555 timer ic

a tiny timing chip that can beep, blink, and gate. two classic modes: astable (square-wave) and monostable (one-shot).

astable (free-running oscillator)

use two resistors (R1, R2) and a capacitor C. standard topology (no diode):

• f = 1 / (0.693 × (R1 + 2R2) × C)
• thigh = 0.693 × (R1 + R2) × C
• tlow = 0.693 × R2 × C
• duty ≈ (R1 + R2) / (R1 + 2R2) (always > 50% in this topology)

example: R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF → f ≈ 6.9 Hz (cute blink).

astable with ≈50% duty (diode trick)

add a diode in parallel with R2 so the capacitor charges through R1 only (via the diode) and discharges through R2. orient the diode so it conducts from the R1 side toward the capacitor node (cathode/stripe toward pins 2/6).

• thigh = 0.693 × R1 × C (charge via R1 + diode)
• tlow = 0.693 × R2 × C (discharge via R2)
• T = 0.693 × (R1 + R2) × C → f = 1 / T
• duty = R1 / (R1 + R2) → choose R1 ≈ R2 for ≈50%

tip: a Schottky diode (e.g., 1N5819) gives a smaller forward drop; then pick R1 ≈ R2 for ≈50% duty.

monostable (one-shot)

press a button to trigger a single pulse.

• pulse width: t = 1.1 × R × C
• use a 0.01 µF on pin 5 (ctrl) to ground for stability.
• debounce the trigger (RC or in software) to avoid retriggers.

wiring tips

• decouple vcc with 0.1 µF near the chip + a larger 10 µF bulk cap.
• pin 3 current: design for ≤ 50 mA on bipolar 555s; buffer bigger loads with a BJT/MOSFET (always for motors/relays).
• for low supply current or 3.3 V operation, use a CMOS 555 (LMC555, TLC555).
• typical timing parts: resistors 1 kΩ–3.3 MΩ; supply: NE555 ~4.5–16 V, CMOS 555 often 2–15/18 V (check datasheet).