Piezos & Electrets Pt 4 – Electronics and a very, very simple PT2399 echo/delay

In the previous post in the piezo series I had prepared 4 percussion instruments, all based on making sounds to be picked up by piezo discs.  In the picture below, the one on the left has 4 lengths of piano wire soldered to a large (50mm diameter) piezo; the second one has a snare from a snare drum attached to a large, shallow tin; the third one has an adjustable rubber-lined clip designed to hold a Latin American-style rainstick; and the right-hand one is a circle of sandpaper with a piezo disc firmly superglued to the back.

In the electret series I had made 2 new instruments in which sounds would be picked up by electret elements, and had identified 2 existing instruments, a xylophone and glockenspiel, which needed amplifying in a similar way.  Each of the instruments had its own appropriate preamp, either a piezo type or electret type, as described earlier in the series.


Next, one of the things I felt percussion instruments would benefit from was a reverb/delay circuit, and I had been looking around for a long time for something suitable.  In particular I was looking for a circuit that would be inexpensive, simple to put together, and easily repeatable in different units I might construct or circuit-bend.

Once I came across the PT2399 chip, I knew I’d found the answer.

According to the datasheet, the PT2399 is ‘an echo audio processor IC utilizing CMOS Technology which is equipped with ADC and DAC, high sampling frequency and an internal memory of 44K.  Digital processing is used to generate the delay time, it also features an internal VCO circuit in the system clock, thereby making the frequency easily adjustable.  PT2399 boast very low distortion (THD<0.5%) and very low noise (No<-90dBV), thus producing high quality audio output.  The pin assignments and application circuit are optimized for easy PCB layout and cost saving advantage.’

I managed to get a number of them at about 11p or 12p each, and researched the simplest circuits I could find to make a suitable delay unit.  I found one here: http://www.hqew.net/events/news-article/13932.html and put it together.

I simplified the circuit even more, and adjusted some of the capacitor values to lengthen the delay time and keep the noise to a minimum, and ended up with this first version:

I haven’t labelled it, but the volume pot is 10k.  Ideally, this should be a log pot, although lin pots are usually cheaper and easier to come by these days.  The component values are all very standardised, as I bought these values in bulk – using either resistors or capacitors in series or parallel, you can get close to other typical values.  These values worked fine for me in this context, however.  The value of 50k (lin) was chosen for the ‘Delay Time’ control as it gave a very wide range of delay times.

In this configuration, the repeat function is fully on, but when the ‘Delay’ control is turned fully anti-clockwise, there are no repeats.


One of the slight problems with the PT2399 is its very strict voltage requirements: below about 4.5v it’s unlikely to work; above 6v and it will probably be damaged.  I tested it with three 1.5v batteries, which was my original intended power supply for the percussion instruments, and it worked fine.

I first wanted to build a stand-alone circuit, however, and for this I used the usual 9v PP3 battery and a voltage regulator to reduce 9v to 5v, which is the PT2399’s preferred supply.

The voltage regulators were small DC-DC modules I bought for about 45p each, so didn’t increase the cost of the device too much.  These modules will accept an input voltage of up to 28v, and the output voltage can be adjusted from about 1v to 20v.

This view of the finished circuit shows how I put it together: there is no circuit board, but most of the components are soldered to the pins of the i.c. socket holding the PT2399.

Made in this way, the circuit would take up very little space in, for example, the restricted space of a circuit-bent keyboard or toy.  The 10uF DC-blocking capacitors and the volume control might also not be needed in some applications.  It might be somewhat lo-fi, but should be fine in the situations in which I plan to use it.

I decided to house this stand-alone circuit in one of the plastic jewel boxes I had used before for the Active Tone Control and Low-Pass Filter, as mentioned above, and the Touch-Radio.  Because of the minimal component count and no circuit board, even with the voltage regulator module, there was no problem fitting everything in the box:

The completed unit looked like this:


After trying the circuit out in this way, I decided to put one of these units into each of the percussion instruments, and in the case of the piezo-based ones, I made up the preamp circuit boards with the PT2399’s included.

