or the potentiometer to be selected.

I chose two knobs which fitted the space available on the top surface, drilled holes and fixed them in place.


That was everything I planned to do with the Carousel keyboard for now, so I carefully closed up the case and screwed it back together.  I had to cut some rectangular holes in the base to make room for the 5 switches, but surprisingly everything else fitted in.



The MIDI CPU Project – 3. Bass Pedals

After building the MIDI CPU box and programming it to accept input from keys, switches and potentiometers, it was necessary to build MIDI instruments to use its features.

As the MIDI CPU accepted input via a 25-way DIN socket, it would just be necessary to equip each instrument with such a socket and link it to the MIDI CPU with a suitable cable.

The first instrument I’d planned was a set of 13-note bass pedals, and the initial SysEx file with which I’d programmed the MIDI CPU was suitable for this application, with the first 13 control terminals, 0 – 12, configured as a complete octave from C0 to C1.

I got the pedals from a local seller on eBay who dismantled and repaired Hammond organs.

They were in excellent condition, and the switches on each of the pedals seemed to be still wired.  They looked like this:

and the actual connections were like this:

Pedal switches

The two tabs on the front linked one side of all the switches together; when a pedal was pressed, this bus would be connected to the other side of that individual switch, the tab on the top.  This was perfect for this application, in which an individual switch connected to 0v would be interpreted by the MIDI CPU as a note command.

My main task, then, was to connect each of the 13 switches to a 25-way socket, in order to pass the switch presses to the MIDI CPU box.

In addition to this, I wanted to have Octave Up and Down and Hold commands available, so there would be 3 further connections to the 25-way socket.

Finally, I would have to construct a housing in which the pedal unit would sit.


There were no circuits as such involved in this project – it was just a case of wiring up the switches and connecting them to the socket.  At the same time I decided to add a couple of extra sockets that would enable the pedals to be used in a different way if necessary.

The additional sockets added were a 15-way socket, which would be compatible with the Superstylonanophone (another MIDI device, with  a built-in MIDI-USB interface):

and a 9-way socket compatible with the Apple IR remote:


I connected the switches to the sockets and, because the enclosure I had planned was designed to have another instrument on top of it (another MIDI foot controller, a Digikick Footar) I connected the Hold and Octave Up and Down connections to 1/4″ sockets so these switches could be external, rather than on the top of the pedals.  All the sockets were housed in a small panel, which would be attached to the rear of the enclosure.


After connecting the switches to the sockets, I found a D25 cable and attached the Bass pedal unit to the MIDI CPU box.  At the moment the MIDI CPU box is connected to a laptop via a Midisport 2×2 MIDI-USB interface – along with the Digikick Footar – and controls software instruments in Apple Logic.

I added 2 or 3 different bass instruments to the Logic set-up and tested the pedals.  All notes worked as they should.

With an external switch I tested the ‘HOLD’, ‘OCTAVE UP’ and ‘OCTAVE DOWN’ functions, which seemed to be working OK, so deemed it safe to screw down the top of the case in which the pedals were housed and fix the panel to the back of the case.


The Animal Band & The Telephone – 2

This post actually concerns The Telephone, to which I decided to make a small further adjustment.

After finishing the Animal Band and The Telephone, I realised it would be a bit limiting to leave them playing in only one key, so I went back to The Telephone and searched for the resistor which controlled the pitch of the notes.

Having found it – by means of the tried-and-tested wetted finger method – I removed it from the circuit board and replaced  the connection with a pair of wires.

In the past I had replaced such a resistor with a potentiometer, enabling continuous – and wide-ranging – adjustment of pitch.  In this instance, however, I just wanted to be able to tune The Telephone to other keys, so instead of a potentiometer I added a 12-way rotary switch, the idea being to tune each step precisely to each key.

The Telephone was in the key of B – B being the lowest note it would play.  Rather than make this the lowest key and have all the others higher, I thought it would be better to have B in the middle, with some of the others lower, and some higher.

After some experimentation, I concluded that the difference in resistance between one note and another was just a few k.  I had a number of spare 10k trimmers, so decided that these could be used.

I connected one of these between each of the 12 pins on the switch.  The last one was connected to the circuit board on one side of where the original 100k timing resistor had been; the pole of the switch was connected to the other side, via a 100k trimmer to bring the resistance roughly into line with the original.  In this way, when the switch was at position 1, it would connect through the 100k trimmer and the first of the 10k trimmers; at position 2 the value of the second 10k trimmer would be added; at position 3 the value of the second and the third would be added, and so on to the end.  The way it worked, the more resistance, the lower the pitch.

In the end, I couldn’t reduce the resistance enough to have B right in the middle – the device wouldn’t operate at the highest frequency required for that – so it ended up in 4th position on the switch.  It was quite OK to go down another 8 semitones so that any key could be selected while The Telephone was being operated by the Bigfoot sequencer.

I found a suitable knob to go on the switch, and put The Telephone back together.  Enabling it to be used in any key would make it a much more versatile addition to the collection.


The Alien Choir

The Alien Choir wasn’t a device I particularly wanted to circuit bend- the alien voices it makes are fine as they are!  – but I wanted to do a couple of things to make is easier and better to use.

The Alien Choir is one of the ‘Finger Beats’ series.  These are recognisable by their distinctive design, and are all operated in a similar way, using light finger pressure in certain places on a colourful illustration on the front.

I don’t have an instruction book with mine, so it’s not clear that all of its functions are working – in particular, recording samples with the built-in microphone – but it makes 8 excellent alien voice sounds.  The volume of the sounds can be turned up or down with the ‘+’ and ‘-‘ buttons on the left-hand side.


First of all, although 3v isn’t an odd or unusual value, I decided to add a socket and dc converter to allow the device to be powered by a mains adapter or larger – e.g. 9v – battery.  To this end I also added a small square of velcro to the back, as I had with other instruments before, to allow one of my 9v battery holders to be attached.

The dc converter was a small and inexpensive (less than 50p) module, which didn’t need any additional components, just an input voltage from 20v down to about 1v higher than the desired output voltage.  The output voltage is set by means of a small trimmer on the board.

In this case I found that the required voltage, as measured on my multimeter, needed to be a little lower (about half a volt lower, in fact) than the anticipated 3v in order for the circuit to work smoothly.  I don’t know if this is something specific to the Alien Choir, a difficulty in measuring the voltage precisely, or what.

Inside the device, I added a switch to choose between the battery or the external socket.  In the following picture  1 indicates the socket, 2 the dc converter and 3 the switch.

The device has a 3.5mm audio out socket (and an audio in, in fact, as can be seen in the lower right-hand corner of the picture above), but I decided to add banana sockets for connection to one of my external speakers.  This always produces a much better sound than the small built-in speakers these devices are fitted with.

The small switch on the right selects either the internal or external speaker.

I hope this will be a useful – and certainly attractive – addition to the ‘Outer Space’ section of my instrument collection.


December 2017
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