555

Do you mean the 555 timer IC (electronics) or the number 555 in math? I can cover either. If you want the timer IC, here’s a quick refresher:

Key points

• Pinout (DIP-8): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control Voltage, 6 Threshold, 7 Discharge, 8 Vcc.
• Modes:
  • Monostable: One-shot pulse when Trigger goes low. Output stays high for a time t ≈ 1.1 × R × C.
  • Astable: Free-running oscillator with no trigger input. Output pulses high/low continuously.
• Notable equations
  • Astable
    • t_high = 0.693 × (R_A + R_B) × C
    • t_low = 0.693 × R_B × C
    • Frequency f = 1 / [0.693 × (R_A + 2 × R_B) × C]
    • Duty cycle = (R_A + R_B) / (R_A + 2 × R_B)
  • Monostable
    • t ≈ 1.1 × R × C (R is the timing resistor, C the timing capacitor)
• Common use cases: LED blink/pulses, PWM, tone generation, timers, debouncing, one-shot pulses.

Quick example

• Astable: R_A = 1 kΩ, R_B = 10 kΩ, C = 100 nF
  • f ≈ 1 / [0.693 × (1k + 2×10k) × 100 nF] ≈ 686 Hz
  • Duty ≈ (1k + 10k) / (1k + 2×10k) ≈ 11k / 21k ≈ 52%
• Monostable: R = 10 kΩ, C = 100 nF
  • t ≈ 1.1 × 10k × 100 nF = 1.1 ms

If you meant something else by 555, tell me which context you want (math, Roman numerals, or another topic), and I’ll tailor it. If you’d like, I can give you a couple of practice problems or walk through a specific circuit you’re analyzing.