I wanted to be able to adjust the number of repeats this time, so I experimented a bit and came up with the following development of the circuit above.  Since making the drawing and further experimenting with the percussion instruments, I changed the ‘REPEATS’ potentiometer to 50k; in some cases, depending on the sound from the instrument itself, I increased the added resistance here from 15k to 20k:

In this case I experienced problems connecting the preamp output directly to the input of the PT2399 circuit; so I added a 2k resistor (actually two 1k resistors in series) between the preamp output and PT2399 input – point ‘A’ in the above diagram.

It was at this point when I noticed that the output sound of the PT2399 didn’t include the sound at the input! . . . So, in order to hear the original as well as the delayed version, I used the other half of the TL072 as a simple mixer – it’s a dual op amp chip, and only one of them is used for the piezo preamp.

The circuit looked like this:

Although, in fact, the PT2399 output required 200k-500k, depending on the device, to balance the volume with the ‘dry’ signal.  The power connections – 4.5v to pin 8, 0v to pin 4, and 2.25v to pin 5 – were already in place for the half of the TL072 used for the piezo preamp).


I added one of these boards to each of the four piezo percussion units, plus a volume control between the mixer and the output socket – the odd one out being the one for the rainstick, which was designed to be a stereo device.  This meant doubling up each of the elements of the circuit: 2 preamps, 2 PT2399’s and 2 mixers; and using dual potentiometers for delay time, repeats and volume.  As the stereo preamp used both halves of the TL072, a second one was needed for the mixer left and right channels.

Here’s a sound file of the ‘Snare’ instrument, the Piano String instrument and the Sandpaper instrument.  Bear in mind that these are not being struck with any force: on the contrary, they’re being tapped very lightly with a thin disposable wooden coffee stirrer.


I wanted to do one more thing with the Sandpaper instrument before moving on.  The scratching sound covered a wide frequency range, and I thought it would be interesting to vary the sound by putting it through a low pass filter.

I had made a couple of these filters before, in the Optical Theremin, and as a stand-alone unit, using the 741-based filter from the Music From Outer Space website.  These were so simple and effective, I thought I’d use the same one again.  Here is the circuit:

This circuit was placed after the mixer stage described above.  I added a 1k resistor at the input, and at the output a 1M preset and a 10uF capacitor.  The only change I made to the circuit as shown was  to use a 500k potentiometer in place of the 1M potentiometer for the cut-off frequency, and didn’t include a ‘Fine’ control.

This picture shows how few components are needed to make an excellent filter.  There are two circuits on this small board:

This filter proved very effective, and sounded like this:


I then moved on to the instruments with the plastic bottles and the electrets.  I glued the electrets to the acrylic tubes, and the tubes to the wooden bases, then connected the electrets to the circuit boards I had prepared, each containing both a preamp and a PT2399 Echo circuit.

This is what they sounded like:

I was basically satisfied with the sounds I’d obtained.  The next article in the series describes how I finished the instruments off.


Electret microphones Pt 2 – Practical applications

After constructing some preamp circuits for electret microphones, as described in the first article in this series, I started to look at different uses for them.

First of all, I had some conventional instruments to amplify – a xylophone and a glockenspiel; secondly, I wanted to make percussion instruments from some plastic bottles.


I had a collection of plastic bottles, which would be suitable for tuned (or semi-tuned) percussion.  I sawed the ends off, leaving them at different lengths – and therefore sounding at different pitches – and prepared a framework to attach them to.  This consisted of small square trays which I bought, and 2x2cm wood, which I cut to length.

I then glued the bottles to each side of the central post:

Each bottle would have an electret microphone inside.  The electret elements were salvaged from part of a job lot of voice memo recorders which I bought in bulk on eBay.  These were said to be non-working, but their only problem seemed to be that the coin-type batteries had run down.  The electret element can be seen in position at the top of the right-hand picture below.

The electrets were to be mounted inside the plastic bottles on short lengths of rigid acrylic tubing.

