# How to Design and Build a Simple Op Amp Monostable Multivibrator Circuit

An operational amplifier or operational amplifier is one of the most commonly used components in any electronic design. It is a versatile device that can be used in a variety of applications. We also covered basic op amp circuits such as summing amplifiers, difference amplifiers, instrumentation amplifiers, voltage followers, op amp integrators, and more. In this tutorial, we will create an op amp based monostable multivibrator circuit with all its calculations and tests. So let's get started right away.

What's Monostable Multivibrator

A monostable multivibrator is also called a single short multivibrator, it has only one stable state and the other state is aquasi-stable state. In the former design, we took the occasion to make a NE555 grounded monostable multivibrator circuit. You can check that out if you're looking for commodity like that. By stable state in a multivibrator, we mean that at that moment, the affair is high or low. Now, when a driving palpitation is applied, the multivibrator changes its affair from a stable state to aquasi-stable state. And after a certain time' T', which is determined by the circuit factors, the multivibrator returns to its original stable state automatically. In other words, no external triggering signal is needed to induce this rear transition. The circuit remains in this state until another driving palpitation is applied. These multivibrators are also called one- shot, single cycle, single- step, or unified multivibrator. Time ‘ T ’ after which the circuit returns to its original state is called gate- range and hence the multivibrator is also known as the gating circuit or detention circuit. This circuit is used to produce the beats of variable range at the required moment. The image below will give you a better idea about the content.

Monostable Multivibrator with Op-amp - Working

In order to fully understand the working principle of the monostable multivibrator, you first need to understand the working principle of the Schmitt trigger circuit, and in a previous post on the Astable multivibrator circuit, we have discussed the topic of Schmitt trigger in extreme details. You should check that out if you do not know about a Schmitt trigger circuit.
If you have read the article about an astable multivibrator circuit, you already know about it. As you can see by slightly modifying an astable multivibrator circuit, or by just putting a diode in place, we have converted the astable circuit into a monostable circuit. And as you can see in the above image, a diode is connected in parallel to the capacitor and the triggering signal is applied at that node where R1 and R2 get connected.
So, let’s understand the working principle of this circuit. Let's consider the output is at positive saturation voltage or the output of the op-amp is positive, then the voltage at node A will be (R2/(R1+R2)) * Vsat, and whenever the output is at positive saturation voltage, the diode D1 will become forward biased, and the voltage across the capacitor C1 will be the forward voltage drop across the diode.
So, at the inverting node, the voltage will be equal to the forward voltage drop across the diode. For this scenario, as the output voltage is greater than the inverting node, the output of the op-amp will be at the positive saturation voltage. Now we will apply a trigger pulse at node A, and whenever this negative trigger signal has applied, the output at the non-inverting node will be less than the voltage at the inverting node. And the output of the op-amp will switch from the positive saturation voltage to the negative saturation voltage. And the circuit will go into the quasi-stable state. Now the voltage at the non-inverting node will be equal to -XVsat.
As the output voltage is equal to the negative saturation voltage, the diode will become reversed biased and the capacitor will start charging towards the negative saturation voltage. Now whenever the voltage at the inverting node goes below the voltage -XVsat, then once again the output of the op-amp will become positive saturation voltage because, at that time, the non-inverting node will be slightly less than the inverting node. So, the output will switch from negative saturation voltage to positive saturation voltage. And for this time T, only the output will stay on the quasi-stable as soon as the output reaches the positive saturation voltage, the diode will become forward biased and this cycle will continue. So this is the basic working principle of the monostable multivibrator. Now as we have understood the working principle, we can now move onto calculating the time duration of the pulse.

Op amp monostable multivibrator circuit diagram
The complete schematic of the op amp based monostable multivibrator circuit is given below.

That concludes the project, a snapshot of the output is shown below.