In this project, you're going to make an electronic piano. Its keyboard consists of keys S8 - S1, which can produce a complete octave. You can try to play your favourite song.
After wiring the circuit, turn power ON.
Before playing the piano, you need to tune it. Press S8 (the lower doh), and hold it depressed for a while. You'll see the LED light up and hear a piano-like "pong" sound. Rotate the control volume to adjust the speaker tone to "doh." The tone fades out, so you need to press S8 a number of times.
Now, you're ready to play music. S8, S7, S6..... and S1 correspond to doh, ray, me.....and doh. For better results, you should hold each key depressed for some time rather than releasing it quickly. U1 is a NAND multivibrator.
When one of keys S1 - S8 is pressed, a resistor R4 - R10 is connected, and U1 oscillates at the frequency associated with the pressed key.
When a key is pressed, the oscillating output of U1 is given to Q2. Also, when U1 starts oscillating, a trigger pulse is sent through Q1 and U2 to Q3, so C4 is charged.
When C4 is discharging, if the key is still depressed, the oscillating sound fades out like a piano sound. The fading sound passes through the power amplifier U3 and emits from the speaker.