Check out the video of the cigar box synth.
We thought that a tutorial on making a complete instrument from start to finish would be the best way to demonstrate the Critter Board in action.
The instrument is a cigar box synthesizer. It is made out of a Critter Board, a small audio amp, and some other components we had kicking around. Since it is a synthesizer, we use the version of the Critter Board with on-board flash (no card socket).
The project utilizes all the features that make the Critter Board unique, such as the volume control header, the sound output header (you can listen on the speaker, or plug it in to a bigger amp), and most of the I/O lines.
After digging up a 12 key key-pad, we thought it would be nice to make a 1 octave synthesizer. We then added another button above the keypad as a register button. If it is held down before pressing any of the 12 keys, the key will play an octave higher. So it is really a 2 octave synth.
There are 3 control knobs. One is tuning, shifting the entire range of the keys. The other 2 control harmonicity and modulation index of the synthesizer.
The Critter Board has plenty of processing power to handle four independent 8 bit FM voices, and read the key-pad and control knobs. In addition to the source code, we also provide a pre-compiled .hex file. If you make all the same connections as we have done, you should be able to just upload the .hex file, and have a working 4 voice synth.
Downloads:
The synth is also available for purchase!
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The completed synth.
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The cigar box is nice and deep, making it easy to work with.
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This small amplifier kit is available from QKits for $6.95. Its a perfect size amp for a project like this, and even comes with a speaker.
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We had a 12 key key-pad from allelectronics lying around. The keypad isn't encoded, really just 12 individual buttons. Some 10K pots, and a nice button. In the picture is a little joystick from mouser, but decided not to use it cause its a pain to mount.
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Layout.
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The other great thing about cigar boxes is that you can cut them with a knife, don't even need a saw.
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Drill some holes, and cut another hole for the speaker...
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The speaker had no mounting holes, so we used washers and screws. Some epoxy for extra measure, since the wood is thin.
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Wiring. Start with the ground wire since it gets connected to the most things.
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VCC (3.3 volts in this case) gets connected to the 3 control pots.
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First measure and cut the 12 wires for the keypad. Then solder them on to the key pad before attaching them to the Critter Board.
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Before attaching anything to the Critter Board, we need to take care of something. There is a wire jumper soldered across the volume control header. Since we would like to attach a potentiometer to control overall volume, we remove this jumper.
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Carefully cut the wire. Then with a soldering iron and some needle nose pliers, remove the two pieces.
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Jumper removed.
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This is a pain. For the most part the LPC2138 doesn't have internal pull-up resistors on the I/O pins, so a 10K resistor must be connected from each key wire to Vcc.
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We attach a resistor to each pad where the key switch wires attach (and the wire for the lone button), and then solder the other end of all the resistors together to Vcc.
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Careful not to drip any solder on the board if doing it this way. (it takes a lot of solder to get that buch of resistors together)
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Gnd and Vcc wires get attached to the power header.
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In this pic, the blue and red wires are taking raw power (Gnd and Vin) to the audio amp circuit. The two yellow wires behind them are connected to the volume control pot.
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After connecting the pots to the Critter Board, and connecting the sound out to the amplifier, and finally the amp to the speaker, we are done! With everything wired up, there is just enough room for the USB programmer to fit in. We will attach the Critter Board to the cigar box with screws when programming is done.
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There is acutally lots of room left over for cigars.