555

Do you mean the 555 timer IC, or something else with the number 555 (e.g., a math problem)? If you want the 555 timer, here’s a quick refresher you can use for exams. Tell me if you’d like a worked example.

Quick 555 timer cheat sheet

• Pins and basics

  • 1 GND | 2 Trigger | 3 Output | 4 Reset | 5 Control Voltage | 6 Threshold | 7 Discharge | 8 Vcc
  • Reset (pin 4) must be high for normal operation.
  • Control voltage (pin 5) defaults to 2/3 Vcc; a small capacitor (e.g., 0.01 µF) to ground is often added.

• Modes

  • Astable (oscillator): continuously cycles between high and low without any external trigger.
  • Monostable (one-shot): a single output pulse is produced in response to a trigger.

• Key formulas

  • Astable mode

    • f (frequency) = 1.44 / ((RA + 2 RB) C)
    • T_high (output high duration) = 0.693 (RA + RB) C
    • T_low (output low duration) = 0.693 RB C
    • Duty cycle = T_high / (T_high + T_low) = (RA + RB) / (RA + 2 RB)
    • Components: RA is the resistor between Vcc and pin 7, RB is between pin 7 and the capacitor node, C is the timing capacitor to ground.

  • Monostable mode

    • Pulse width T = 1.1 R C
    • Typical wiring: a resistor R between Vcc and the timing capacitor node, capacitor C from that node to ground, and the node connected to Trigger/Threshold as appropriate. Reset is tied high.
    • Trigger (pin 2) goes low to start the timing interval; output goes high for duration T.

• Common values and limits

  • Supply voltage typically 4.5V to 15V (check specific part variant).
  • Output can source/sink tens to a couple hundred milliamperes depending on supply and load.
  • Control voltage pin 5 is optional; adding a small decoupling cap helps stability.

If you want, I can:

• Walk through a couple of worked examples with chosen resistor values and calculate f, T, and duty cycle.
• Create a quick practice problem set for exam prep. What would you like next?