Chips
555 timer
A classic chip for blinking, beeping and timing. It charges a capacitor through resistors and flips its output each time the capacitor voltage crosses 1/3 and 2/3 of the supply, so two resistors and a capacitor set the timing.
Pins
- TRIG (2) / THR (6)
- : The two comparator inputs, watching the capacitor voltage at 1/3 and 2/3 of VCC.
- DIS (7)
- : Discharge: a transistor that empties the capacitor.
- OUT (3)
- : Can drive about 200 mA, enough for an LED, buzzer or small relay.
- RST (4) / CTRL (5)
- : Reset (tie high if unused) and control voltage (add 10 nF to ground).
An 8-pin chip (DIP or SOIC). The 556 holds two 555s; the CMOS version (TLC555, 7555) uses far less power.
What to look for inside
- This is astable mode, where it oscillates on its own. The top trace is the capacitor voltage and the bottom one is the output.
- The capacitor charges through R1 and R2. When it reaches 2/3 of the supply, an internal comparator flips the output LOW and opens the discharge transistor.
- Now it discharges through R2 only. At 1/3 of the supply the other comparator flips the output HIGH again, and the cycle repeats.
- Bigger resistors or a bigger capacitor mean a slower blink.
The numbers that matter
| Spec | Typical |
|---|---|
Supply voltage The classic NE555 does not work from 3.3 V. | 4.5 V to 16 V (NE555) · 2 V up (CMOS) |
Output current Can drive small loads directly. | ≈ 200 mA |
Max frequency Fine for blinking, tones and simple PWM. | ≈ 100 kHz to 500 kHz |
Where you’ll use it
- Blinking LEDs and beeping buzzers.
- Delay timers (monostable: one pulse per trigger).
- Simple PWM for dimming or motor speed.
Common mistakes
- Leaving RESET (pin 4) floating. Tie it to VCC.
- Using electrolytic capacitors for precise timing. Their tolerance is ±20 %.
Lessons that use it
No 555 lesson yet. Its timing comes from the RC charging shown in lesson 2.2.
Go deeper
Astable timing. 0.693 = ln 2, the time to charge from 1/3 to 2/3.
Frequency.