555

Do you mean 555 in math, or the 555 timer IC in electronics, or something else (area code, Roman numeral, etc.)? Here are quick notes you can use right away for both math and electronics. Tell me which you want to dive into or give me a problem you’re stuck on.

Math (555)

• Prime factorization: 555 = 3 × 5 × 37
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555 (8 divisors)
• Sum of divisors: 912
• Binary: 555 = 1000101011₂
• Roman numeral: DLV

Electronics (555 timer IC)

• Modes: astable (for oscillation) and monostable (one-shot)
• Astable frequency: f ≈ 1 / [0.693 × (RA + 2RB) × C] = 1.44 / [(RA + 2RB) × C]
  • T_high = 0.693 × (RA + RB) × C
  • T_low = 0.693 × RB × C
  • Duty cycle D = T_high / (T_high + T_low) = (RA + RB) / (RA + 2RB)
• Monostable pulse width: t_on ≈ 1.1 × R × C
• Quick example: RA = 1 kΩ, RB = 4.7 kΩ, C = 100 nF
  • f ≈ 1.44 / [(1k + 2×4.7k) × 100 nF] ≈ 1.44 / (10.4k × 100 nF) ≈ 961 Hz

Which topic do you want to work on? I can give you a couple of practice problems and solutions if you’d like.