This LED will glow when we press PUSH button Switch and automatically switch OFF after T second. Here is the practical demonstration of the Monostable mode of 555 timer IC, where we have connected a LED to the output of the 555 IC. Remember one thing, while designing this circuit, that Trigger pulse at PIN 2 must be shorter enough to the OUPUT pulse, so that the capacitor gets enough time to charge and discharge. Quasi stable state only remains for 1.1*R1*C1 seconds and then it automatically switch back to stable state. So now we can see that MONOSTABLE mode has only one stable state and require a negative pulse at PIN 2, for the transition to Quasi stable state. T= 1.1*R1*C1 Seconds where R1 is in OHM and C1 is in Farads. This duration of quasi stable state is given by the below formulae: So 555 IC automatically fall back to the stable state (LOW) after the time determined by the RC network. And secondly, Transistor Q2 becomes ON, and capacitor starts discharging to the ground, through the Discharge PIN 7.Firstly, Upper comparator becomes HIGH and Flip flop gets Resets and the OUTPUT of the chip at PIN 3 becomes LOW.Now when capacitor starts charging and reaches to the voltage slightly greater than 2/3 Vcc, voltage at Threshold PIN 6 becomes greater than the voltage at inverting end (2/3Vcc) of Upper comparator, again two things happens: This state is called the quasi stable state and remains for some time (T). And second thing is, Transistor Q1 becomes OFF, and Timing capacitor C1 get disconnected from the Ground and start charging though the Resistor R1.First is, Lower comparator becomes HIGH and Flip flop gets Set and we get HIGH OUTPUT at PIN 3.
![use tank circuit as timer use tank circuit as timer](https://i.pinimg.com/originals/e6/98/9b/e6989bac4b94b99914f074389fcb7346.jpg)
We know that Non-inverting end of Lower Comparator is at 1/3Vcc, so when we apply negative (< 1/3Vcc) voltage to the Trigger PIN 2 by connecting it to Ground (through a PUSH button switch), two things happens: Operation is simple, initially 555 is in stable state i.e. Operation of Monostable Multivibrator mode of 555 timer IC: Power or VCC: It is connected to positive voltage (+3.6v to +15v). Whenever the output goes low or the flip-flop gets reset, the discharge pin is pulled to ground. DISCHARGE: This pin is drawn from the open collector of transistor. Since the transistor (on which discharge pin got taken, Q1) got its base connected to Qbar. Then a voltage equal to or greater than VCC*(2/3) (i.e.6V for a 9V supply) will reset the flip-flop. The threshold pin is drawn from positive input of comparator1. THRESHOLD: Threshold pin voltage determines when to reset the flip-flop in the timer. Normally this pin is pulled down with a capacitor (0.01uF), to avoid unwanted noise interference with the working. Control Pin: The control pin is connected from the negative input pin of comparator one. This pin is connected to VCC for the flip-flop to stop from hard resetting. Reset pin is directly connected to MR (Master Reset) of the flip-flop. Reset: There is a flip-flop in the timer chip. This is output pin where Load is connected. OUTPUT: This pin also has no special function. If this pin is connected to ground (or less than Vcc/3), the output will be always high. With the comparator two output high we get high voltage at the timer output. The comparator two output is connected to SET pin of flip-flop.
![use tank circuit as timer use tank circuit as timer](https://1.bp.blogspot.com/-LJkRqL_mI18/XTA8C6-JHmI/AAAAAAAABRw/iaQlhzslL3w310UDArs4aJXukoq6c6rSwCLcBGAs/w1200-h630-p-k-no-nu/555-based-water-level-controller-watermark.png)
TRIGGER: Trigger pin is dragged from the negative input of comparator two. Ground: This pin should be connected to ground. Before going through below, you should know about 555 timer IC and its PINs, here is the short description about its PINs. This external triggering is given by connecting the Trigger PIN 2 to the Ground using a PUSH button. 555 IC automatically switches back to stable state after some time, this time, for which the 555 stays in quasi stable state, is determined by the time constant of RC network in the circuit. An external triggering is required for transition from Stable to unstable state. 555 timer IC remains in Stable state until the external triggering is applied. As the name indicates, only one state is stable and the other one is called unstable or quasi stable state.
![use tank circuit as timer use tank circuit as timer](http://www.talkingelectronics.com/projects/SpotMistakes/images/Page18/TankCircuit.gif)
Monostable multivibrator (MMV) mode of 555 timer IC is also called Single shot mode.