555

Do you mean the 555 timer IC, or something else with the number 555 (math, code, etc.)? If you’re prepping for exams, here’s a quick 555 timer refresher you can use right away.

If you mean the 555 timer IC:

• Pinout (DIP): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 Vcc
• Modes:
  • Astable (oscillator, free-running): no external trigger needed; makes a square wave.
  • Monostable (one-shot): produces a single pulse when triggered.
• Key formulas (standard astable with R1 between Vcc and Discharge (pin 7), R2 between Discharge and Capacitor (pin 2/6), C from Capacitor to GND):
  • t_high = 0.693 (R1 + R2) C
  • t_low = 0.693 R2 C
  • f = 1 / [0.693 (R1 + 2 R2) C]
  • Duty cycle D = (R1 + R2) / (R1 + 2 R2)
• Quick example (typical values):
  • Choose C = 10 nF, R1 = 50 kΩ, R2 = 50 kΩ
  • Then f ≈ 1 / [0.693 (50k + 2×50k) × 10nF] ≈ 1 kHz
  • D ≈ (50k + 50k) / (50k + 2×50k) ≈ 2/3 ≈ 66.7%
  • Note: to get 50% duty in a standard astable you’d use diodes or a different configuration; with R1 = 0 you’d get close to 50% but practical designs use a diode network or a 556/other arrangement.

If you meant something else (e.g., math 555, or a different context), tell me and I’ll tailor it.

Would you like a worked problem or a concrete design for a specific frequency or duty cycle?