Simple 555 Timer Monostable Circuit

Rating: 1.00/10 (35Votes cast)

Category: Oscillator Circuits / Monostable Circuits
Views: 2351
Rank: 0

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.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. visit page.

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. http://circuitschematic.blogspot.com/2010/08/pushbutton-555-timer-monostable-circuit.html

Related circuits

Monostable photocell circuit

A photocell circuit provides automatic threshold adjustment. Monostable action prevents undesired retriggering of the output. With only one op amp IC, the circuit offers: Automatic adjustment of its trigger level to accommodate various light sources, changes in ambient light and misalignments; A...

Astable/Monostable oscillator using 555 IC

The 555 is a highly stable device for generating accurate time delays or oscillation. Aditional terminals are provided for triggering or resetting if desired. In the time delay (monostable) modeof operation the time is precisely controlled by one extrernal resistor and one capacitor. For stable...

Monostable-operation

In this mode, the timer functions as a one shot. The external capacitor is initially held discharged by a transistor internal to the timer. Applying a negative trigger pulse to pin 2 sets the flip-flop, driving the output high, and releasing the short circuit across the external capacitor....

Monostable oscillator

The output of a video amplifier is differentiated before being fed to a Schottky comparator. The propagation delay is reduced to typically 10ns. The output pulse width is set by the value of C, lOOpf giving a pulse of about 90ns duration.

Slow op amp makes fast multivibrator

A classic one-shot multivibrator using an op amp (Figure 1a) needs a relatively long-duration triggering pulse, even though the circuit is in theory a slope-triggered configuration. This limitation can be a problem when you want to use a "spare" op amp from a dual or quad package (LM324, LM358,...

Controller IC and one-shot form resonant controller

Resonant power supplies are popular because of high efficiency, low noise, and compactness. You can implement a resonant buck or boost converter using a single switch. The regulation of the output in such a converter derives from using a constant on or off time and a variable frequency.

Linear voltage ramp generator

In the monostable mode, the resistor can be replaced by aconstant current source to provide a linear ramp voltage

Monostable-multivibrator

The circuit illustrates the usefulness of the HA-5151 as a battery-powered monostable. In this circuit, the ratio is set to .632, which allows the time constant equation to be reduced to: T=R1*C1. D2 is used to force the output to a defined state by clamping the negative input at +0.6 V....

Articles & Projects

60 Second Timer - ElProCus

Now we shall analyse the working of the 555 timer IC2. IC2 is connected in monostable mode. In monostable mode, the circuit will provide a HIGH output only for a defined time period T2 after it is triggered which is defined by ...

Monostable Multivibrator using 555 Timer - electroSome

This is the circuit diagram of 555 Timer wired in Monostable mode. 8th pin and 1st pin of the 555 timer are used to given power Vcc and Ground respectively. 4th pin is the Reset pin of 555 Timer, which is active low so it is ...

IC 555 TIMER-MONOSTABLE OPERATION CIRCUIT (1) Electrical ...

IC 555 TIMER-MONOSTABLE OPERATION CIRCUIT,Ask Latest information,Abstract,Report,Presentation (pdf,doc,ppt),IC 555 TIMER-MONOSTABLE OPERATION CIRCUIT technology discussion,IC 555 ...

433mhz transmitter controlled by 12v timer - All About Circuits Forum

If you can find a timer that controls a 12V signal, you could use that signal to control power to the key fob or perhaps to act like the fob switch. There will likely be a 555 timer circuit (a one-shot, or monostable multivibrator) ...

555 Timer as Monostable Multivibrator | schematic knowledge

A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, ...

diagram circuit: 555 timer Mono stable one shot circuit

The two circuits beneath allegorize application the 555 timer to abutting a broadcast for a agreed bulk of time by acute a cursory N/O advance button. The ambit on the larboard can be acclimated for continued time periods ...

555 Timer as Astable Multi vibrator or Monostable - General Circuits