555

Do you want help with the 555 timer IC (electronics), or did you mean something else by “555”?

If you’re studying the 555 timer, here’s a quick refresher:

Monostable (one-shot)

• Trigger: pin 2 goes below 1/3 Vcc
• Output: goes high for t = 1.1 × R × C
• R and C: external resistor from Vcc to discharge pin (pin 7) and timing capacitor from threshold/trigger (pins 6/2) to ground

Astable (oscillator)

• No trigger; continuous pulses
• t_high = 0.693 × (R_A + R_B) × C
• t_low = 0.693 × R_B × C
• f = 1 / (t_high + t_low) = 1.44 / ((R_A + 2R_B) × C)
• Duty cycle D = t_high / (t_high + t_low) = (R_A + R_B) / (R_A + 2R_B)

If you want, tell me a concrete problem (values for R and C, desired frequency, etc.) and I’ll work through it step by step.