555

Do you mean:

• the 555 timer IC (electronic circuits), or
• the number 555 in math/number theory, or
• something else?

If you’re prepping for exams and it’s the 555 timer IC, here’s a quick cheat-sheet:

• Pinout (common 8-pin DIP): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control (often with a 10 nF to GND), 6 Threshold, 7 Discharge, 8 Vcc.

• Astable mode (produces a square wave):

  • t_high = 0.693 (R1 + R2) C
  • t_low = 0.693 R2 C
  • f ≈ 1.44 / [(R1 + 2R2) C]
  • Duty cycle = (R1 + R2) / (R1 + 2R2)

• Monostable mode (one-shot, triggered by a negative pulse on Trigger):

  • t ≈ 1.1 R C
  • R is the resistor from Vcc to Discharge (pin 7), C is the capacitor from Threshold/Trig (pins 6/2) to GND
  • Example: R = 10 kΩ, C = 100 μF → t ≈ 1.1 s

• Tips:

  • Use a small decoupling cap (e.g., 0.1 μF) close to Vcc and GND.
  • Pin 5 (Control) can be left open or tied to a capacitor (e.g., 10 nF) to reduce noise.
  • You can add a diode in series with R2 in astable to adjust charge/discharge paths if you need asymmetric duty cycles.

If you meant something else by 555 (like a math problem about the number 555, e.g., factorization 555 = 3 × 5 × 37), tell me which topic and I’ll tailor it.