I want to show some of the modifications you can do to any toy to turn it from what might simply be an annoyance to a tool for glitchy, noisy awesomeness. The techniques here are pretty easy--even if you don't have much experience with electronics. All you need is a willingness to make mistakes, to repeatedly pop batteries in and out when you crash the processor and need to reset things, and a desire to make strange sounds.

This instructable, and its videos, show two of the simplest modifications that can be done: pitch bending and body contacts. I've deliberately not shown some of the more complicated methods, like triggering samples using a timer circuit, that you can learn once you've got the basics down.

Most steps have videos that carefully go through the processes described in the text. I see them as vital tools to help you see how this is down within a real circuit.

Difficulty: beginner to immediate
Skills needed: basic electronics (know about resistors, power, ground, switches), soldering (although this is an easy-enough project to get or build soldering skills)

Step 1: Materials and Tools

You need a few things to begin with, of course. If you have never worked with electronics before you'll have to spend more money to get things like a soldering iron--but I bet you can get everything you need for under $30.

Onto the list of things you need!

Toy: Obviously you need a toy to modify--one that you don't mind opening up and (potentially) making inoperable if something bad happens. I recommend the usual places: thrift stores, surplus stores, etc. But...please, please, please, PLEASE don't go to a major retailer and purchase something new to modify. There are enough toys out there on the market being discarded---we don't need to give multinational corporations higher profits by buying something new when we can reuse and modify something that already exists. So go to your thrift store and scrounge around; you might not find something immediately, but be patient for the right toy to come along.

The best toys are the ones that aren't complicated: only a few sounds, a few buttons, etc. Anything that has complicated behavior is going to have complicated circuitry that is both going to be more difficult to modify and more likely to simply screw things up without making useful sounds. A good beginning toy to bend would be a stuffed animal that makes a few sounds...open up the toy and you'll find inside a plastic case that has an extremely basic chip. In my case, I chose a toy designed for toddlers that has only a few buttons and sounds--meaning the circuit inside will be relatively straightforward.

And it should go without saying (I hope)...but don't bend anything that requires a connection to the wall or mains! ONLY BEND TOYS THAT USE 9V BATTERIES OR LESS!!! I'm certainly not responsible if you hurt yourself doing this...you shouldn't, if you bend battery powered toys. But no warranties, etc. legal mumbo jumbo.

Electronics: For this bend you'll need a random assortment of parts: on/off switches, some resistors, a potentiometer, and wire. To attach things you'll of course need a soldering iron and solder. And to test things out, some alligator clips are really useful.

Other tools: A drill or dremel to cut holes in the casing, and a screwdriver to take the case apart.

The video for this step is me playing around with the toy before any modifications are made.

Step 2: Opening the Circuit and Finding the Bends

Now that you've got your toy, get out your screwdriver and open the thing up. Be careful to keep track of all of the screws! A digital camera is also really helpful--take pictures at various steps in the process, like before removing something major, so that you can retrace your steps backward if necessary.

The best suggestion I can give is to simply experiment: play around with different values of resistors, different connections in the circuit, with capacitors, diodes or inductors, with putting yourself in place of a resistor, etc. A good place to start is to just connect different points in the circuit together with wire and see what happens. And if you end up causing all sound to cease, you might have crashed the processor; just remove the batteries (to reboot the processor) and try again.

Finding the places on the circuit that give you interesting sounds is the black art of circuit bending. There are a few things that can be helpful:

On basic toys like this, it's usually pretty easy to find the resistor that controls the pitch. Often it's near one of the ends of the chip (like it is in this toy) and can be distinguished from other resistors that are near other components (like those near transistors that control audio outputs). Look at the image of the board to see where the resistor is in this particular circuit, and then look for an analogous place in your own circuit.
Power and ground can be easy to find--look first for where the leads from the battery hit the circuit board. Often this will also be the place on the board where the traces are the thickest (widest). You'll connect things to ground to create "voltage dividers" that enable you to change things like pitch.

So many different, strange, and unexpected things can happen when you just play around. But once you've found something cool, and that you can reproduce, be sure to make note of it so that you can return to it when you start to make things permanent.

Step 3: Making and Testing the Pitch Bend

Now that we've figured out where the resistor is that controls the pitch, let's see if we can make something that allows us to vary it. We do this through a potentiometer (also called a pot), which varies resistance as we turn a knob. (These are the things you find in radio or TV dials (at least before they all became digital).) We need to have a pot whose maximum resistance is near to the resistor that we're going to connect to. In this case, the pitch resistor was in the megaohm range, so I chose a pot that had an upper limit of around 5 megaohms.

Next, I connected one lead of the pot to the resistor on the circuit, and another lead of the pot to ground. (Pots have three leads, and you want to use the first two or the last two; which one you chose determines whether turning the knob in one direction increases or decreases the pitch.) And then it's just a matter of testing it out! If things don't sound as you'd like, go ahead and play around with things: different pots, different resistors in series or parallel, etc.

Check out the video for a detailed walkthrough of this modification within the toy.

Step 4: Making and Testing the Body Contact

Since humans can act as big resistors connected to ground, we can put ourselves into the circuit to modify the sound. This step is all about playing around with this. We can use the same location as for the pitch bend. The best way to see how this works is to watch the video.

Step 5: Soldering the Bends to the Circuit Board

Now that we've found a set of interesting bends, it's time to make the connections permanent.

There are a couple of things to keep in mind here:

Make sure you provide long enough leads for your new part (the pot, a switch, etc.) to reach from your mounting location on/in the toy to the place on the circuit board
It's often good to provide a way to override or remove a particular bend from the circuit. That's exactly what I've done with the pitch bend, and I've provided a switch that allows me to choose whether or not to enable the pitch bend (i.e., connect the pot to ground).
Also, a reset switch is a must; this enables quick recovery in case you crash the processor :-)
Check to see that your leads pass through whatever holes you need them to go through before you solder them. I forgot to do this on one of my switches here and had to redo it.

Step 6: Fitting Everything Inside the Case

Now it's time to fit all of your modifications within the case! This is the step that is often forgotten at the beginning of the process, but while deciding what bends to make you should also have in mind how to fit everything inside the case at the end. Sometimes this might entail moving things around inside the case, or making an external box that gives you more room. But in nearly all cases this will require some modification to the existing case, like drilling holes for switches.

And of course, how you modify the case is entirely up to you--you can give it a new paint job, remove everything and place it in an entirely different case, and so on. Look on youtube or flickr for some really inspiring examples.

Step 7: Sounds to Be Heard!

At this point we just attach the case together with the screws (you still have them, right?) and play our modified monstrosity! There are still lots of unexpected things that can happen: crashing the processor might give us a strange loop, very low pitches can make incredible rumbling noises, voices can become unintelligible. And this is the great part of playing with our modified toy! We can now have fun exploring this space of options that was created by our reusing and enhancing of something that was once destined for a boring life of obscurity.

There are a lot of sites on the web that will guide you on more complicated bending methods, as well as highlighting the multitudes of different ways to house your new creation. I can't be exhaustive, but here are a few: Reed Ghazala (one of the first to document and describe the procedure), warranty void, Get LoFi (a blog showcasing a number of projects), and casper electronics.