The pictures below give an idea of how the electrets attach to the tubes, and the tubes attach to the instruments’ bases:

Following on from this post, I will describe the xylophone and glockenspiel which also needed an economical method of amplifying; and then installing the electronics for all the new percussion instruments.


Dismantling a hard drive

Note that in the following post I’m describing taking a hard drive apart in order to reuse some of the parts, but have no intention of putting it back together, or making it work again as a hard drive!

This the 3.5″ hard drive I took apart as my first experiment:

Front & Back IMG_1515

I’m sorry the PCB side is rather blurry, but I was less interested in that than what was inside.

The tools required for the job were a small screwdriver set, from which I mostly used a Torx or ‘star’, which I think was size 8, and a very small Philips or crosshead for a couple of screws inside:

Tools IMG_1525

I also needed a large flat-bladed screwdriver, which I used two or three times.  If you’re going to do the same, you may find a few differences in the details – type and positioning of screws, differently shaped fittings, and so forth, but the principles should be the same.

I first removed the circuit board, and put that aside.  Turning it over, you can clearly see in the first picture the 6 screws round the edge of the ‘lid’ which needed removing.

Having taken these out, it seemed that the top was glued in place as well as screwed, so I went round with the large screwdriver, prying it open.  This almost freed it, but it was only after still experiencing considerable difficulty in getting it completely open that I realised there must be another screw somewhere in the middle, under the label.  I scraped away the label until I found it and took it out.  The arrow shows where it was.

Top IMG_1527

I was then able to take the lid right off and expose the inner workings – of which there aren’t actually that many.

Top removed Captioned IMG_1528

The picture shows the disk or disks, one on top of the other, like a stack of pancakes – except there are gaps between them, and the arm is actually several arms, one for each disk; the arm itself with the delicate heads on the end which read the data from the disks, and with its other end moving between two magnets (not visible at the moment); and finally a flexible plastic multi-way ‘cable’ which joins the arm – and some small circuitry on the arm – to a connector.  This would have poked through a gap in the case to connect to the circuit board  on the other side.

Later on, connections will have to made to some very, very tiny points on the arm, and it’s going to be a lot easier to trace these points back to the connector, so its important not to damage the plastic cable.  The connector could be poked out from the other side, once it two fixing screws had been removed.

The fitting obscuring the back end of the arm wasn’t – in this model of drive anyway – screwed in place, it was just slotted on some pins and just needed prising off.  I believe the magnets in hard drives are made of neodymium.  This is classed as a ‘rare earth’ (although it’s isn’t actually any rarer than, say, copper) and makes very strong magnets.  Just be a bit careful as you remove them, and think where you’re going to put them down as you can be surprised at how firmly they grab lighter metal objects.

This picture shows the top magnet fitting removed:

Top Magnet Out Captioned IMG_1529

Once the top magnet is off, it’s possible to move the arm out of the way and get the disks out.  This involves removing 6 screws around the centre (on top of the motor) and two screws at the side. the disks and the various fittings associated with them all come off in layers.  The disks – there were three of them in this drive, look very much like CD’s, with a large hole in the middle, except they’re made of metal.

I put the disks carefully to one side – I had an idea they might be good for chimes or cymbals – and focused on the arm.

With the top magnet out of the way, it was possible to see the so-called ‘voice’ coil’.  Two impossibly tiny wires connected the voice coil with the arm actuation circuitry, and to test that the arm was working I needed to attach a battery to these wires.

In the end these wires were just too small, so I traced them back through the plastic cable to the connector, where it was slightly easier to get at them.  Attaching the battery leads to the two points produced nothing at 4.5v, but at 9v there was a loud and satisfying clunk, which showed that the arm was working fine.


The procedure with the next disk I worked on, a smaller 2.5″ was exactly the same.  The top came off once I had removed the 6 visible screws and the 7th screw hidden under the label.  I needed a smaller Torx/star screwdriver – a size 5 or 6 – for this disk compared to the other one.


This exposed the single disk and arm:


A single screw in the middle of the motor housing allowed the disk to come out.


