⏲️ 555 Timer Interactive Simulator

Explore how the 555 timer IC works internally

555 Timer Pin Configuration

  • Pin 1: Ground (GND)
  • Pin 2: Trigger
  • Pin 3: Output
  • Pin 4: Reset
555
1 2 3 4 8 7 6 5
  • Pin 8: Vcc (Power Supply)
  • Pin 7: Discharge
  • Pin 6: Threshold
  • Pin 5: Control Voltage
Pin Functions:
  • Trigger (Pin 2): Starts the timer when voltage drops below 1/3 Vcc
  • Threshold (Pin 6): Resets the timer when voltage exceeds 2/3 Vcc
  • Output (Pin 3): Provides the timer output signal (HIGH or LOW)
  • Discharge (Pin 7): Discharges the timing capacitor
  • Control Voltage (Pin 5): Allows manual control of threshold voltage
  • Reset (Pin 4): Forces output LOW when grounded

Internal Schematic

Voltage Divider (3 × 5kΩ) Vcc 5kΩ 2/3 Vcc 5kΩ 1/3 Vcc 5kΩ GND Comparator 1 + - Pin 6 (Threshold) Reset Comparator 2 + - Pin 2 (Trigger) Set SR Flip-Flop R S Q Q̄ 1 Pin 3 (Output) T GND Pin 7 (Discharge)
How it works:

The voltage divider creates two reference voltages: 2/3 Vcc (for Comparator 1) and 1/3 Vcc (for Comparator 2). These comparators control the SR flip-flop, which determines the output state and the discharge transistor.

Comparator Operation

9.0 V
4.0 V
4.0 V

Comparator 1 (Threshold)

Positive Input: 4.0 V (Pin 6)

Negative Input: 6.0 V (2/3 Vcc)

Output: LOW

Outputs HIGH when Pin 6 > 2/3 Vcc
This resets the flip-flop

Comparator 2 (Trigger)

Positive Input: 3.0 V (1/3 Vcc)

Negative Input: 4.0 V (Pin 2)

Output: LOW

Outputs HIGH when Pin 2 < 1/3 Vcc
This sets the flip-flop

Comparator Rules:
  • If positive input > negative input → Output is HIGH
  • If positive input < negative input → Output is LOW
  • Comparator 1 triggers reset when threshold exceeds 2/3 Vcc
  • Comparator 2 triggers set when trigger drops below 1/3 Vcc

SR Flip-Flop Operation

LOW
LOW
SR Flip-Flop
LOW
HIGH
Truth Table:
S (Set) R (Reset) Q Q̄ State
0 0 No Change No Change Memory
1 0 1 0 Set
0 1 0 1 Reset
1 1 Invalid Invalid ⚠️ Forbidden

Warning: Both S and R should never be HIGH at the same time!

Pin 7 Discharge Operation

555 Output State

Click to toggle output state

Discharge Transistor

OFF

When Output = HIGH → Transistor is OFF
When Output = LOW → Transistor is ON

Capacitor

Discharged (0%)

How Pin 7 Works:
  • Pin 7 is connected to a transistor controlled by the flip-flop output
  • When Output = HIGH: Transistor is OFF, capacitor can charge
  • When Output = LOW: Transistor is ON, connects capacitor to ground, allowing discharge
  • This mechanism controls the timing interval of the 555 timer