What is Schmitt Trigger | How It Works

In this tutorial we will learn what is Schmitt Trigger and how it works. You can watch the following video or read the written article below.

Overview

The Schmitt Trigger is a logic input type that provides hysteresis or two different threshold voltage levels for rising and falling edge. This is useful because it can avoid the errors when we have noisy input signals from which we want to get square wave signals.

So for example, if we have a noisy input signal like this, that is meant to have 2 pulses, a device that has only one set point, or threshold, could get incorrect input and it could register more than two pulses as shown in this illustration. And if we use the Schmitt Trigger for the same input signal we will get a correct input of two pulses because of the two different thresholds. So that’s the primal function of the Schmitt Trigger, to convert noisy square waves, sine waves or slow edges inputs into clean square waves.

Types of Schmitt Triggers

There are many logic ICs that have built-in Schmitt Triggers on their inputs, but also it can be built using transistors or easier using an Operational Amplifier, or comparator and just adding some resistors to it and a positive feedback.

Operational Amplifier based Schmitt Trigger

Here we have an op-amp which inverting input is connected to the ground or zero volts and the non-inverting input is connected to a voltage input, V_IN. So this is actually a comparator and compares the non-inverting input to the inverting input or in this case the input voltage V_IN to 0 V. So when the V_IN value is below 0 volts the output of the comparator will be the negative V_CC and if the input voltage is above 0 volts the output will be positive V_CC.

Now if we add a positive feedback by connecting the output voltage to the non-inverting input with a resistor between them and another resistor between the V_IN and the non-inverting input we will get the Schmitt Trigger. Now the output will switch from V_CC– to V_CC+ when the voltage at the A node will cross 0 volts.

That means that now by adjusting the values of the resistors we can set at what value of the V_IN input the switch will occur using the following equations. We get these equations with the following relationships. The current “i” through this line equals V_IN – V_A divided by R₁ as well as V_A – V_OUT divided by R₂. So if we replace the V_A with zero, as we need that value for the switch to occur, we will get that final equation. For example if the output is -12 volts and the V_IN input is negative and rises, the switch from -12 V to +12 V will occur at 6 volts according to the equation and the values of the resistors and vice versa when the V_IN input is high and declines the switch from +12 V to – 12V will occur at -6 volts.

Non-Symmetrical Schmitt Trigger

In order to get two different non-symmetrical thresholds, we can use this circuit of an inverting single powered Schmitt Trigger. Here the V_REF voltage is the same as the V_CC of the op-amp. Now because the V_IN input is connected to the inverting input of the op-amp when its values will reach the upper threshold, the output will switch off to 0 volts, and then when its values will decline to the lower threshold, the output will switch on to 5 volts.

Here’s an example of how we can calculate the thresholds. The V_REF and the V_CC will be 5 volts and the three resistors will be the same 10k ohms. So what we need to calculate now is the voltage at the A node. In the first case when the output is 0 V our circuit will look like this, a simple voltage divider and the value of the V_A will be 1.66 V. This means that the V_IN input needs to decline below that value in order the output to switch on to 5 volts. Now with this 5 volts at the output the circuit will look like this. The value of the V_A will be 3.33 V. This means that the V_IN input needs to rise above that value in order the output to switch off to 0 volts.

Transistor based Schmitt Trigger

We can also build a Schmitt Trigger using transistors. You can learn how the Transistor Schmitt Trigger works in my next tutorial. Click here to visit it.