555

Do you mean 555 in one of these contexts?

• 555 timer IC (electronics)
• The number 555 in math/number theory
• something else (e.g., Roman numeral DLV)

If you want a quick study sheet, here are brief cheat notes for the two most common:

1. 555 timer IC (exam prep)

• Pinout (typical): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 Vcc
• Key basics: compares at 1/3 Vcc and 2/3 Vcc
• Modes:
  • Astable: continuously oscillates (no external trigger)
    • t_high = 0.693*(R1+R2)*C
    • t_low = 0.693R2C
    • T = t_high + t_low = 0.693*(R1+2R2)*C
    • f = 1/T
  • Monostable: one-shot pulse on trigger
    • t = 1.1RC (R and C chosen for desired pulse width)
• Control voltage pin (2/3 Vcc reference) can be overridden by node at pin 5
• Common gotchas: reset pin must be tied high; discharge pin usage; grounding and decoupling.

2. 555 in math/number theory

• 555 = 5 × 111 = 3 × 5 × 37
• Prime factorization: 3, 5, 37
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555
• Properties: odd composite; divisible by 3 and 5 (sum of digits 15)
• Roman numeral: DLV

Tell me which context you want, and I’ll go deeper (with worked examples or practice questions).