part · Level 6 · Logic · active
What a 555 timer chip does
555 timer chip — The most famous chip ever — it blinks, beeps, and keeps time.
What it is
The 555 is an eight-legged black chip that has been made by the billion since 1972. It does one thing: it watches a capacitor charging and discharging and flips its output between high and low at the moments you set with two resistors and that capacitor. That one trick gives you a blinker, a tone, a stopwatch, a pulse for a servo, or a clock for a counter chip.
A capacitor is a part that stores charge and fills up gradually through a resistor — the bigger the resistor and the bigger the capacitor, the slower it fills. The 555 turns that slow fill into timing.
It runs happily from 4.5 to 15 V, so your 3×AA pack is fine, and its output can drive an LED (with its resistor) or a buzzer directly — up to about 200 mA.
How it works
Pins, counting anticlockwise from the dot or notch: 1 GND (−), 2 Trigger, 3 Output, 4 Reset (tie to +), 5 Control (leave, or 10 nF to −), 6 Threshold, 7 Discharge, 8 VCC (+).
Inside are two comparators watching the capacitor voltage against ⅓ and ⅔ of the supply. In the common astable wiring, the capacitor charges from + through R1 and R2 in series; when it reaches ⅔ V the output goes low and pin 7 drains the capacitor through R2 alone until it falls to ⅓ V; then the output goes high and it starts again. The times:
- high (charging): t₁ = 0.693 × (R1 + R2) × C
- low (discharging): t₂ = 0.693 × R2 × C
- frequency: f = 1 ÷ (t₁ + t₂) = 1.44 ÷ ((R1 + 2·R2) × C)
With R1 = 1 kΩ, R2 = 100 kΩ and C = 10 µF: t₁ = 0.693 × 101 000 × 0.00001 = 0.70 s, t₂ = 0.69 s, so it blinks about 0.72 times a second. Swap the capacitor for 10 nF and the same resistors give about 700 Hz — a tone. Because charging uses both resistors and discharging only one, the high time is always longer than the low time.
The output can source or sink up to 200 mA, so an LED needs its usual resistor: (4.5 − 2.0) ÷ 220 ≈ 11 mA.
The numbers
| Supply | 4.5–15 V — NE555; the CMOS TLC555/LMC555 goes down to 2 V |
|---|---|
| Output current | up to 200 mA — source or sink — drives LEDs and small buzzers directly |
| Trip points | ⅓ and ⅔ of supply — set by three equal resistors inside |
| Astable frequency | 1.44 ÷ ((R1 + 2·R2) × C) — duty cycle (R1 + R2) ÷ (R1 + 2·R2) |
| Timing range | microseconds to minutes — keep R1 ≥ 1 kΩ and R1 + R2 ≤ 1 MΩ |
See it working
The 555 blinker from the library, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Pin 4 (Reset) left floating. The chip stops at random. Fix: wire pin 4 to +.
- R1 too small. When pin 7 discharges it shorts R1 across the supply; below about 1 kΩ it overheats. Fix: R1 ≥ 1 kΩ.
- Chip in backwards. The notch/dot marks pin 1; power reversed can kill it in seconds. Fix: check the notch before the battery.
- Blink rate nothing like the formula. Electrolytic capacitors can be ±20 % off and change with age. Fix: measure, or use the 555 calculator and accept a rough answer.
- LED across the output with no resistor. Same rule as always — 220 Ω.
Questions people ask
- What does a 555 timer chip do?
- It flips its output between high and low at times set by two resistors and a capacitor. Wired one way it blinks or beeps forever (astable); wired another it gives one pulse of a set length when triggered (monostable).
- How do I calculate the frequency of a 555?
- For the astable circuit, f = 1.44 ÷ ((R1 + 2·R2) × C). With 1 kΩ, 100 kΩ and 10 µF that is about 0.72 Hz. The 555 frequency calculator on this site does the arithmetic and runs the circuit live.
- Can a 555 run from 3 AA batteries?
- Yes. The standard NE555 needs 4.5 V or more, so a fresh 3×AA pack is right at the edge; it works, but a 4×AA pack (6 V) is safer, and the CMOS versions run down to 2 V.
Projects that use this part
No project uses it yet — it arrives at Level 6, and the projects for that level are still being written and verified.
verified against: Texas Instruments — NE555 datasheet · Wikipedia — 555 timer IC · Kuphaldt — Lessons in Electric Circuits, 555 timer experiments