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Resistor values can be tweaked to adjust the pitch. The switch on the bottom left controls pitch register. Flip up for lower notes and down for higher notes (you can experiment with different value caps to lower or raise the note register or in other words change the range of frequencies of the oscillating signal output when each momentary switch is pressed). You can calculate the capacitor and resistor values by the desired frequency of signal generated. An 'A' note, when played on a piano, is the sound produced by striking a key which triggers a mechanism which hits a string inside. The string vibrates back and forth 440 times, vibrating the air around it which resonates in the piano; that's the sound you're hearing. So the frequency of the sound wave for that 'A' note is 440Hz. But what if you want to play an 'A' note that's an octave lower? Well that 'A' has a frequency of 220Hz. Since we have 5 switches, it makes sense to select the pentatonic scale to determine the intervals between each note of our tone generator. Let's make it a minor pentatonic. So the notes will be a c d e g. Now we pick the register we want our tone generator to play in. I think audible tones have a frequency between 20Hz - 20KHz but I don't remember. For the key and scale we have chosen, let's start with 440Hz for the 'A' note which will be the lowest pitch (with 'G' being the highest pitch). Build the circuit but only connect one push button and pot in place of the series resistor. Use a tuner to measure the pitch/frequency of the signal through the speaker, and with the potentiometer carefully fine tune the signal to approximately 440Hz. If you want to raise the note an octave up, increase the frequency; for an octave down, lower the frequency. You can find charts online displaying information on notes in each register and correlating frequency values. Or if you have a piano just hit the key for the note you want and adjust the potentiometer until pitch from the tone generator sounds the same. This can be very easy or very difficult to hear depending on how developed your ear is or your level of intuitive understanding of harmonic intervals (notes played in unison). I haven't built this myself so I'm not sure if it's easier to calculate values from desired frequency of wav or just to tune it by ear with a piano and pots and caps. I'll make a edit after I give it a try.
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