555

Do you mean one of these, or something else with “555”?

• 555 timer IC (electronics): quick refresher

  • Pinout (8-pin): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 VCC
  • Astable (free-running): frequency f ≈ 1.44 / [(RA + 2RB) C], duty cycle ≈ (RA + RB) / (RA + 2RB)
  • Monostable (one-shot): pulse width t ≈ 1.1 × R × C
  • Quick example: RA = 1 kΩ, RB = 9 kΩ, C = 100 nF
    • f ≈ 1.44 / [(1k + 2×9k) × 100 nF] = 1.44 / (19k × 100 nF) ≈ 758 Hz
    • Duty ≈ (1k + 9k) / (1k + 18k) ≈ 10k / 19k ≈ 52.6%

• Math/number 555: quick notes

  • 555 = 3 × 5 × 37
  • In decimal: digits sum = 15
  • Last digit is 5, so divisible by 5; divisible by 3 because digits sum to a multiple of 3

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