Removing two screws freed the unit in the bottom right, which connected the arm with the circuit board on the other side.


I prised off the top magnet (bottom left), just to make sure the construction was the same as the 3.5″ drive – which it was.

I replaced the magnet and tested the drive with a battery.  I’d already removed a plastic retainer on the right, by the heads at the end of the arm; I also had to remove the plastic piece which you can see on the left, which was restricting the arm’s movement.  However, once I had done this, applying 9v to the appropriate pins on the connector reliably operated the arm.

In fact, this smaller mechanism would also work with 4.5v, which was a useful discovery.  If parts of the circuitry which would operate it had to be quite low – say, 5v – it could be handy if the arm would also work at the same voltage.


So, I had verified that an arm from an inexpensive broken hard drive could operate as an electrically-operate striker, like a solenoid; that arms from both 3.5″ and 2.5″ drives would work; that the 3.5″ drive arm would operate at 9v; and the 2.5″ drive arm would operate at 4.5v.  It will be some time before I get round to the next part of this project, but the next thing will be to see how the arms could be set up to operate outside the hard drive housing.


Inductor pickups 1

After looking into piezo elements and electret microphones, I decided to try another kind of pickup described by Nic Collins, the magnetic inductor.

Collins describes using a device marketed as a telephone pickup coil – I don’t know what the situation is now, but it used to be illegal to attach recording devices directly to the telephone network, so a telephone pickup coil has a suction cup on one end to stick to the telephone handset, and a 3.5mm mono plug on the other end to connect to an amplifier or recording device.

I bought a few of these for about £1.50 each from CPC.

I wanted to see what was inside, so I sawed the top off one of them and took out the contents of the plastic enclosure.

It appeared to be a simple inductor coil, rather like a small guitar pickup, but with no indication of its value.

Inductors are the the third ‘passive linear circuit elements that make up electronic circuits,’ (https://en.wikipedia.org/wiki/Inductor) along with resistors and capacitors.  Their values are expressed in Henries (H), although 1 Henry is quite a large value and the the majority of inductors encountered in circuits fall in the millihenry (mH) range.

I was expecting to find other components inside the telephone coil enclosure, having read here: http://hydrophones.blogspot.co.uk/2010/09/induction-coil-pick-up-now-available.html that ‘whilst most other coils on the open market have certain limitations placed upon them, these adapted coils [for sale on that site] can detect a wider range of sounds that the technology is capable of . . . adapting the coils not only increases the range of frequencies but also boosts the overall signal level.’  There was no further information about the nature of the limiting circuitry, or the means of boosting the signal level, but perhaps the coils I bought were not the type with limitations, I couldn’t tell.

Nor could I tell what value of inductor could perform the function of the telephone coil.  An article here: http://www.unterzuber.com/tap.html on how to make a telephone coil recommends 100 turns of 28AWG wire on ferric core; another here: http://www.circuitstoday.com/telephone-pickup-preampliifer (sic) recommends 3000 – 5000 turns of 0.4mm on a plastic former.

There is a company called Knowles, which manufactures ‘telecoils’: inductors which can be built into hearing aids to amplify sounds created by an induction loop (http://www.knowles.com/eng/Products/Hearing-aid-components-accessories/Telecoils).  However, these are rare and quite expensive (£2 – £7 each from a component supplier with a minimum order requirement and delivery cost – more than the Telephone Pickup itself).  The values of these covered a very wide range, from about 30mH to over 900mH.

So, in order to experiment, I first had the coil from the Telephone Pickup.  I also bought some smaller inexpensive inductors of different values.  These were (on the left) 100mH, costing a little under 40p each, and (on the right), different values between 1mH and 10mH, costing a little under 4p each.

I also had a guitar pickup, which works on exactly the same principle, as mentioned above, but has 6 separate magnetic poles within it.  This was about £2:


The preamp I had used recently for the electret micophones seemed to work fine, so I used this and experimented with the various different inductors, passing them over my laptop keyboard, a good source of magnetic noise.

These were the 3 inductors I compared: the Telephone Pickup Coil is on the left, the 100mH in the middle, and the 10mH on the right.

