Simple 555 Timer Monostable Circuit


Rating: 0.00 / 5 
Monostable Circuits / Oscillator Circuits
The two circuits below illustrate using the 555 timer to close a relay for a predetermined amount of time by pressing a momentary N/O push button. The circuit on the left can be used for long time periods where the push button can be pressed and released before the end of the timing period. For shorter periods, a capacitor can be used to isolate the switch so that only the initial switch closure is seen by the timer input and the switch can remain closed for an unlimited period without effecting the output.

Simple 555 Timer Monostable Circuit - img1

In the idle state, the output at pin 3 will be at ground and the relay deactivated. The trigger input (pin 2) is held high by the 100K resistor and both capacitors are discharged. When the button is closed, the 0.1uF cap will charge through the button and the 100K resistor which causes the voltage at pin 2 to move low for a few milliseconds. The falling voltage at pin 2 triggers the 555 and starts the timing cycle. The output at pin 3 immediately moves up to near the supply voltage (about 10.4 volts for a 12 volt supply) and remains at that level until the 22 uF timing capacitor charges to about 2/3 of the supply voltage (about 1 second as shown). Most 12 volt relays will operate at 10.4 volts, if not, the supply voltage could be raised to 13.5 or so to compensate. The 555 output will supply up to 200mA of current, so the relay could be replaced with a small lamp, doorbell, or other load that requires less than 200mA. When the button is released, the 0.1uF capacitor discharges through the 100K and 2K resistors. The diode across the 100K resistor prevents the voltage at pin 2 from rising above the supply voltage when the cap discharges. The 2K resistor in series with the 22uF cap limits the discharge current from pin 7 of the timer. This resistor may not be necessary, but it's a good idea to limit current when discharging capacitors across switch contacts or transistors.


  • Hackaday
    Hackaday You could use another 555 astable (using the same as the clock but changing the values). But a common choice is to use an astable and 555 as an Atari punk console (we have a detailed article on the punk console which ...
     14 January 2016
    Experimenting with an HF , I needed to control the varactor diode voltage in precise increments over a 2V-10V range. A buffered potentiometer was the obvious choice, and connecting two pots in series to give coarse/fine control (or using a
     30 November 2015
  • Hackaday
    Hackaday The 555 operation is almost identical to the astable , however the trigger input is not connected to the timing capacitor. This means the does not “self trigger”, requiring an input (in this case from the astable
     17 September 2015
  • PR Newswire (press release)
    NEW YORK, June 18, 2015 /PRNewswire/ -- This report analyzes the worldwide markets for Electronic Components in US$ Million by the following Product Segments: Resistors (Semi-Fixed Variable, Carbon Variable, Other Variable, Carbon Film Fixed, Me
     18 June 2015
  • IEEE Spectrum
    IEEE Spectrum Immediately following the Second World War, electrical engineers grappled with a fundamental but open question: How should electronic digital computers be built? What kind of switch would serve best for logic ? And what should b
     19 March 2014
  • EE Times
    EE Times One could also use the comparator to create a one-shot or mutiviator for a special application. Regular multiviators or programmable are another possibility, while voltage controlled and alignable Xtal (crystal
     4 December 2013
  • works on low supply voltagesThis sawtooth-, drawing less than 3.2 μA and working at under 1V, is a useful building block that fits the bill for extremely low-power consumption and operation to low supply voltages. It could be
     15 May 2013
  • EE Herald
    Linear has introduced the LTC6993, a tiny “one-shot” pulse generator, the latest member of the TimerBlox family of timing devices The devices one-shot function, also known as a , is used for accurate event timing
     10 September 2010
  • (subscription) (subscription) With no flame present, the flame detector is essentially an open so that any charge on C3 will stay until the next clock cycle of the astable 33ms later. The TPS65552A senses the output voltage by looking at the f
     17 January 2008


Leave Comment

characters left: