555

Do you mean the number 555 or the 555 timer IC? Here are quick notes for both. Tell me which you want and your level (e.g., undergrad).

If you mean the number 555:

• Prime factorization: 3 × 5 × 37
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555
• Sum of digits = 15, so 555 is divisible by 15 (555 = 15 × 37) — a Harshad number
• In binary: 1000101011₂
• In hex: 0x22B
• Roman numeral: DLV
• It’s a 3-digit repdigit (all digits the same)

If you mean the 555 timer IC:

• Common configurations: astable (free-running oscillator), monostable (one-shot), bistable
• Rough formulas:
  • Astable: f ≈ 1.44 / [(R1 + 2R2) C], Ton ≈ 0.693 (R1 + R2) C, Toff ≈ 0.693 R2 C
  • Monostable: t ≈ 1.1 × R × C (triggered pulse width)
• Pinout basics (typical): Vcc = pin 8, GND = pin 1, Trig = pin 2, Output = pin 3, Reset = pin 4, Threshold = pin 6, Discharge = pin 7, Control = pin 5

Would you like a deeper dive into one of these (and maybe a couple of practice problems)?