The Legendary NE555: Timer and Oscillator

The NE555 is a timer integrated circuit capable of generating delays, pulses, and oscillations with very few external components. It remains one of the most beloved chips because it is cheap, robust, and highly educational.

Designed in 1971 by Hans Camenzind, this small 8-pin chip is almost the Swiss Army knife of electronics. Need a clock? A timer for a stairway light? A tone generator? An analog PWM? The 555 can handle all those tasks.

In this article, we will learn its two master configurations: monostable mode (one-shot) and astable mode (continuous oscillation).

What’s inside the black box?

The 555 is a fascinating hybrid. Inside it coexists:

1. Two analog comparators: (Like the LM393).
2. A digital flip-flop: (A 1-bit memory to save the state).
3. A discharge transistor: To empty the capacitor.
4. Three 5k resistors: which form an internal voltage divider (hence the urban legend that it is called “555”).

Thanks to this structure, the 555 is dedicated to monitoring the voltage of an external capacitor. When the voltage crosses 1/3 of the supply voltage or 2/3 of the supply voltage, the 555 changes its output.

Monostable mode (the timer)

Imagine the typical push-button for a stairway light: you press once, the light turns on, and after a while it turns off by itself. That is a monostable circuit (“one stable state”).

• Normal state: Output off (LOW).
• Trigger: Upon receiving a low pulse on the trigger pin (2), the output turns on (HIGH).
• Timing: The output stays on while an external capacitor charges through a resistor.
• End: When the capacitor reaches 2/3 of the voltage, the 555 cuts off and returns to rest.

The timing formula

The nominal time the output is on (T) depends on the resistance (R) and the capacitor (C), in addition to their tolerances and the actual thresholds of the circuit:

If you use ohms for and farads for , the time will come out directly in seconds.

Example: We want an LED to stay on for 5 seconds after pressing a button. We use a capacitor of 47 F. What resistance do we need?

With a resistor of 100 k and a capacitor of 47 F, we have an analog timer without writing a single line of code.

Astable mode (the oscillator)

In astable (no stable state) mode, the 555 output continuously oscillates between HIGH and LOW. It generates a square wave.

The trick is to connect the circuit so it re-triggers itself.

1. The capacitor charges.
2. Upon reaching 2/3 Vcc, the 555 turns the output off and starts discharging the capacitor.
3. Upon dropping to 1/3 Vcc, the 555 turns back on and starts charging.
4. Infinite loop.

Frequency and duty cycle

Here we use two resistors (, ) and one capacitor (C).

The frequency (blinking speed) is:

Why use a 555 today?

You are probably thinking you can do this with a microcontroller in two lines of code. And you are right. But the 555 lives on for several reasons:

1. Price: It costs pennies.
2. Robustness: It has no firmware to crash and needs no boot time. It works instantly.
3. Voltage: It operates from 4.5V to 15V. A microcontroller at 12V needs a regulator; the 555 connects directly.
4. Output current: the bipolar version can handle high currents, but the voltage drop and dissipation increase significantly. For relays or motors, it is better to use a transistor and the corresponding protection.

But above all, it is still mentioned due to a nostalgia factor and because it is an interesting tool for learning electronics.