555

Do you mean:

• the number 555 (math properties), or
• the 555 timer integrated circuit (electronics)?

Here’s a quick cheat sheet for both. Let me know which you want to go deeper into.

1. If you mean the number 555 (math)

• Prime factorization: 555 = 3 × 5 × 37
• Divisibility: divisible by 3, 5, 15, 37, 111, 555
• Decimal properties: a repdigit (all digits the same), palindrome
• Sum of digits: 5 + 5 + 5 = 15
• Roman numerals: DLV
• In binary: 555 = 1000101011₂

2. If you mean the 555 timer IC (electronics)

• What it is: a versatile timer/oscillator IC used for delays, pulses, and PWM
• Pinout (8-pin DIP): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control Voltage, 6 Threshold, 7 Discharge, 8 Vcc
• Modes:
  • Monostable: one-shot pulse; t ≈ 1.1 × R × C
  • Astable: free-running oscillator; f ≈ 1.44 / ((RA + 2RB) × C); duty cycle D ≈ (RA + RB) / (RA + 2RB)
  • Bistable: flip-flop mode
• Key notes: Reset is active low; Control Voltage can modify timing via a capacitor on pin 5; discharge transistor on pin 7 discharges the timing capacitor
• Common uses: LED blinkers, pulse generation, timers, PWM signals

Tell me which one you want to study, and any specific topic or problem you’re preparing for.