This is what they sounded like, running over the same small area of the laptop keyboard (the most interesting section!):

It seemed to me that the Telephone Pickup Coil was the most sensitive; the 100mH a little less sensitive, but still effective –  perhaps more effective in some areas where there was a great deal of noise; and the 10mH was a little too insensitive.  On balance, I thought the Telephone Coil was probably the one to use in a best quality application, but the 100mH would be fine in cases where cost was a major factor.


For the next round of experiments I made some stereo inductor pickups: in one case I took the coil out of a second Telephone Pickup and hot glued a pair of them to a shaped piece of scrap acrylic; in the other I mounted two 100mH inductors side by side to a smaller acrylic strip.  I also tried out the standard-sized guitar pickup.

These three pickups were a great improvement on the single inductor pickups, and sounded like this, with the guitar first, then the stereo Telephone Coils, then the stereo 100mH inductors:

I had to be careful with the guitar pickup, as  bringing it near the laptop seemed to cause strange effects – the screen going blank, for example, which was not good . . . However, this wouldn’t be a danger in the application I hand in mind for it.

In fact, I would recommend NOT experimenting with a guitar pickup like this.  Because of the magnets in it, it could possibly damage sensitive circuits like laptops, mobile phones, etc. – stick to the passive inductors, like the other ones I used.

In the next post in this series, I will describe an instrument I made based on inductor pickups.


Piezos Pt 3 – Amplifying and creating instruments

After preparing the discs and the buffer/amplifier in Part 2 of this series, I looked around for different instruments that could effectively be amplified and recorded with the use of piezo elements.

I also tried a few inexpensive commercial piezo contact mics, like these:

The top one (Cost: approx £1.50) has quite a large piezo disc inside a plastic cover and a sticky pad to fix it to the surface which is to be amplified; the bottom one (Cost: £1.25) is built into a (not-too-strong) plastic clip, with a foam pad to protect the disc.


The more solidly the piezo is connected to the sound-producing surface, the better the sound obtained.  In other words, it’s best if the disc can be glued to the surface.  I did this with some of the non-valuable items:

However, I wasn’t keen on making this permanent addition to my instruments, and  looked for different ways of making temporary connections.  Each of these could be useful in different circumstances:

This double-sided tape is described as ‘removable’, and is less likely to damage either the piezo element or the instrument it’s stuck to, so is a good choice – although, at about £7.00, was rather expensive.   I hope to make use of it elsewhere in the house!  It’s also more suitable for a one-off performance or recording.  I’ve also read that Blu-Tac works in a similar way, although my experience is that it can leave marks; elsewhere I’ve read that insulating tape can be used – that might also be less sticky than conventional sellotape, but unlike the double sided tape, would have to go over the top of the piezo disc.

I also bought some clamps of different sorts:

The spring clips were very strong, so I would guess I’d have to be careful using them so as not to damage the piezos.  I have read of people using soft pads – made of felt or foam rubber, for  example – to put over the piezos when using them with strong clips.

In this way I had a variety of different methods of attaching the piezos to items I wanted to amplify or record.  The items themselves could be either acoustic instruments that just needed appropriately amplifying; or items that were not musical instruments, but which could be amplified – perhaps changing their sound, or revealing a hidden sound in the process – by attaching a piezo.


As they are used to pick up sounds from vibrations in solid surfaces, the best acoustic instruments to work on would be those with sounding boards, such as guitars, zithers or other stringed instruments, several of which I had in my collection; as for non-musical instruments, this would be a matter of experimentation!  The advantage of using a piezo contact mic in these examples would be that, unlike a conventional microphone which picks up airborne sounds, the contact mic wouldn’t easily be affected by the nearby sounds of the player, other instruments, or external noises in the recording environment – e.g. traffic or the people next door.

First of all, the more conventional instruments.  These are just a few of the various things I tried the piezo mics with.  At the top are bells and a rainstick; at the bottom are a rattle (not perhaps, strictly speaking, an instrument!) and a zither.

