555

Do you mean the 555 timer IC? If so, here’s a quick refresher. If you meant something else (e.g., a number problem), tell me and I’ll tailor it.

555 timer (NE555) quick reference

• Pinout (8-pin DIP):
  • 1 GND
  • 2 Trigger
  • 3 Output
  • 4 Reset (active low)
  • 5 Control Voltage (often 0.01 uF to ground)
  • 6 Threshold
  • 7 Discharge
  • 8 Vcc
• How it works (internal): two comparators feed an SR latch; threshold and trigger levels are 2/3 Vcc and 1/3 Vcc, respectively.
• Modes:
  • Astable (free-running): no external trigger; generates a square wave.
  • Monostable (one-shot): requires a trigger to produce a single timed pulse.
• Key formulas:
  • Astable
    • t_high = 0.693 (RA + RB) C
    • t_low = 0.693 RB C
    • f = 1 / [0.693 C (RA + 2 RB)]
    • Duty cycle D = (RA + RB) / (RA + 2 RB)
  • Monostable
    • Pulse width T = 1.1 R C
• Practical notes
  • Reset pin (4) should be tied to Vcc if not used.
  • Control voltage (5) often bypassed with a 0.01–0.1 µF cap to ground to prevent noise.
  • Use a load or decoupling cap on Vcc (typical 0.1 µF near the IC) for stability.
  • The discharge transistor on pin 7 pulls the timing node to ground during the low part of the cycle.
• Typical applications
  • Timers, oscillators, PWM, debouncing, pulse-width control, waveform generation.

Would you like a worked example with specific component values, or adapt this to your course (e.g., electronics, EE, or digital systems)? If you have a circuit/problem, share the values and I’ll solve it.