• Everyone:
• Insides
• Monostable
• Astable
• Motor Speed
• PWM Dimmer
• Short Pulses
•  

Everyone

Data Sheet

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.

Falstad Simulation

For the Falstad Circuit Simulation, CTRL+Click 555 Monostable
In options, check European Resistors and uncheck Conventional Current.
Click the logic input low and high again to trigger the monostable.

Alternatively view 555_Monostable.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

reviseOmatic V3     Contacts, ©, Cookies, Data Protection and Disclaimers Hosted at linode.com, London