555

Do you mean 555 in electronics (the 555 timer IC), or the number 555 in math, or something else?

If you meant the 555 timer IC, here’s a quick refresher:

• Pinout (common DIP8): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 Vcc.

• Comparators: upper comparator trips at 2/3 Vcc; lower comparator trips at 1/3 Vcc.

• Modes:

  • Monostable: one-shot pulse on a trigger. Pulse width t ≈ 1.1 R C.
  • Astable: continuously oscillates. Relations:
    • t_high = 0.693 (R1 + R2) C
    • t_low = 0.693 R2 C
    • Frequency f = 1 / [0.693 (R1 + 2 R2) C] = 1.44 / ((R1 + 2 R2) C)
    • Duty cycle D = t_high / (t_high + t_low) = (R1 + R2) / (R1 + 2 R2)

• Quick tips: use a reset pin pulled high, keep control pin (pin 5) with a small capacitor if you need to reduce noise, and choose R/C values to keep currents reasonable and waveform clean.

If you meant math (the number 555):

• Prime factorization: 555 = 3 × 5 × 37
• In hex: 0x22B; in binary: 1000101011; in Roman numerals: DLV

Tell me which interpretation you want, or share your exam topic, and I’ll tailor it (including practice problems or a mini cheat-sheet) to your course.