555

Do you mean the 555 timer IC, or something else with the number 555?

If you want the 555 timer refresher, I can give quick notes and practice problems. For example:

• Pinout (1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 VCC)
• Modes:
  • Monostable: t ≈ 1.1 × R × C
  • Astable: f ≈ 1 / [0.693 × (R_A + 2R_B) × C], TH ≈ 0.693 × (R_A + R_B) × C, TL ≈ 0.693 × R_B × C
• Typical applications: LED flasher, tone generator, simple PWM

Would you like a short problem set, or a worked example (e.g., choose RA, RB, C to get a target frequency or pulse width)? Or if you meant a math problem involving the number 555, tell me and I’ll tailor accordingly.