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.
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