555

Not sure what you want with "555". Do you want:

• Electronics: 555 timer IC (how it works, astable/monostable modes, key formulas)
  • Quick notes: pinout (1 GND, 2 Trig, 3 Out, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 Vcc)
  • Astable: f ≈ 1/(0.693*(RA+2RB)C); t_high = 0.693(RA+RB)C; t_low = 0.693RB*C
• Math / number theory: properties of 555
  • 555 = 3 × 5 × 37; not prime
  • Divisible by 5 and 3; digits sum to 15
• Roman numerals: 555 = DLV
• Something else (e.g., culture, emergency number, a code)

Tell me which topic you’re studying, and I’ll tailor a quick explanation or a practice problem. If you want, I can also give you a 5-minute cram sheet or a short solved example.