NE555 with discrete components

To better understand how NE555 works and how to use it in reality, I’ve decided to re implement it using discrete components.
Looking at it’s datasheet you can see that its internals are shown but there are no details. I had to proceed by trial and error.


I’ve used NI Multisim as electronic simulator since my high-school years. It’s quiet useful to do this kind of “trial and error” job without wasting real materials and time.

As you can see from the above image, I drawed an astable oscillator with time constant Tau determinate by R4-R5-C2.

The 555 is called that way because of the three 5kOhm resistors inside its die.
They are implemented into my schematic as R1-R2-R3 and I have choosed a standard 4.7kOhm value. Their role is to act as voltage divider giving to U2A – pin 2/3 Vcc and on U2B + pin 1/3 Vcc. C1 “stabilize” the 2/3 Vcc point but it can be also substituted by other circuits capable to wisely unbalance the voltage divider for some particular purpose.

The simulation confirms that circuit works as expected. I have cleaned the circuit and exported as PDF that you can download here ne555.