555

Could you clarify what you want with “555”? Do you mean:

• 555 timer IC basics (monostable/astable operation), or equations for timing
• a math problem involving the number 555
• something else

If you’re studying electronics for exams, here’s a quick refresher you can start with:

• Pinout (common 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 when triggered. Timing t ≈ 1.1 × R × C
  • Astable: free-running oscillator
    • THigh = 0.693 × (R_A + R_B) × C
    • TLow = 0.693 × R_B × C
    • Period T = THigh + TLow = 0.693 × (R_A + 2R_B) × C
    • Frequency f = 1 / T = 1 / [0.693 × (R_A + 2R_B) × C]
    • Duty cycle D = THigh / T = (R_A + R_B) / (R_A + 2R_B)

If you’d like, I can generate a short practice set (with solutions) tailored to your course. Tell me your topic and level.