This is simple music keyboard circuit. The work of the circuit this origin frequency sound circuit model astable multi vibrator. When build the circuit suits power supply the circuit will still don’t work. But if we press switch S1 the circuit will begin the work. By have current flow through R9 get along well with B of Q2, make Q2 begin the work before, then have current flow through SP1, R11 and LED1 at the same time. If we press switch S1-S7 will all right have current flow through R1-R7.
The switch at we press to change R2 reach charging give with C2. Which must use period of until full time which make Q1 work bias. By have current flow through R8 and born feeling numb loads to reach at C1 make voltage at Q2 be down make Q2 stop work now SP1 still not loud and LED1 not bright. Because have no current flow through enough C2 charge full have current flow get along well with B of Q2 make Q2 will begin the work again and Q1 as a result stop work by work alternate back and such forth until born frequency signal upwards. If press switch other the frequency that happen different go to. If observe meet that R1-R7 build serial and have S1-S7 will formed choose the resistance possibly that the resistance that change to affect the frequency.