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555 Data Sheet from Texas Instruments        555 Monostable in the Lushprojects Simulator - Momentarily click the trigger or reset switches.

The 555 Monostable output is a single accurately timed pulse. There is one stable state.

Key Facts

• When triggered, the monostable produces a single accurately timed pulse. The output goes high for the timed period and then goes low again.
• The monostable has one stable state and one unstable state.
• In the circuit below, the 10k pull-up resistor pulls pin 2 high.
• Pressing the switch sends a LOW trigger pulse to pin 2.
• This turns on the output for a time T = 1.1 R C
• The duration of the output pulse is called its period.
• The reset switch can be pressed to end the timing earlier than normal.

How it Works - Exam Answer

• The timer starts when the trigger voltage drops below Vs / 3.
• The output goes high and the discharge transistor turns off. Trigger pulses are ignored.
• C charges through R towards Vs.
• When the voltage across C reaches 2/3 of Vs, the timer resets. This takes 1.1 R C Seconds.
• The output goes low.
• The discharge transistor turns on and quickly discharges C to zero volts.

Pulse Time

The square wave output is high for the following period ...

T = 1.1 R C

Timing Diagram

• The trigger pulse (a) turns on the output for a fixed time.
• Trigger pulse (b) turns the output on again.
• Trigger pulse (c) is IGNORED because the output is already on.

Something to Build

This produces a 1.1 second pulse when triggered. The LED should come on for 1.1 seconds. The black wire on the left is an improvised switch.