How does the electronic piano work ?

Hi everyone .. 
I have done this circuit last week with my friends 
but I still couldn't understand the concept of how it's work
I just need a few and simple line to get it 
because when I read some data sheet about it I got confused >_<

please help me 

this is the circuit what I've used

Picture of How does the electronic piano work ?
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CSE251 (author) 5 years ago
Higgs Boson thank you very much for your answer ^-^

I think now I got the useful of the capacitors .. that IC freq depends on the charge discharge of C on pins 6 and 2 as "Richarrie" said.. and then the sound of the speaker depends on the charge discharge of C that connected in pin 3
<< is this right ?

lemonie... I think when we press two buttons together they will work as one button with the two resistors of them and give me one tone .. this is what in my mind
but how can I calculate the frequency of each button ?
which formula can I use !

also there is another point ... what will happen if I didn't connect a capacitor to the speaker ? should we always use a capacitor in the output ?
Higgs Boson5 years ago
When you press a button it lets power through the resistor to the thresh hold of the 555. when you press more buttons, you add more resistance to the system, and the slower the 10 uf capacitor charges and discharges into the speaker, and the lower the frequency.
CSE251 (author) 6 years ago
so now the first button from the right will take a high voltage which give us a high frequency " High pitch sound "
and the last button in the left has least voltage because the value of the resistor will increase .. that's mean it will give a low frequency " low pitch sound "
that what's you mean right ??

thank you for your reply ^-^
lemonie CSE2516 years ago
I think you've got the idea there.
You'll also understand why it doesn't give different notes by pressing more than one button?

CSE251 (author) 6 years ago
every thing is clear now ^^
thaaaanks a lot

rickharris6 years ago
Yes more or less the frequency of the 555 oscillator is controlled by the charge discharge time of the capacitor on pins 6 - 2 this in turn is controlled by the value of the resistor(s) the bigger the resistor the slower the charge and so the lower the frequency.
lemonie6 years ago
It's pretty simple.
Close 1 switch, then add up the resistors between that and the 555 - what's the difference with different switches?
And what is the 555 doing?