555

Do you mean the 555 timer IC (electronics), or is 555 referring to something else for your exams?

If you want the 555 timer refresher, here’s a quick outline:

• Pins and basics

  • Pin 1 GND, 2 TRIG, 3 OUT, 4 RESET, 5 CTRL, 6 THRS, 7 DIS, 8 VCC
  • Inside: comparators, SR latch, discharge transistor

• Modes

  • Astable (free-running)
    • Components: RA, RB, C
    • t_high = 0.693 × (RA + RB) × C
    • t_low = 0.693 × RB × C
    • Frequency f = 1 / [0.693 × (RA + 2RB) × C]
    • Duty cycle D = (RA + RB) / (RA + 2RB)
  • Monostable (one-shot)
    • Trigger below 1/3 Vcc
    • Output goes high for t = 1.1 × R × C (where R is the external resistor)
    • Threshold uses 2/3 Vcc

• Tips

  • Reset pin should be tied high
  • Use a decoupling cap near Vcc (e.g., 0.1 µF)
  • Control pin (5) can be left at 2/3 Vcc or bypassed with a small cap
  • If you need near 50% duty in astable, use a diode to separate charge/discharge paths

• Quick example

  • RA = 10 kΩ, RB = 100 kΩ, C = 100 nF
  • t_high ≈ 0.693 × 110k × 100nF ≈ 7.6 ms
  • t_low ≈ 0.693 × 100k × 100nF ≈ 6.9 ms
  • f ≈ 1 / (7.6 ms + 6.9 ms) ≈ 68 Hz
  • Duty ≈ (110k) / (210k) ≈ 52%

Would you like:

• a set of practice problems on 555 timer design, or
• help solving a specific 555 timer problem you’re studying, or
• a more detailed step-by-step derivation for either mode? I can tailor it to your course level.