Schmitt Trigger: How it Works and Applications

Hey friends, welcome to the Kohiki.com ALL ABOUT ELECTRONICS. So, in this article, we are going to talk about the Schmitt Trigger. And we will understand what is this SchmittTrigger, what is the purpose of using this Schmitt Trigger in electrical and electronics systems, and how it can be designed.

Limitation of the comparator circuit

Now, in the last article of the comparator, we have seen that it can be used to compare the two voltage levels. But the problem with this comparator is that if the input signal is noisy, in that case, your output will be get affected. And you will not get the desired output.

So, let’s say we have one comparator and to this comparator, we have applied these input signals V1 and V2. Now, here assume that this signal V1 is the input signal and this signal V2 is the reference signal. And assume that these signals are ideal signals.

Schmitt Trigger How it Works and Applications

So, in that case, as V1 is greater than V2, so you should get a constant high output voltage. But if this signal V1 is noisy, in that case, it is possible that it can affect your output voltage. So, let’s say because of the noise, this V1signal looks like this. So, if you observe this signal V1, then you can notice over here that this signal V1 crosses this signal V2 at these two locations. So, for the time period for which this signalV1 is less than V2, for that duration you will get low voltage.

So, because of the noise, you will see the transition in the output voltage. Or we can say that output will be get affected because of the noise in this input signal. So, we can say that this comparator is not immune to the noise. And because of that, it can affect your output. Now, this kind of problem can be avoided by using the Schmitt Trigger.


What is Schmitt Trigger and how it works

 Now, this Schmitt Trigger is nothing but the comparator with the hysteresis. It means that this has two threshold voltages. One is the upper threshold voltage, which is for low to high transition. And second is the lower threshold voltage, which is for high to low transition. And this is the symbol of the Schmitt Trigger.

What is Schmitt Trigger and how it works

So, now let’s understand how this SchmittTrigger circuit works. So, here, let’s say the signal which is represented in red color is the input signal. And these two are the upper and the lower threshold voltage for the given Schmitt Trigger.

So, now let’s see how this Schmitt Trigger will respond to this given input signal. And here, let’s assume that the initial voltage across this is zero. So, as you can see over here, initially the input signal is zero, and gradually it is increasing. So, the output of this Schmitt Trigger will remain low till the point this input signal crosses this upper threshold voltage.

What is Schmitt Trigger and how it works

So, up to this point, your output signal will remain zero. And from that point onwards, your output signal will remain high. Now, if you observe over here, once this input signal crosses this upper threshold voltage, then after it deviates around this upper threshold voltage. But then also, your output signal will remain high. And it will go to the low voltage only when this input signal goes below this lower threshold voltage.

So, at this point, once again your output will become low. And it will remain low till the point the input signal crosses this upper threshold voltage. So, again at this point, your output will become high. And it will remain high after that point. So, as you can see over here, even if your input signal varies around this upper and the lower threshold voltage then also your output signal will not get changed. And it will remain constant.

So, we can say that this Schmitt Trigger provides noise immunity over this band. And the difference between this upper and the lower threshold voltage is known as the hysteresis voltage of the Schmitt Trigger. So, basically, this hysteresis voltage defines the noise immunity of the given Schmitt Trigger.


Hysteresis Curve of Inverting and Non-Inverting Schmitt Trigger

So, if you observe this example, there are four parameters that are required to define the Schmitt Trigger. The first two are high and the low output levels of the Schmitt Trigger and the remaining two are the upper and the lower threshold voltages, which will decide the triggering instance for the Schmitt trigger.

So, these four parameters define the characteristics of the Schmitt Triggers. And the characteristics of the Schmitt Triggergraphically can be represented by this transfer characteristic curve. So, here on the X-axis, we have input voltage and on the Y-axis we have the output voltage. So, now for the given example, let’s draw the transfer characteristic of the Schmitt Trigger.

Hysteresis Curve of Inverting and Non-Inverting Schmitt Trigger

So, if your input signal is less than the upper threshold voltage level of the Schmitt Trigger, in that case, your output will remain low. And as soon as it crosses this upper threshold voltage level then your output will become high. And if the input signal goes beyond this upper threshold voltage level then also your output will remain high. Now, suppose if your input signal starts reducing then also your output signal will remain high. And it will remain high till the point your input signal goes below this lower threshold voltage level.

Hysteresis Curve of Inverting and Non-Inverting Schmitt Trigger

So, as soon as your input signal goes below this lower threshold voltage then again your output will become low voltage. And if your input signal goes below this lower threshold voltage then also your output will remain low. So, this is how the Schmitt Trigger can be represented using these transfer characteristics. So, this transfer characteristic is also known as the hysteresis curve for the Schmitt Trigger.

So, now here if your output voltage levels and VL are equal and opposite in polarity, in that case, your transfer characteristics curve will be symmetric with respect to the Y-axis. And by providing the reference voltage as an input to this Schmitt Trigger, we can shift this curve either to the right or the left-hand side.

