555

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

If you meant the 555 timer (common exam topic), here are quick basics:

• Astable mode (free-running):

  • t_high = 0.693 (R_A + R_B) C
  • t_low = 0.693 R_B C
  • f = 1 / [0.693 (R_A + 2 R_B) C]
  • Duty cycle D = t_high / (t_high + t_low) = (R_A + R_B) / (R_A + 2 R_B)

• Monostable mode (one-shot):

  • Pulse width T = 1.1 R C (where R and C are the timing resistor and capacitor)

If you meant the number 555:

• Prime factorization: 555 = 3 × 5 × 37
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555
• Sum of digits: 5+5+5 = 15

If you had a specific problem (circuit values, calculations, etc.), tell me which interpretation and the given data, and I’ll work through it.