To better understand the NE555 timer’s internal operation and practical applications, I decided to rebuild it using discrete components. Although the datasheet shows a block-diagram of its internals, it lacks detailed explanations—so I relied on trial and error to figure everything out.
Since my high-school days I’ve used NI Multisim as my electronic simulator. It’s perfect for ‘what-if’ experiments without wasting real components or time.
In the schematic above, I’ve implemented an astable oscillator whose time constant τ is set by R4, R5, and C2:
The NE555 gets its name from the three 5 kΩ resistors inside its silicon die. In my discrete version they’re represented as R1, R2, and R3—each chosen as a standard 4.7 kΩ value. These form a voltage divider that provides reference levels at one-third and two-thirds of VCC for the dual comparators. Capacitor C1 decouples the two-thirds node, though you could replace it with another circuit to shift the divider ratio for special functions.
The simulation confirms the discrete circuit behaves exactly like a real NE555. I cleaned up the schematic and exported it as a PDF, which you can download here: ne555_discrete.pdf.