555 Timer Monostable Mode

Push a button and a camera flash fires for exactly 5 milliseconds. Set a delay before a relay trips in an industrial timer. Both use the same circuit: the 555 timer in monostable mode, which produces a single output pulse of precise, programmable duration. The NE555 IC is one of the most widely used analog ICs ever made.

Core Concept

The NE555 has two internal comparators. The upper comparator monitors pin 6 (threshold) and compares it to 2/3 VCC. The lower comparator monitors pin 2 (trigger) and compares it to 1/3 VCC. An internal SR flip-flop stores the output state. In the stable resting state, pin 2 is held above 1/3 VCC, and the discharge transistor holds pin 7 low, keeping the capacitor discharged.

When the trigger pin (pin 2) is pulled below 1/3 VCC by a negative pulse, the lower comparator sets the flip-flop. The output (pin 3) goes HIGH and the discharge transistor turns off, releasing pin 7. The timing capacitor then begins to charge through R toward VCC. The output stays HIGH throughout this charging period.

When the capacitor voltage reaches 2/3 VCC, the upper comparator resets the flip-flop. The output returns to LOW, the discharge transistor turns on, and the capacitor discharges rapidly back through pin 7 to ground. The circuit returns to its stable state, waiting for the next trigger. This entire sequence produces exactly one pulse of duration T = 1.1RC, regardless of how long the trigger was held low, as long as the trigger pulse is shorter than T.

Key Equations

Pulse width (output HIGH duration):

T = 1.1 × R × C where R is in ohms and C is in farads. This formula comes from the time for C to charge from 0V to 2/3 VCC through R.

Derivation (for reference):

T = -RC × ln(1 - 2/3) = RC × ln(3) ≈ 1.0986 × RC ≈ 1.1 RC

Threshold comparator reference: 2VCC/3. Trigger comparator reference: VCC/3. Both are set by an internal voltage divider of three equal resistors (5 kΩ each), giving the IC its name: 555.

Given:
  NE555 supply VCC = 9V
  R = 100 kΩ = 100 × 10^3 Ω
  C = 10 µF = 10 × 10^-6 F
  Required: pulse width T

Why this formula:
  T = 1.1 × R × C is the standard NE555 monostable formula.
  It comes from the capacitor charging from 0 to (2/3)VCC through R.

Formula:
  T = 1.1 × R × C

Substitution:
  T = 1.1 × (100 × 10^3) × (10 × 10^-6)

Calculation:
  T = 1.1 × 10^5 × 10^-5
  T = 1.1 × 10^0
  T = 1.1 seconds

Verify with exact formula:
  T = RC × ln(3) = (100 × 10^3 × 10 × 10^-6) × 1.0986
  T = 1 × 1.0986 = 1.0986 s ≈ 1.1 s (confirmed)

Final Answer:
  Output pulse duration T = 1.1 seconds

Exam Tip: GATE questions on the 555 monostable almost always require you to recall T = 1.1RC without derivation. Know that the factor 1.1 comes from ln(3) ≈ 1.0986. A second trap: the trigger must be a negative-going pulse below VCC/3. If the trigger stays LOW longer than T, the output does not extend beyond T in a standard monostable. It resets at 2VCC/3 regardless of trigger state. Also remember pin assignments: pin 2 = trigger, pin 6 = threshold, pin 7 = discharge, pin 3 = output.

Key Properties

• The output pulse width T = 1.1RC is independent of supply voltage VCC. This is because the threshold (2VCC/3) and trigger level (VCC/3) both scale with VCC.
• Supply voltage range for NE555 is 4.5V to 16V. The CMOS version (TLC555) works from 2V to 15V with much lower quiescent current.
• The trigger pulse must drop below VCC/3 to initiate the timing cycle. A brief pulse of even a few microseconds is enough.
• Pin 5 (control voltage) can shift the threshold from 2VCC/3 to any desired value. A 0.01 µF capacitor to ground on pin 5 filters noise from the internal reference.
• Pin 4 (reset) is active LOW. Holding pin 4 low at any time immediately terminates the output pulse and resets the circuit.
• The timing range is from a few microseconds (using small R and C) to several hundred seconds (using large R and C, up to 10 MΩ and 1000 µF).
• Internal three-resistor divider (each 5 kΩ) sets both comparator references. This gives the 555 its name.

Quick Revision

• Monostable has one stable state (output LOW). Trigger produces one pulse, then returns to stable.
• Pulse width T = 1.1 × R × C. Derived from T = RC × ln(3).
• Trigger: pin 2 must go below VCC/3. Output HIGH starts immediately.
• Capacitor charges through R to 2VCC/3 at pin 6. At this point, output goes LOW.
• Pin 7 (discharge) shorts the capacitor to GND when timing cycle ends.
• T is independent of VCC because threshold and trigger levels are both VCC fractions.
• Reset (pin 4) is active LOW. Always tie to VCC unless intentional reset is needed.
• Exam trap: Using T = RC (without the 1.1 factor) and getting a 10% error. The correct factor is 1.1 (or exactly ln3 ≈ 1.0986). This factor comes from the capacitor charging to 2/3 of VCC, not to full VCC.