The 555 timer is an old IC from the 1970s, but with just two resistors and a capacitor you can make an LED blinker, an alarm tone, or a delay circuit, so it is still a staple of beginner practice. This article covers the NE555's pinout, how astable operation works, the frequency and duty-cycle formulas, how to decide component values, and what to watch out for when building it on perfboard. The 555 timer calculator does the calculations for you.
Pinout
The NE555 is an 8-pin DIP. With the notch at the top, the upper left is pin 1 and the pins are numbered counterclockwise (Reading DIP IC Pin Numbers).
| Pin | Name | Role |
|---|---|---|
| 1 | GND | Ground |
| 2 | TRIG | Trigger. When the voltage falls below 1/3 of the supply, the output goes High |
| 3 | OUT | Output |
| 4 | RESET | Pulled Low, it forces the output Low. Tie to +V if unused |
| 5 | CTRL | Control voltage. Bypass to ground with 10 nF if unused |
| 6 | THRES | Threshold. When the voltage exceeds 2/3 of the supply, the output goes Low |
| 7 | DISCH | Discharge. When the output is Low it connects to ground and discharges the capacitor |
| 8 | VCC | Supply (4.5 to 16 V for the NE555) |
The astable circuit
Astable is operation where the output flips between High and Low on its own. Pins 6 and 2 are tied together so both watch the capacitor voltage.
- The capacitor charges through R1 + R2. The output is High while the voltage is above 1/3 of the supply and until it reaches 2/3.
- When the voltage reaches 2/3, the internal circuit switches the output Low and connects pin 7 to ground. The capacitor discharges through R2.
- When the voltage falls below 1/3, the output goes High again and charging starts.
Formulas
High time t_H = 0.693 × (R1 + R2) × C Low time t_L = 0.693 × R2 × C Period T = 0.693 × (R1 + 2 × R2) × C Frequency f = 1 / T ≈ 1.44 / ((R1 + 2 × R2) × C) Duty cycle D = (R1 + R2) / (R1 + 2 × R2)
The duty cycle is always above 50%. It gets closer to 50% when R2 is much larger than R1.
Deciding component values
The convenient order is to start from the frequency you want, pick C first, and then fit the resistors.
- Decide C. For slow blinking use an electrolytic of 10 µF or more, and for fast oscillation use a 100 nF to 1 µF film or ceramic.
- Set R1 to 1 kΩ or more (if it is too small, it strains the discharge pin).
- Calculate R2 from the period you need.
R2 ≈ (T ÷ (0.693 × C) − R1) ÷ 2. - Fit to standard values and recalculate to confirm.
| R1 | R2 | C | Frequency | Duty | Use |
|---|---|---|---|---|---|
| 1 kΩ | 6.8 kΩ | 100 nF | about 988 Hz | 53% | Driving a buzzer (alarm tone) |
| 1 kΩ | 6.8 kΩ | 10 µF | about 9.9 Hz | 53% | Fast LED blinking |
| 1 kΩ | 10 kΩ | 10 µF | about 6.9 Hz | 52% | Blinker |
| 10 kΩ | 68 kΩ | 10 µF | about 0.99 Hz | 53% | A clock at a 1-second beat |
| 10 kΩ | 68 kΩ | 22 µF | about 0.45 Hz | 53% | Slow blinking |
The values in the table are theoretical values from the formulas. Because of the tolerance of electrolytic capacitors (±20%), real values can differ by 10 to 20%.
Lighting an LED from the output
Pin 3 can source or sink up to about 200 mA. The simplest way to light one LED from a 5 V supply is to connect pin 3 through an LED resistor (330 Ω) and the LED to ground. The resistor calculation is in LED Current-Limiting Resistors. To light two alternately, put one LED between pin 3 and GND and the other between +V and pin 3. When pin 3 is High the first lights, and when it is Low the second lights.
Tips for building on perfboard
- Plug the 555 into an IC socket. That prepares you for orientation mistakes and replacement.
- Put a 100 nF between the supply pin (8) and ground (1) close to the IC (Power Rails, Grounding, and Decoupling Capacitor Placement).
- Do not forget to tie pin 4 (RESET) to +V. If it floats, it resets at random.
- Putting a 10 nF capacitor from pin 5 to ground makes it less sensitive to supply noise. It works if you leave it out.
- Mind the polarity of the electrolytic capacitor. Usually the + side is the pin 6 and 2 node.
- Tie pins 2 and 6 together with a short wire.
Common mistakes
| Symptom | Cause | Fix |
|---|---|---|
| Does not work at all | Pin number mix-up, power pins reversed | Recheck the notch direction and pin 1 |
| Output stuck High or Low | Pins 2 and 6 not tied together | Connect the two pins |
| Occasionally stops or resets | Pin 4 floating | Connect pin 4 to +V |
| Frequency differs from the calculation | Electrolytic and resistor tolerance | Measure the part values, fine-tune with a potentiometer |
| IC is hot | Discharge pin overcurrent (R1 too small), power connected backwards | Increase R1 and check polarity |
Checking in Patterndo
Patterndo's built-in ICs include the NE555, so you can lay out the circuit above as is and simulate it. Hover over the output pin to watch its HIGH and LOW state change with time (Using Patterndo Simulation). Experimenting by changing component values and seeing how the frequency changes lets you learn the formulas by feel.
Summary
The frequency of a 555 astable circuit is 1.44 ÷ ((R1 + 2R2) × C), and the duty cycle is always over 50%. Keep R1 at 1 kΩ or more, tie pin 4 to +V, tie pins 2 and 6 together, and put a 100 nF close to the supply. If you need an operation that turns on just once, continue with 555 Timer Monostable Circuit, and for the principle of how a capacitor value sets time, RC Time Constants.