The 555 has been in production since 1972 and still turns up in more student projects than any other chip. It is worth knowing properly, because a panel that sees one in your circuit will ask you how it works.
Inside are two comparators, a flip-flop, a transistor and a chain of three equal resistors — the chain is where the name comes from. The resistor chain divides the supply into thirds, and the whole device does one thing: it charges a capacitor and watches it cross 1/3 and 2/3 of the supply.
The pin-out
| Pin | Name | What it does |
|---|---|---|
| 1 | GND | Ground. |
| 2 | Trigger | Watches for the voltage falling below 1/3 Vcc. That sets the output high. |
| 3 | Output | Swings between ground and about 1.7 V below Vcc. Good for 200 mA. |
| 4 | Reset | Forces the output low when pulled below about 0.7 V. Tie it to Vcc if you are not using it. |
| 5 | Control | Direct access to the 2/3 Vcc reference. Leave it alone and fit 10 nF to ground. |
| 6 | Threshold | Watches for the voltage rising above 2/3 Vcc. That resets the output low. |
| 7 | Discharge | An open transistor to ground, switched with the output. It is what empties the capacitor. |
| 8 | Vcc | 4.5 V to 16 V for the standard bipolar NE555. |
Astable: a free-running oscillator
Tie pins 2 and 6 together and the chip chases its own tail. The capacitor charges through R1 and R2 until it hits 2/3 Vcc, the output goes low, and it then discharges through R2 alone into pin 7 until it falls to 1/3 Vcc — at which point it starts again. It never stops.
f = 1.44 / ((R1 + 2 x R2) x C)
- f in hertz, R in ohms, C in farads
- High time: t_high = 0.693 x (R1 + R2) x C
- Low time: t_low = 0.693 x R2 x C
A 1 Hz flasher, the usual first build. Take C = 10 uF, R1 = 47k and R2 = 47k:
f = 1.44 / ((47000 + 94000) x 0.00001) = 1.44 / 1.41 = 1.02 Hz
| R2 | Frequency | Feels like |
|---|---|---|
| 1k | about 12 Hz | A visible flicker |
| 10k | about 4.8 Hz | A fast blink |
| 47k | about 1.4 Hz | A steady blink |
| 100k | about 0.68 Hz | Roughly a second and a half per cycle |
To change the frequency by a factor of ten, change C by a factor of ten. Use the resistors for fine adjustment and the capacitor for the range.
555 timer calculator
Open on its own pageBoth circuits. It warns when R strays outside the range the chip is happy with.
Tie pin 4 to Vcc and fit 100 nF between pins 8 and 1, right at the chip. A floating reset pin and a missing decoupling capacitor are the two reasons a 555 that should oscillate does not.
Monostable: one pulse, of a set length
Here the 555 sits still with its output low until pin 2 is briefly pulled below 1/3 Vcc. Then the output goes high, the capacitor charges through R, and when it reaches 2/3 Vcc the output drops back and stays there. One trigger, one pulse. This is what a timer circuit, a door-chime hold and a debounced button all use.
T = 1.1 x R x C
- T in seconds, R in ohms, C in farads
- The 1.1 is ln(3) — one third to two thirds is a longer climb than the astable's
For a 10 second pulse with C = 100 uF: R = 10 / (1.1 x 0.0001) = about 91k, so fit 100k and get 11 seconds. Keep the trigger pulse shorter than the output pulse — if pin 2 is still held low when the time is up, the output stays high.
Choosing R and C sensibly
- Keep R between about 1k and 1M. Below 1k the discharge transistor on pin 7 has too much current through it and runs hot. Above 1M the chip's own input leakage becomes comparable to the charging current and the timing drifts.
- Reach for the capacitor for long times, not a huge resistor. For minutes, use a big C.
- Avoid electrolytics where the timing must be accurate. They are typically +/- 20 % and leak, so a calculated 10 seconds may be 8 or 13. If it matters, use a film capacitor.
- Check the dielectric on ceramics. A Y5V capacitor loses much of its value with temperature and applied voltage, and the frequency will not match your arithmetic.
It is not oscillating. Check these four.
- 1
Pin 4 is floating
Reset is active low and an unconnected pin picks up noise, so the chip resets at random or sits dead. Tie pin 4 directly to Vcc. This is the single most common 555 fault on a breadboard.
- 2
No decoupling capacitor
The 555 draws a sharp current spike every time its output changes state — enough to dip the supply on a breadboard and retrigger itself. Fit 100 nF between pins 8 and 1, physically next to the chip. On long-lead breadboard wiring, add 10 uF as well.
- 3
Pin 5 left bare in a noisy circuit
Control voltage is the 2/3 reference itself. Noise on it moves the threshold and the output jitters. A 10 nF capacitor from pin 5 to ground fixes it and costs nothing.
- 4
Pins 2 and 6 not actually joined
In an astable they must be connected together and to the capacitor. It is easy to place them in adjacent breadboard rows and assume it. Check continuity with a meter rather than by eye.
If you need very low power, or the supply is below 4.5 V, use the CMOS version — the 7555 or TLC555. Same pin-out, same formulas, microamps instead of milliamps of supply current, and it does not slam the rail on every transition. It will not source 200 mA, which is the trade.