555

Do you mean:

• the number 555 (math problems), or
• the 555 timer IC (electronics), or
• something else?

If you’re prepping for exams, here are quick notes for the two most common:

1. 555 timer IC (electronics)

• Modes: astable (oscillator) and monostable (one-shot).
• Pinout (typical): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 Vcc.
• Astable timing:
  • t_high = 0.693 (R1 + R2) C
  • t_low = 0.693 R2 C
  • f = 1 / [0.693 (R1 + 2 R2) C]
  • Duty cycle = (R1 + R2) / (R1 + 2 R2) × 100%
• Monostable timing: t_high ≈ 1.1 R C (R is the external resistor, C the external capacitor)
• Useful notes: varies with supply, output can source/sink limited current, control voltage pin allows duty-cycle tweaking.

2. Number 555 (math)

• Factorization: 555 = 3 × 5 × 37.
• Prime factors: 3, 5, 37 (all to the first power).
• Divisibility: divisible by 3 (3 + 5 + 5 = 13, wait — sum is 15, so divisible by 3); divisible by 5 (ends in 5); divisible by 37 (since 3×5×37 = 555).
• In other representations:
  • Binary: 555 = 1000101011₂
  • Hexadecimal: 555 = 0x22B
  • Roman numerals: DLV
• Quick exercise ideas:
  • Number of divisors: (1+1)(1+1)(1+1) = 8.
  • Sum of divisors: (1+3)(1+5)(1+37) = 4 × 6 × 38 = 912 (if you need a quick check).

If you tell me your course (EE, physics, CS, math) and what you want (concepts, problem sets, or a quick quiz), I can tailor a concise study sheet or generate practice questions.