it has 2 speakers which both play different sound at the same time, which makes an interesting sound and it is tunable using the trimmer potentiometers so you could in theory make an accurate scale, the notes are also layed out like on a piano.
Another feature is a potentiometer that controls pitch bend and a rotary switcjh to change the capacitors on the organ, changing the sound.
Step 2: What you'll need
2x 555 timer chips
10k + 100k potentiometers
20 2k trimmer potentiometers
10n, 100n, 47n and 2 10uf capacitors
large perfboard (or you could make a printed circuit board)
assorted wire and battery clip
glue, cable tie and cork
probably desoldering pump/wick
screwdriver for the trimmers
Step 3: The piano circuit
The 100n capacitor is replaced by the rotary switch, with three capacitors connected. the wire from 6/2 goes to one of the middle termnals and other capacitors you want to use are connected to the other terminals of the switch and to ground. I used 100n, 47n and 10n capacitors but you could use more different values.
Step 4: The keyboard
The top layer of buttone is set out like a pianos black notes so there are larger gaps between the bases of the buttons, so instead of using solder to birdge the bases i used a piece of wire that went to each button and the pins 2/6. The green wires are where the bottom layer is sonnected to the top, look at the 5th picture to see where they go as it is hard to explain.
Step 5: The Atari punk module
EDIT: the picture is not mine, it is from http://www.robthefiddler.com/
Step 6: Finishing off
you could make an enclosure but im not sure how you could mount the buttons unless you made the whole thing completely differently, I like it without an enclosure. I put hot glue on the bases of the potentiometers to make them more stable and easier to use with your little finger while playing the keys with the rest of your hand, it you put both potentiometers next to each other this would not be necessary. i used a cable tie to attach the power cable to one of the holes in the perfboard and glued the switch and speakers on with bostik.
I power it with 4 AA battery which produce about 5v and make it loud, a 9v is ok if your just using the keyboard but it isnt usually powerful enough for both 555s.