555

Do you mean the 555 timer IC? Here’s a quick exam-ready refresher plus a simple example. Tell me what you’d like next (practice problems, more derivations, or another topic).

Key ideas about the 555 timer (8-pin IC)

• Pinout (NE/µA series, 8-pin DIP): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control Voltage, 6 Threshold, 7 Discharge, 8 Vcc.

Modes

• Astable (free-running oscillator): no trigger required.

  • t_high = 0.693 (R_A + R_B) C
  • t_low = 0.693 R_B C
  • Frequency: f = 1 / (t_high + t_low) = 1.44 / ((R_A + 2 R_B) C)
  • Duty cycle D = t_high / (t_high + t_low) = (R_A + R_B) / (R_A + 2 R_B)

• Monostable (one-shot): output goes high for a fixed time when triggered.

  • Trigger input (pin 2) must go below 1/3 V_cc to start timing.
  • Timing interval: t = 1.1 R C
  • After t, output returns low when threshold reaches 2/3 V_cc.

• Bistable (latch): two stable states controlled by trigger and reset; no inherent timing.

Important values and notes

• Threshold and trigger levels are 2/3 V_cc and 1/3 V_cc, respectively.
• Control pin (pin 5) can be used to modify the threshold via an external capacitor (often 10 nF to ground recommended) to improve noise immunity.
• Power supply range: typically 4.5 V to 15 V (check your part variant).
• Decoupling: place a small capacitor (e.g., 0.1 µF) close to V_cc for stability.

Quick worked example

• Astable example: R_A = 10 kΩ, R_B = 30 kΩ, C = 0.01 µF (10 nF)
  • t_high = 0.693 (10k + 30k) × 10 nF = 0.693 × 40k × 10e-9 ≈ 0.000277 s
  • t_low = 0.693 × 30k × 10 nF ≈ 0.000208 s
  • f ≈ 1 / (0.000277 + 0.000208) ≈ 2.06 kHz
  • Duty ≈ 0.000277 / (0.000277 + 0.000208) ≈ 0.57 (57%)

Would you like:

• More practice problems (pick monostable or astable)?
• Derivations for a specific circuit?
• A quick cheat sheet without explanations, or a flashcard set for exam prep?