[circuit bending for dummies]
frequently asked questions


1. what is circuit bending ?

definition 1 : using cheap electronic circuits as a basis for musical instruments through the use of both theory and experimentation.

definition 2 : modifying electronic toys and cheap electronic instruments by connecting various points of their circuits in ways that were not meant to be, so as to modify the existing sounds, create unexpected effects or imprevisible noises…



2. what do i need to do that ?

hardware. patience. knowledge. imagination.

  • hardware : low wattage soldering iron/pen, pliers, cutters, wire, screwdrivers, drill, components, etc.
  • patience : you’ll usually have to make lots of tests to find good modifications, and then to make sure that these mods don’t interfere with each other. not to mention the actual planning/soldering/drilling.
  • knowledge : you won’t go far without at least some basic understanding of electronics. go read a book.
  • imagination : unless you find it exciting to redo what others have done before without adding anything, some imagination is required to get satisfactory results. vary the devices you work on. find new mods. think about the interface (even if the mods themselves are the same, two bent devices can be really different).

if you’re new to this, you should read ghazala’s tutorial.


3. huh, what did you say about the hardware ?

here’s a brief list of things that are either essential or useful to modify electronic devices :

  • low wattage soldering iron/pen. 10-25 watts should be ok. don’t use a plumbing soldering iron.
  • digital multimeter. one of the most useful devices i can think of. resistors and voltages are your friends.
  • a dremel-like mini-drill. great if you don’t want to make holes in plastic with a knife.
  • one or two boxes of ‘jeweller’ screwdrivers. these should be very cheap and are really useful.
  • lots of crocodile clips wires. also cheap and also very useful.
  • a desoldering pump. essential to remove badly placed solder.
  • pliers, cutters, screwdrivers, etc.


4. what are good circuit bending targets ?

the most *popular* devices are the texas instruments speak and xxx series speaking toys and the casio sk-x series keyboards.

why ?

because reed ghazala made an article about the sk1 in experimental musical instruments that introduced lots of people to circuit bending and has modified many s&s since the early 80’s.
this is not the only reason, of course, as these things are full of hidden treasures and are also quite robust. and not so expensive (well, at least they shouldn’t be).

but these devices are far from being the only good bending “targets”. basically anything that satisfies the following criterias can be mutated into a new instrument :

  • produces sounds. basic beeps are boring. speech and samples are better.
  • works on batteries. this is not necessary but definitely recommended. electricity is nice as long as it doesn’t exceed 12 volts.
  • was made in the golden age of electronic devices. i.e. mainly the 80’s/early 90’s.

brief explanation : before that, most consumer products didn’t have enough complexity as far as electronics goes. most of the products made after this period contain miniaturised circuits that don’t offer as many possibilities. this is a general guideline, exceptions exist (lots of them in fact)…

  • is not too precious. messing with the bowels of a toy or a keyboard can seriously damage the aforementioned bowels. don’t try it on something you really care for without having a backup.

this being said, here are some toy brands that usually produce nice and useful things : vtech, creatoy, texas instruments. casio keyboards are usually interesting too, but don’t feel restricted to these ones only. many 80’s digital drum machines provide great results too.


5. what should i avoid ?

anything that :

  • works on 110/220V, if you don’t know how to locate the power supply area and avoid it. don’t even think of opening a tv for example.
  • is expensive. don’t try to bend things that cost lots of money. or at least keep in mind that you could destroy them and that their value will certainly decrease after your digital surgery.
  • contains a dsp. dsp means digital signal processor, i.e. a chip that does all sorts of calculations/processing and that creates/modifies signals, to put it simply. you must write code to use a dsp, you don’t connect components to its pins to control it. big big difference. so, don’t open this kind of things and mess with it unless you want to destroy it.

examples : mp3 players, korg electribe, waldorf q

  • is actually analog. as in analog synthesizer. circuit bending is good for digital devices, with cheap and robust ICs, not analog circuits. this kind of instruments should be modified after reading and understanding their schematics. in case of doubt, ask knowledgeable people. and leave this synthi a alone…

examples : juno xxx, tr-606, ms20, pro-one.