So, the Schmitt Trigger, which has this kind of transfer characteristic curve is known as the non-inverting Schmitt Trigger. Because here, once your input signal crosses this upper threshold voltage level then your output will become high and when your input signal is less than this lower threshold voltage, at that time your output will become low. So, we can say that this transfer characteristic curve is the curve for the non-inverting Schmitt Trigger. Similarly, we can have a hysteresis curve for the inverting Schmitt Trigger.


Transfer Characteristics Of inverting Schmitt Trigger

 So, in the case of this inverting Schmitt Trigger. whenever your input signal is less than the upper threshold voltage level at that time your output will be high. And once, it crosses the upper threshold voltage level, then your output will become low voltage. And if we go beyond this upper threshold voltage level, then also your output will remain low. Now, suppose this input signal starts reducing then also your output will remain low. And it will remain low till the point your input signal crosses the lower threshold voltage level.

Transfer Characteristics Of inverting Schmitt Trigger

So, once this input signal crosses this lower threshold voltage level, then once again your output will become high. So, this is the transfer characteristic curve of the inverting Schmitt Trigger. And this curve can be shifted either to the right or left side by providing the external reference voltage as an input to this inverting Schmitt Trigger.

So, this is the symbol of the inverting Schmitt trigger. So, if you observe over here, the hysteresis curve inside this triangle is the hysteresis curve for the inverting Schmitt Trigger. Or it can be even represented by adding the bubble at the notch of the non-inverting Schmitt Trigger.


Design of Inverting Schmitt Trigger (with Derivation)

So, now that is being said, now let’s see how we can design this inverting and the non-inverting Schmitt Triggers. So, there are many ICs are available which can be directly used as Schmitt Trigger. But this Schmitt Trigger can also be designed by using the op-amp and the comparators. Or even it can be designed by using transistors.

So, in this article, we will understand how this inverting and non-inverting Schmitt Trigger can be designed by using the op-amp. So, this is the example of inverting Schmitt Trigger which is being designed using the op-amp. So, here the input is applied to the inverting terminal and there is positive feedback from output to the input side. So, now let’s understand how this circuit will act as an inverting Schmitt Trigger.

So, here assume that the voltage at this non-inverting terminal is equal to V+. Now, for this circuit whenever your Vin is greater than V+, at that time your output voltage will become low voltage. And whenever your Vin is less than V+, at that time your output will be high.

So, first of all, let’s find the expression for this V+. And it can be found by applying the KCL at this node. So, applying KCL, we can write, V+ minus zero divided by R1, plus V+ minus V out divided by R2 that is equal to zero. That means the summation of these two currents is equal to zero. And if we simplify it then we will get the expression for the V+ as, R1 plus R1 divided by R1+R2 times this voltage Vout.

Design of Inverting Schmitt Trigger (with Derivation)

Now, here initially assume that the output of this Schmitt Trigger is equal to high voltage. That is VH. And in that condition, let’s assume the voltageV+ that is equal to V1. So, we can write this voltage V1 as R1 divided by (R1+R2) times this voltage VH.

So, when your input signal Vin is greater than this voltage V1, in that case, the output of this op-amp or Schmitt Trigger will become low voltage. So, now whenever your input signal Vin is greater than this voltage V1, in that case, your output will become low. And this voltage V1 is known as the upper threshold voltage of this inverting Schmitt Trigger.

So, we can say that the upper threshold voltage for the inverting Schmitt Trigger is equal to R1 divided by R1+R2, times this voltage Vh. Now, once this condition is satisfied, at that time your output of this Schmitt Trigger will become low voltage. And it will remain low till the point when this input signal Vin is less than this voltage V+.

Now, once this output of this Schmitt Triggers is low that case your V+ will become R1 divided by (R1+R2) times this voltage VL. And let’s say in this condition this voltageV+ as V2. Now, in this condition, the output of this Schmidt Trigger will become high only when this input signal Vin is less than this voltageV2. So, whenever this condition is satisfied at that time the output of this Schmitt Trigger will become again high.

So, this voltage V2 is known as the lower threshold voltage of the Schmitt Trigger. So, we can say that the lower threshold voltage of this Schmit trigger is equal to R1 divided by (R1 + R2 ) times this voltage VL. And the upper threshold voltage is equal toR1 divided by (R1 +R2) times this voltage VH. And the difference between this upper and the lower threshold voltage is known as the Hysteresis voltage of the Schmitt Trigger. And basically, this hysteresis voltage defines the noise immunity of the Schmitt Trigger.

So, let’s say for example, in this condition is equal to 12 V and VL is equal to -12V. R1 is equal to R2 is equal to R.So, in this condition, the upper threshold voltage for the given Schmitt Trigger will become 6V. And the lower threshold voltage will become-6V. And the hysteresis will be equal to 6 minus-6. That is equal to 12V. So, in this way, we can design the inverting Schmitt Trigger by using the op-amp. And we can even shift the transfer curve either to the right or to the left by providing the external reference voltage as an input to this inverting Schmitt Trigger.