The following sound file illustrates how these sounded:

In these experiments only the zither was recorded in stereo by using two piezos, but a stereo effect would certainly bring something to some of the others – for example the rainstick.


Next, some uses of the piezos that created new instruments in themselves.  Both of these made sounds that, when picked up by the piezos were very different from the way they sounded in the room.

The first one uses a small snare – normally used for a snare drum.  This was purchased very cheaply (about £1.30) and attached to a wide, flat tin.  A 50mm piezo disc was superglued to the middle of the tin.

The second one is just a 50mm piezo disc with 4 lengths of piano wire soldered to it.  I’m not 100% certain of the diameter of the wire: it said ‘G0’  (i.e. ‘G zero’) on it, and was bought from a (classical) music shop about 30 years ago.  If it means Music Wire gauge 0, this would make it about 9mm, something like a thin top E guitar string, which is about what it seems to be.  The 4 lengths are approximately 6″, 9″, 12″ and 18″.

This sound file illustrates first the ‘snare’ instrument, then the ‘string’ instrument:

It’s surprising how different the sound through the piezo is, compared to the natural sound, especially the one with the soldered strings, which makes hardly any noise at all.  The small preamp also plays a part in preserving the lower frequency sounds.

The picture below shows, on the left, the three – I don’t know what to call them – strikers or activators, which I used to make the sounds from the snare instrument: the one on the left is a home made beater or mallet, made from a length of dowel and a wooden bead; the middle one is a wooden coffee stirrer, much more delicate; and the one on the right is a small cleaning brush.

Activators IMG_1281

It’s a good idea to make a collection of these if you’re going to make piezo instruments, as the way you interact with the instrument can make a big difference to how it sounds.  The picture on the right shows some more things I use, as well as pipe cleaners, fire-lighting spills and small emery boards.  This writer has a very impressive collection: https://skoglosa.wordpress.com/2015/05/06/contact-mic-guidelines/!


The instruments required quite a bit of physical construction, since a framework was needed to support the sounding parts.  I bought some small square trays to serve as the bases, and cut lengths of 2x2cm wood for the uprights.

After glueing and screwing these together, I was able to attach the sounding parts:

The two on the right are the ones described earlier; the one on the left is simply a sandpaper disc with a 35mm piezo glued to the back.  The narrow space underneath the base can be seen in this picture.  The batteries and electronics would have to fit in this space.


After the physical construction, it was time to add the electronics.  I’ll describe this in the next part of the series.


Electret microphones Pt 1- General

After starting my recent piezo project, I decided to look into electret microphones – there are many situations in which a piezo-type contact mic is less suitable than a mic which detects sounds in the air.

Electret capsules are very cheap, and need just a few additional components to get them to work.  I bought a number of these for about 10p – 11p each:

It’s possible to get them without any wires – long or short – attached, but I preferred these.  The smaller ones, illustrated at the top, were very small – 4mm in diameter; the larger ones were 10mm.  I also bought a few that were in between, at 6.5mm.

Most people working in electronic music will be aware of the importance of microphones, and  I have some quite expensive ones for different amplifying and recording purposes; but there are various situations in which a very low-cost method of picking up and amplifying sometimes quite small sounds can be all that’s needed.

One renowned electronic music composer for whom the microphone became extremely important for a time was Karlheinz Stockhausen.

In summer 1964, Stockhausen said, ‘I searched for ways to compose – flexibly – also the process of microphone recording. The microphone, used until now as a rigid, passive recording device to reproduce sounds as faithfully as possible, would have to become a musical instrument and, on the other hand, through its manipulation, influence ALL the characteristics of the sounds . . .’

At the same time, he had been experimenting with a large tam-tam (a percussion instrument very similar to a gong), ‘using a great variety of implements – of glass, cardboard, metal, wood, rubber, plastic – which I had collected from around the house.’

