NE555 electronic keyboard

 
The NE555 is a precision timing circuit capable of producing precise time delays or oscillations. In the time-delay or monostable modes of operation, the timing interval is controlled by an external resistor and capacitor network. In the a-stable mode of operation, the frequency and duty cycle can be independently controlled by two external resistors and one external capacitor. (The timing time and frequency duty cycle are determined by external capacitors and resistors)
 
 
The working principle of NE555
 
The function of the NE555 timer is mainly determined by two comparators. The output voltage of the two comparators controls the state of the RS trigger and the discharge tube. When a voltage is added between the power supply and the ground, when the 5th pin is suspended, the voltage at the inverting input of the voltage comparator A1 is 2VCC/3, and the voltage at the non-inverting input of A2 is VCC/3. If the voltage at the trigger input TR is less than VCC/3, the output of comparator A2 is 1, which can set the RS trigger to 1 and make the output OUT=1. If the voltage at the threshold input TH is greater than 2VCC/3, and the voltage at the TR terminal is greater than VCC/3, the output of A1 is 1, and the output of A2 is 0, which can set the RS trigger to 0 and make the output OUT=1.