Here is a schematic of a simple 555 organ circuit.The circuit is consist of two parts first 555 circuit of organ and second lm386 amplifier for driving 8 ohms speaker. The circuit will generate slightly different tone by pressing each switch from S1 to S7 and work like a piano. Use 100K small
variable resistors for R1 to R7. The frequency of tone can be changed by adjusting the variable resistors R1 to R7, adjust them like piano tone 1, 2, 3, 4, 5...., and then change these variable resistors to fix resistors if required. More switches with variable resistors can be added before S1 for more tones. You can also change tone of all switch from S1 to S7 at once by adjusting the variable resistor R9. Apply 9 to 12 volt DC to power the circuit.