NE555 from discrete components

“To better understand how the NE555 works and how to use it practically, I’ve decided to reimplement it using discrete components. Looking at its datasheet, you can see that its internals are shown, but there are no detailed explanations. I had to proceed through trial and error.


I’ve used NI Multisim as an electronic simulator since my high-school years. It’s quite useful for doing this kind of ‘trial and error’ work without wasting real materials and time.

As you can see from the above image, I’ve drawn an astable oscillator with a time constant Tau determined by R4-R5-C2.

The 555 is named that way because of the three 5kOhm resistors inside its die. They are represented in my schematic as R1-R2-R3, and I have chosen a standard 4.7kOhm value. Their role is to act as a voltage divider, providing U2A – pin 2/3 Vcc and U2B + pin 1/3 Vcc. C1 stabilizes the 2/3 Vcc point, but it can also be substituted by other circuits capable of wisely unbalancing the voltage divider for some particular purposes.

The simulation confirms that the circuit works as expected. I have cleaned up the circuit and exported it as a PDF, which you can download here: ne555.