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Lots of people use 555 timer to make musical synthesis. I want to try something different. The XR-2209 is a monolithic voltage-controlled oscillator that can generate simultaneous triangle and square wave outputs over a frequency range of 0.01Hz to 1MHz. Its frequency is determined by resistor R1 to R13 and capacitor C1 (see schematic .f=1/ RC). By varying the value of R1/R13, we can get 13 different tones. I use 13 sets of reflective sensor QRD1114 and mosfet 2N7000 to turn on/off R1/R13. The reflective sensor QRD1114 consists of an infrared emitting diode and an NPN silicon phototransistor. When a white paper is placed on the top of the sensor, the phototransistor output moves from a high voltage state to a low voltage state, and the mosfet will be turned off. By placing a scantron on the top of the sensors, only one mosfet will be turned on each time. The mosfet will be on/off when the scantron is moving.