‘One day’, he continues, ‘I took some equipment from the WDR Studio for Electronic Music home with me. My collaborator Jaap Spek helped me. I played on the tam-tam with every possible utensil and during this, moved the microphone above the surface of the tam-tam. The microphone was connected to an electrical filter whose output was connected to a volume control (potentiometer), and this in turn, was connected to amplifier and loudspeaker. During this, Jaap Spek changed the filter settings and dynamic levels, improvising. At the same time, we recorded the result on tape.

‘The tape recording of this first microphony experiment constitutes for me a discovery of utmost importance . . . Actually, this moment was the genesis of a live electronic music with unconventional music instruments. On the basis of this experiment I then wrote the score of Mikrophonie I. Two players excite the tam-tam using a great variety of implements, two further players scan the tam-tam with microphones . . . Two further players – seated in the auditorium at the left and right of the middle – each operate an electrical filter and two potentiometers. They, in turn, reshape the timbre and pitch . . . dynamic level and spatial effect . . . and the rhythm of the structures . . .’

The score includes instructions for the placing and movement of the microphones, just as it includes instructions for the tam-tam players and the filter operators, so the microphones can be regarded as essential instruments in the performance of the piece.

One of the interesting features of the use of microphones in the piece is, as Stockhausen wrote: ‘normally inaudible vibrations (of a tam-tam) are made audible by an active process of listening into [them with a microphone].’ The reviewer Albrecht Moritz (http://home.earthlink.net/~almoritz/mikrophonie1.htm) states: ‘There are several passages in Mikrophonie I where this process is exclusively employed, foregoing stronger excitement of the tam-tam which would produce the commonly heard sounds. A result is that, if you would play back these passages to persons whom you would leave in the dark about the source of the sounds, probably most or even all of those listeners – including musicians – would not be able to guess it.’

Elsewhere Moritz says that ‘the audibility of most sounds that are created on the tam-tam in Mikrophonie I appears to strictly depend on the microphonic amplification. Among these are scratching noises, produced by treating the surface with not only metallic, but also other kinds of objects. Strangely “rolling” sounds can be generated on the surface, sounds evocative of rustling of silver paper, and many other astounding sounds . . . Quite frequently there are dark, roaring, sometimes growling, yet in volume often rather soft, undercurrents of sound that appear to stem from only local resonances of the tam-tam plate, generated by gentle use of a beater or as a result of other treatment, a sound phenomenon most likely audible only because of microphonic amplification as well.’


These electret capsules won’t work, however, just by connecting them to a mixer or amplifier – they have a small built-in preamp inside them which needs power to operate it.  This means the positive lead to the capsule must have a few volts of power running to it – 3v to 9v, typically – for the capsule to work.   An interesting article on this topic can be found at http://www.epanorama.net/circuits/microphone_powering.html.

The minimum circuitry required to get a sound from the capsule is this:

Using a prototype of this circuit with the Taurus amplifier, the level of the signal was perfectly good enough without any additional circuitry.

Sometimes, however, more output is needed, and I found a suitable circuit which was simple but effective at http://www.instructables.com/id/Pre-amp-to-electret-mic/.  It was based on a single transistor, a 2N3904, obtainable at less than 5p each by buying a bag of 50, plus 3 resistors and 2 capacitors.   The circuit for one channel looked like this:

I made up a batch of them all at the same time on a spare piece of veroboard.  These 12 circuits cost no more than about £2 – £2.50 altogether.

I took some of these and made up some stereo circuits, adding an output socket, 9v battery clip and volume control pot to each one:

With this I was able to experiment with different microphone combinations. No other components were needed, except the electret capsules themselves.  In some cases I connected the two wires from the capsule directly to input and ground on the board, at other times I attached input sockets.

In all cases I was surprised at the quality of the sound I was able to get for such a low cost.  The circuit also worked with the piezo mics/pickups I had made earlier, although the output didn’t seem to have as much lower frequency content.

In the next article in the series I’ll describe some of the practical applications for which I used these electrets.




The Blue Cow

Having just finished a couple of projects designed for automatic control via the Bigfoot sequencer, I was looking for a similar toy that could be played manually.

The Blue Cow seemed to be exactly the kind of thing I was looking for. which could be worked on with a combination of modification and circuit-bending.

