Circuit

Schmitt Trigger: How Schmitt Trigger Works? (Inverting & Non-Inverting Circuit Diagrams)

A Schmitt trigger is a multivibrator with two stable states and it is a form of comparator circuit that has hysteresis or different input switching levels to change the output between the two states. The Schmitt trigger required differential amplifier at core. In tis tutorial we will study about the Schmitt trigger, implementation with using transistor and Op-Amp, how it work and important applications.

What is a Schmitt Trigger?

what-is-a-schmitt-trigger

it is an electronics circuit that adds hysteresis to the input-output transition threshold with the help of positive feedback to the non-inverting input of a comparator or differential amplifier. And it was invented by Otto Schmitt in 1930’s. It is uses two different input threshold voltages to avoid a noise in the input signal and this dual-threshold is known as hysteresis. It is a Bi-stable Multivibrator and its output remains in either of the stable states indefinitely. This Bi-stable operation of the Schmitt Trigger requires an amplifier with positive feedback or regenerative feedback with a loop gain i.e. greater than one. The is also name of Schmitt trigger.

The above figure shows a noisy input signal, Schmitt trigger is convert noisy signals into clean square pulses with sharp leading and trailing edges. The noisy signal may be sine triangular square or any of the periodic signal. It is commonly use for removing noise in digital signal

Schmitt trigger basics

This circuit is widely used for many years and it was invented by Otto Schmitt. He is an American scientist. It is switches at different voltage level depending upon the moving from high to low or low to high. The symbol of Schmitt trigger is shown below.

Schmitt-trigger-symbol-Schmitt trigger circuit

Basically, the Schmitt circuit can easily converted into the stander converter circuit. It uses positive feedback by the additional resistance; therefore the circuit is called regenerative comparator circuit. The Schmitt circuit can be design by both of the Operational amplifier (Op-Amp) and Transistor. And these two are described below:

  • Operational amplifier (Op-Amp)
  • Transistor

Schmitt trigger circuit diagram

The Schmitt circuit diagram is a fast operating voltage level detector or it is also known as comparator circuit. When the input voltage level arrives and this input voltage level is detected by circuit elements, the output voltage quickly shift from maximum positive level and its maximum negative level.

Circuit-diagramSchmitt trigger circuit using Op-amp

The design of Schmitt circuit can be use operational amplifier in two way i.e. inverting and non-inverting way. If the input signal is applied on the inverting terminal of Op-Amp, so it is known as Inverting Schmitt Trigger, and if the input signal is applied on the non-inverting point of Op-Amp, so it is known as non-inverting Schmitt Trigger. Depending on the where the input signal is applied.

Schmitt trigger inverter

In the inverting Schmitt circuit the input signal is to applied to the inverting terminal of the operational amplifier with the positive feedback signal from the output of the inverting input. The non-inverting signal is to be ground. The circuit diagram is shown below;

inverting-circuit

Now we are going to understand the working of the Schmitt trigger inverter. The reference voltage is “V” and the input voltage “Vin”. When we applied the input voltage at the Vin terminal of Schmitt trigger inverter. If the applied voltage “Vin” is grater then reference voltage “V”, the output will be low. If applied voltage “Vin” is less then reference voltage “V”, the output will be high.

Vin > V       Vout = VL

Vin < V       Vout = VH

Now, we calculate V, by applying Kirchhoff’s current law.

schmitt-trigger-inverter-calculatiion

Now the output of the Schmitt is high when Vin < V       Vout = VH       and   V = V1

The V1 will be      V1 = (R1 / R1 + R2 ) VH

When the input signal is greater than V1 the output will be low and the Hence, V1 is an upper threshold voltage (VUT).

VUT = (R1 / R1 + R2 ) VH

Output of the Schmitt trigger is low when Vin > V       Vout = VL and   V = V2

The V2 will be       V1 = (R1 / R1 + R2 ) VL

When the input signal is lower than V2 the output will be low and the Hence, V2 is an upper threshold voltage (VUT).

VLT = (R1 / R1 + R2 ) VL

Schmitt trigger Non-inverting

In non-inverting Schmitt the input signal is applied to the non-inverting terminal of the operational amplifier with positive feedback from the output to the input terminal. And the inverting terminal of the Op-Amp is connected to the ground terminal. The figure of non-inverting Schmitt is shown below.

When v is greater than zero the output of Schmitt will be high and voltage V is less than zero the output will be low.

Vin > 0       Vout = VL

Vin < 0       Vout = VH

Now we find the voltage “V” by applying the KCL at node;

schmitt-trigger-non-inverting-calculation

Now let us assume the output of operational amplifier is low. Hence the output voltage is VL. And voltage V is equal to V1.

Vout = VL    and    V = V1

Now V1 =   [(R2 / R1 + R2 ) Vin  +  (R1 / R1 + R2 ) VL ]

When V1 is greater than zero the output will be high.

(R2 / R1 + R2 ) Vin  > – (R1 / R1 + R2 ) VL

Vin  >  – (R1 /R2 ) VL

Schmitt Trigger using Transistor

The transistor based circuit can be made by with the help of two transistors. The circuit diagram of Schmitt trigger transistor based is shown below.

transistor-based-schmitt-trigger

Input voltage is “Vin

Reference voltage, “Vref” = 5V.

