555

Do you mean the 555 timer IC? If yes, here’s a quick refresher you can use for exams.

Key basics

• Pinout (NE555/common): 1 GND, 2 TRIG, 3 OUT, 4 RESET, 5 CTRL, 6 THR, 7 DISCH, 8 VCC.

Two common configurations

• Monostable (one-shot)
  • Trigger a low pulse on pin 2; output goes high for a time t.
  • Timing: t ≈ 1.1 × R × C
  • Typical wiring: R from Vcc to DISCH (pin 7) and node to C (to ground) connected to THR/TRIG; DISCH turns off during timing.
• Astable (oscillator)
  • No external trigger; output toggles continuously.
  • Timing:
    • t_high ≈ 0.693 × (R1 + R2) × C
    • t_low ≈ 0.693 × R2 × C
  • Frequency: f ≈ 1.44 / [(R1 + 2R2) × C]
  • Duty cycle: (R1 + R2) / (R1 + 2R2)

Tips

• Use a decoupling cap near Vcc (e.g., 0.01–0.1 µF) to stabilize supply.
• Control voltage pin (5) is normally at 2/3 Vcc; you can add a 10 nF cap to ground to stabilize it if you’re modulating frequency.

Quick practice (example)

• Astable: R1 = 4.7 kΩ, R2 = 47 kΩ, C = 100 nF
  • t_high ≈ 0.693 × (4.7k + 47k) × 100e-9 ≈ 0.68 ms
  • t_low ≈ 0.693 × 47k × 100e-9 ≈ 0.0033 s ≈ 3.3 ms
  • f ≈ 1.44 / [(4.7k + 2×47k) × 100e-9] ≈ a few hundred Hz

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