This is a synth i built a while ago. It's designed by Dino Segovis (Youtube: Hackaweek). It's monophonic meaning that it plays one note at a time. If more than one button is pushed, the added resistance value will produce a higher tone instead of two tones.

Mine has a problem with the amplifier but i have bypassed it to another amplifier and it works just fine. I recommend watching Dino Segovis's video on how he built it: The SQONKBOX 55 Cigar Box Organ With A 555 Timer

Step 1: Basic Operation

There are two 555 timers. The first one outputs a square wave and the second one can be set to modulate that signal. Like an LFO!

The frequency of the first 555 is determined by a resistance value set by pushing the different buttons.

Step 2: Materials

2 555
1 Lm386
3 8 pin dip socket
9v Battery
9v Battery Holder
1 On/Off Switch
1 LFO Switch
1 Speaker
13 Momentary Pushbuttons
Black/Red Wires
13 1k Ohm Trimpotentiometer
1 1k Ohm Potentiometer
1 100 Log Ohm Potentiometer (I could not find one so I used one as close to 100Ohm as possible. This is used for the volume control so it just makes it work over a shorter range i.e. more sensitive)
2 100 Ohm Resistor
1 10 Ohm Resistor
1 LDR
1 LED
2 100 uf Capacitor
1 1 uf Capacitor
1 4.7 uf Capacitor
1 10 uf Capacitor
1 0.01 uf Capacitor
1 0.047 uf Capacitor
1 220 uf Capacitor

Step 3: Tools

Soldering iron
Shoe box
Box cutter
Spray paint
Electrical tape
Hot glue gun

Step 4: Preparations

I had to paint some of the buttons white since I could only find black ones at the store.

Make holes for the momentary pushbuttons in a piano key pattern. Also make holes for the switches and potentiometers.

Step 5: Pushbuttons

Solder one of the outer legs of all the trimpots to one of the legs of all the momentary pushbuttons. The trimpots are used as simple variable resistors.

Step 6: Connecting the Trimpots

The trimpots gets connected in parallel as shown in the picture.

Please refer to the circuit schematic for instructions on how to connect them to the circuit.

Step 7: Circuit

The electrical tape contains the LED and the LDR. The LED outputs the modulation signal from the second 555 timer. The LDR picks up that signal on the control pin of the first 555 timer.

I use sockets so I protect the ICs from the heat when soldering.

Step 8: Finishing Up and Tuning

Fasten all buttons, switches and potentiometers in the holes you made in the box. I used hot glue for all the components that I couldn't screw in place. The battery holder, speaker and circuitboard I simply taped down to the cardboard box.

To tune the pushbuttons simply use a tuner or a dial tone (440 A) and turn the trimpots until you are satisfied!