Now let’s assume that, initially the input voltage “Vin” is zero, which is applied to base of the transistor of T1. Due to this the transistor “T1” is work in the cut-off region i.e. non-conducting. From the figure, the reference voltage is 5v and we are going to calculate the node voltage Va and Vb by the voltage divider rule. Now the Vb is 1.98 V and the Vb is connected to the base of the transistor of T2. The transistor start conducting and due to and due to this the output of Schmitt trigger is low. The 0.7 V drop at an emitter terminal of T2. So, the base voltage of transistor is 1.28 V. Both of the transistor emitter is connected with each other, hence the both transistor operate at the same level at 1.28 V this means the transistor T1 operate when input voltage is above 0.7 or 1.98V (1.28V + 0.7V). Now input voltage level from the 1.98 and the T1 start conducting. This causes it will cut off the transistor T2. Now we get the output of Schmitt trigger is “High”.

When input voltage start decreasing the transistor T1 will cut off. In this condition the “T2 ” transistor will start conduction i.e. turn “ON”. And the output of Schmitt trigger is “LOW”. Therefore we get two threshold voltage lower threshold at 1.28V and a higher threshold at 1.98V.

Schmitt trigger equations

Or

Schmitt trigger formula

  1. For inverting

Vin > V       Vout = VL
Vin < V       Vout = VH
V = (R1 / R1 + R2 ) VH

When the input signal is lower than V2 the output will be low and the Hence, V2 is an upper threshold voltage (VUT).
VLT = (R1 / R1 + R2 ) VL

When the input signal is greater than V1 the output will be low and the Hence, V1 is an upper threshold voltage (VUT).
VUT = (R1 / R1 + R2 ) VH

  1. Non-inverting

Vin > 0       Vout = VL
Vin < 0       Vout = VH
Vout = VL    and    V = V1
Now V1 =   [(R2 / R1 + R2 ) Vin  +  (R1 / R1 + R2 ) VL ]
When V1 is greater than zero the output will be high.
(R2 / R1 + R2 ) Vin  > – (R1 / R1 + R2 ) VL
Vin  >  – (R1 /R2 ) VL

 Schmitt Trigger IC SN74LS14

The Schmitt trigger IC SN74LS14 are a 14 pin IC. It is used in various applications

Pin detail of IC SN74LS14

Pin Number Pin Name Description
1,3,5,11,13,15 Schmitt Input Pins Input pins
2,4,6,10,12,14 Schmitt Output Pins Output pins
7 Ground Connected to the ground of the system
14 Vcc (+5V) Voltage Supply pin

 

Features

  • It is a Hex Inverter
  • The Operating Voltage is 5V
  • Output current High -0.4mA
  • Output current Low 8.0mA
  • Low Level Hysteresis voltage is 0.25V
  • High Level Hysteresis voltage is 3.4V
  • Hysteresis voltage: 0.8V typically
  • Rise Time 12nS
  • Fall Time 12nS
  • Available in 14-pin PDIP, GDIP, PDSO packages

Schmitt Trigger Applications

  • It is used to convert a sine wave and triangular wave into square waves.
  • It is use to level detector.
  • The Schmitt triggers to remove noise in the digital circuit.
  • The Schmitt trigger is use in function generator.
  • It is use to implement an oscillator.

Conclusion

In this tutorial we study about Schmitt trigger. We learned what a Schmitt Trigger is and its basic circuit using Transistors, Op Amp based Schmitt Triggers (both Inverting and Non-Inverting). We also see important applications of Schmitt Triggers.

Frequently ask question

  1. How does a Schmitt trigger circuit work?

The Schmitt is a circuit that convert sine wave to square wave mans it convert analog to digital. The Schmitt trigger can be implemented by several component however it is mostly implemented by us operational amplifier.

Schmitt-trigger-circuit-work

  1. What is meant by a Schmitt Trigger?

A Schmitt is a multivibrator with two stable states and it is a form of comparator circuit that has hysteresis or different input switching levels to change the output between the two states. It logic input type that provides hysteresis or two different threshold voltage levels for rising and falling edge. This is useful to avoid the noise in the signal. The Schmitt  circuit convert sine wave to square wave mans it convert analog to digital.

  1. What’s the application for an inverting Schmitt Trigger?

The application of Schmitt trigger of inverting is given below

  • It is used to convert a sine wave and triangular wave into square waves.
  • It is use to level detector.
  • The Schmitt triggers to remove noise in the digital circuit. 
  1. What is Schmitt trigger using op-amp?

The design of Schmitt circuit can be use operational amplifier in two way i.e. inverting and non-inverting way. If the input signal is applied on the inverting terminal of Op-Amp, so it is known as Inverting Schmitt, and if the input signal is applied on the non-inverting point of Op-Amp, so it is known as non-inverting Schmitt.

  1. What are the different kinds of Schmitt trigger?

The Schmitt trigger is mainly two types these are given below

  • Inverting
  • Non inverting

If the input signal is applied on the inverting, so it is known as Inverting , and if the input signal is applied on the non-inverting, so it is known as non-inverting.

Also read:- Schottky diode, Zener diode, diode

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