555

Do you mean the 555 timer IC? If so, here’s a quick refresher you can use for exam prep.

Key modes

• Monostable (one-shot): Trigger causes the output to go high for a time t = 1.1 × R × C. After that, it returns low. You set R and C externally.
• Astable (free-running): No trigger needed. The output continuously pulses high/low. The timing:
  • TH (high time) = 0.693 × (RA + RB) × C
  • TL (low time) = 0.693 × RB × C
  • Period T = TH + TL = 0.693 × (RA + 2RB) × C
  • Frequency f = 1 / T
  • Duty cycle D = TH / T = (RA + RB) / (RA + 2RB)

Important notes

• Pinout (typical 8-pin):
  • 1: GND
  • 2: Trigger
  • 3: Output
  • 4: Reset
  • 5: Control Voltage
  • 6: Threshold
  • 7: Discharge
  • 8: Vcc
• Control voltage on pin 5 can be decoupled with a small cap (e.g., 0.01 µF) to stabilize the reference.

Example

• Astable, RA = 1 kΩ, RB = 10 kΩ, C = 100 nF:
  • TH = 0.693 × (1k + 10k) × 100 nF ≈ 0.000762 s
  • TL = 0.693 × 10k × 100 nF ≈ 0.000693 s
  • T ≈ 0.001455 s, f ≈ 689 Hz
  • D ≈ 0.762 ms / 1.455 ms ≈ 52%

If you meant something else by “555” (e.g., a math problem, a countdown, or something in a course), tell me and I’ll tailor it. Want me to generate a few practice problems or a quick cheat sheet for the 555 timer?