There are buttons which make the sounds of a cow, cowbell, cat, dog and pig; two switches – the ducks’ beaks – which play a short musical sequence and light up the 6 LEDs; and three mechanical controls.  One, a snail which moves slowly across the cow’s back, makes no electronic sound, but there are two others which do: a rotating ball which makes the cowbell sound as it turns, and a rotating control with finger-holes, which moves the cow’s tail and plays a tune with a bell-like tone.


I opened the back of the device and took a close look at the PCB inside.

The first thing I noticed was an extra switch which hadn’t been used.  It had the word ‘sheep’ next to it, so I eagerly connected a pair of wires to it and, sure enough, a baa emerged.  So the first thing I did was add an extra switch on the front of the cow to produce this new sound.


After that I searched for the resistor which would affect the pitch of the sounds.  It took a while, but I found it, removed it from the board, and attached two wires in its place, running to a potentiometer so that the pitch could be varied.

In this view of the circuit board 1 indicates the previously unused switch, and 2 indicates the position of the pitch/timing resistor.  The wires now connected to these two sections are on the underside of the board.

It took a little experimentation to find the correct value for the potentiometer, to ensure sufficient pitch variation, but not to increase or decrease the resistance so much that the device crashed.  This is the normal thing when replacing a timing resistor.  In this case, a 250k potentiometer was the best value, with a 100k trimmer to adjust the minimum resistance.

The potentiometer went on the front of the cow.


I unscrewed  the sections behind the 4 main sound buttons, and checked the additional circuit boards which were connected to the main board by a series of red wires.  There were 14 of these these wires, which went directly to the sound-producing chip.

Some of these wires went to LEDs beneath the buttons, so  I tested the others by connecting them in pairs, and found various combinations which would produce almost all available the sounds, including the newly-discovered sheep.  If I included one of the connections of the switches under the ducks’ beaks, all the sounds were available, and I began thinking of extra ways to access them.

First of all I added four tilt switches, so different sounds would be produced as the device was moved around.  These were a simple and cheap type – about 10p each – but very effective.  Constructed in a small can with two legs, not unlike an electrolytic capacitor in appearance, an internal connection is made when the can is tipped from horizontal to vertical.

I glued the switches inside, left, right, top and bottom, with a different sound connected to each.  The arrows in the following picture indicate two of the switches in situ.


One awkward thing about the circuit – especially with the tilt switches in place – was that there was no on/off switch: the power was on – and the device ready to make noises – as soon as batteries were inserted.

So, I decided to add one.  I found a nice one, described as an SPST illuminated rocker:

The odd thing about it, as an SPST, is that it has 3 contacts.  It took me some time to work out, but its normal function is to switch +v from one outside contact to the centre; the other outside contact – not connected to the switch – is for a 0v connection to the internal LED.  In this case, the power lead from the batteries connects to the ‘off’ side of the switch, the lead to the circuit board connects to the centre, and the 0v lead connects to the ‘on’ side of the switch.  In this way, when the switch is turned to the ‘on’ position, power is connected to the circuit board and the switch lights up – very attractive!

I liked these switches so much, I bought several of them at about 30p each.  They were labelled as suitable for 12v – their origin is for use in the automotive industry – but lit up fine from the Blue Cow’s 4.5v.

There’s quite  variety of illuminated switches available, all of which would beautify a project where a +V switch was required.  There were also square ones:

although I always find these more difficult to mount than round ones.  The one on the right looks interesting, as it’s a centre off with a latching switch one way and a momentary switch the other.

One UK outlet has a whole variety of these, including toggle switches, push switches and pull switches:

At this time I also added a small SPDT switch to change the output from the small internal speaker to 4mm external speaker sockets or to an audio out socket.

In the  following picture 1 = the internal/external speaker select switch; 2 = the speaker select switch inside; 3 = the external speaker sockets; 4 = the audio out socket.


Here’s a short demo of the Blue Cow.  In this case it was connected to an external speaker and the sound recorded via microphones.


March 2018
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