555

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

If you’re after the 555 timer IC:

• What it is: a versatile timing/oscillator IC used for monostable and astable modes.
• Quick pinout (common 8-pin DIP): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control Voltage, 6 Threshold, 7 Discharge, 8 Vcc.
• Key formulas:
  • Astable: f ≈ 1 / [0.693 × (R1 + 2R2) × C]
    • T_high = 0.693 × (R1 + R2) × C
    • T_low = 0.693 × R2 × C
  • Monostable: T ≈ 1.1 × R × C
• Typical use: blinking LEDs, timer circuits, PWM, tone generators.
• Quick example: with R1 = 1 kΩ, R2 = 10 kΩ, C = 100 nF, f ≈ 1 / (0.693 × 21k × 100 nF) ≈ 687 Hz.

If you meant the number 555 in math:

• 555 = 3 × 5 × 37; divisible by 3 and 5.
• Binary: 555 decimal = 1000101011₂.
• Roman numeral: DLV.
• Other quick facts or a specific problem you want to tackle?

Tell me which one you’re after (or share a problem), and I’ll tailor the explanation or work through a practice problem.