6. any other general advices ?

  • try to have a clean space dedicated to this activity. having to move the tools and instruments every time you want to work on them can quickly lead to instant boredom, or at least lessened motivation.
  • write down everything you find when testing the devices. alas, human memory has its limits.
  • thoroughly test your mods before actually drilling holes to add the switches/pots/etc. discovering that activating two mods at the same time make the unit crash can be frustating when you have already butchered the case and soldered everything.


7. how can i add a jack output to a toy/keyboard/washing machine ?

this question is really frequently asked and actually deserves its own entry in this faq. btw, note that you can add an output to anything that already has a speaker without modifying it.

my favorite method for this is to use a ‘closed’ 1/4″ (or 1/8″) jack socket. this allows to have the internal speaker automatically disconnected whenever a jack is inserted in the output. there are 3 points on these connectors : ground + 2 signal ones. these 2 signals are connected with each other when no plug is inserted, and get disconnected when you plug a jack. one of these points touches the jack when it’s inserted, this is the one that should carry the signal to the output. let’s call this one ‘a’, the other ‘b’ and the ground ‘c’. so just disconnect the + wire going from the circuit to the speaker (don’t disconnect the ground one) and connect it to point a. then connect b to the + point of the speaker.

and don’t forget to connect the speaker ground to the jack ground (c). to do this nicely, you should also add a resistor between points a and c. check the resistance of the speaker with a multimeter (should be around 10-20 ohms) and use a resistor of similar value.

if you find out that there’s too many high frequencies when using the jack output (these frequencies may usually not be noticeable via the crappy speaker), connect a capacitor to ground from the output. try different values to see which one gives the best effect/sound ratio.


8. the volume of the sound output i added to my toy/instrument is really too high. did some gremlin mess with the circuits or is it normal ?

this is because the output you added is at speaker level and your amp/headphones/mixer wants line levels. there are good explanations of this everywhere, let’s just say that in this case you need to attenuate the signal if you want your toy and your mixer to live happily without having to torture these poor volume controls.

this can be done with 2 resistors and some wire :

  • connect speaker (or jack output) ground to line ground
  • connect a 1k res between line out & line ground
  • connect a 10k res between speaker out & line out

this should give you 20db of attenuation, and replacing the 1k res with a pot/trimmer should allow you to control the volume. a bigger res than 10k will provide a greater attenuation.


9. any interesting things about resistors ?

basically this :

  • resistors in series add their values.
  • for resistors in parallel use this formula : 1/rt = 1/r1 + 1/r2

(with rt being the total resistance)

this shows that the total value of two resistors in parallel is always less important than the lower of the two values. here’s an example that shows why adding a pot in parallel with an existing resistor won’t always have the intended effect.

assume that there’s a 10k res somewhere in the circuit and that this resistor controls the pitch of the sound. the first thing anyone wants to do is to use a variable resistor to control the pitch (pot, ldr, etc.).

there are basically two ways to do this :

a) add something in parallel to the resistor
b) replace the resistor with a variable one

let’s try these two options :

a) in order to have a real effect, we usually need the value to vary greatly. we saw before that if we add another resistor in parallel, the total value won’t be greater than the value of the res that’s already here. let’s say that this is ok in our case (though it’s usually not the case) and that we just want a pot to control the resistance from 0 to 10 kohm. you can test this either mathematically or with actual components, and you’ll see that most of the values are concentrated in a small range of the pot (first 20% for a linear pot, first half for a log one). this is not good.

b) well, there’s not much to say. this needs more work as you have to remove the old resistor and to solder a pot in its place. in our case, a 100k pot could be a good value for example. a wise thing to do is to keep the resistor and to set up a switch that can route the current either through the pot or through the res (to be able to have the unmodified sound too).


10. i’ve just found/bought/stolen a speak and spell. what can i do with it now ?

the best thing to do would be to experiment, i.e. see what you can find yourself. there are lots of things to do on these little things so be creative.

i’ve also put some diagrams explaining how to get the most common bends here. they are intended as a basis for your own findings, an incentive to do more research yourself and not a way to make easy money on ebay though.