Design of Non-Inverting Schmitt Trigger (with Derivation)

So, now that is being said, now let’s see how we can design the non-inverting Schmitt trigger by using this op-amp. So, here, we have applied the input at the non-inverting terminal and there is positive feedback from the output on the input side. So, now let’s understand how this circuit will act as a non-inverting Schmitt trigger. So, once again assume that the voltage at this node is equal to V+. And here if you observe, the inverting terminal is at ground potential. So, whenever, V+ is greater than zero, in that case, the output of this op-amp will be high. And whenever, the V+ is less than zero, in that case, the output of the op-amp will be below.

So, first of all, let’s find the expression for this voltage V+. So, once again, let’s apply KCL at this particular node. So, applying KCL we can write, V+ minus Vin, divided by R1, plus V+ minus Vout divided by R2, which is equal to zero. Or we can say that V+ multiplied by, one divided by R1 plus one divided by R2, that is equal to Vin divided by R1 plus, Vout divided byR2. And if we simplify it then we will get the expression of V+ as R2 divided by R1 +R2. times this input voltage Vin plus R1 divided by (R1 +R2) times this voltage Vout.

So, this is the expression of the V+ in terms of the input voltage and the output voltage. Now, here, initially assume that the output of this op-amp is low, that is Vout is equal to VL. And in that condition, let’s say this voltageV+ as voltage V1. So, the expression of V1 will become R2 divided by R1+R2 times this input voltage Vin, plus R1 divided by R1+R2 times this voltage VL. So, whenever this voltage V1 is greater than zero, that case, the output of this op-amp will become high.

Design of Non-Inverting Schmitt Trigger (with Derivation)

So, we can say that for this condition R2divided by R1+R2 times this voltage Vin should be greater than, _VL multiplied by the R1divided by R1+R2. Or we can say that Vin should be greater than-R1 divided by R2 times this voltage VL. So, whenever this condition is satisfied in that case, the output of this op-amp will become high.

So, this voltage is known as the upper threshold voltage for the Schmitt Trigger. So, we can say that the upper threshold voltage is equal to -R1 divided by R2 times this voltage VL. Now, here, we are assuming that the low voltage level for this op-amp is negative. so, if you see the upper threshold voltage will become positive. So, this is the expression for the upper threshold voltage for the non-inverting Schmitt TRigger.

So, whenever this condition is satisfied, in that case, the output of the op-amp will become high. And once this output voltage will become high, in that condition the expression for the voltage V+ will get changed. So, in this condition when the output voltage V out is high then let’s say the voltage V+ is equal to V2. And for this condition, the V2 will becomeR2 divided by R1+R2 times the input voltage Vin, plus R1 divided by R1+R2 times this voltage Vh. So, here, basically, we have replaced the output voltage Vout by VH.

So, now in this condition, the output of the Schmitt Trigger will become low only when this V2 is less than zero. So, we can say that for this condition, this voltage R2 divided by R1+R2 times this voltage Vin should be less than – R1 divided by R1+R2 times this voltage VH. Or we can say that the input voltage Vin should be less than -R1 divided by R2 times this voltage VH.

So, once this condition is satisfied in that case, once again the output of this Schmitt Trigger will become low voltage. And for this input voltage for which this condition is satisfied is known as the lower threshold voltage for the non-inverting Schmitt trigger. So, we can say that the lower threshold voltage is equal to -R1 divided by R2 times this voltage VH.

So, in this way, the expression for the upper threshold voltage is equal to -R1 divided by R2 times the voltage VL and expression for the lower threshold voltage are equal to -R1 divided by R2 times the voltage VH. And here we are assuming that VH and VL are equal and opposite in polarity. So, in this way, we can design this non-inverting Schmitt Trigger by using this op-amp.


Application of Schmitt Trigger

Now, we can shift this transfer curve either to the right or left side by providing the external reference voltage. So, if you provide the external reference voltage then according to the polarity of this reference voltage, we can shift this transfer curve either to the right or to the left side. So, in this way using this op-amp circuit we can design this inverting and the non-inverting Schmitt Triggers.

So, this Schmitt Trigger is particularly useful when we want to compare the two voltage levels and we know that the input is noisy. So, this Schmitt trigger can be used as a level comparator and it can be used in analog to digital conversions. Apart from that this Schmitt Trigger can be used as a wave shaping circuit.

So, if you have a sine wave or a triangular wave and if you want to convert that wave into the square wave then using the SchmittTrigger we can convert it. Apart from that if you observe over here, we are providing positive feedback to the op-amp to design this Schmitt Trigger.

So, using this Schmitt Trigger we can even design a multivibrator or an oscillator. So, these are the few applications in which this Schmitt TRigger can be used. So, in the next article, we will take some examples and through the examples, we will see the different applications in which this Schmitt trigger can be used.


FAQ


YouTube Video

So here are a youtube video based on Schmitt trigger which was uploaded by How To Mechatronics


So, I hope in this article you understood these inverting and the non-inverting Schmitt Triggers.

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