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
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
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.
Step 3: Making and Testing the Pitch Bend
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
Step 5: Soldering the Bends to the Circuit Board
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
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!
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.