What is a Schmitt Trigger? Working and Applications

Basically, the Schmitt trigger is a multivibrator with two stable states, and the output stays in one of the steady states until further notice. The change from one stable condition to the other condition takes place as the input signal activates approximately. The operation of multivibrator requires an amplifier with positive feedback with loop gain above unity. This circuit is frequently used to change square waves by gradually differing boundaries toward sharp edges used in digital circuits, as well as switch debouncing. This article discusses what a Schmitt trigger, Schmitt triggers working with a circuit diagram with working & applications.

What is a Schmitt Trigger?

Schmitt trigger can be defined as it is a regenerative comparator. It employs positive feedback and converts sinusoidal input into a square wave output. The output of Schmitt Trigger swings at upper and lower threshold voltages, which are the reference voltages of the input waveform. It is a bi-stable circuit in which the output swings between two steady-state voltage levels (High and Low) when the input reaches certain designed threshold voltage levels.

These are classified into two types namely inverting Schmitt trigger and non inverting Schmitt trigger. The inverting Schmitt trigger can be defined as an element of output is connected to the positive terminal of the operational amplifier. Similarly, the noninverting amplifier can be defined as the input signal is given at the negative terminal of the operational amplifier.

What are UTP and LTP?

The UTP and LTP in Schmitt trigger using op-amp 741 are nothing but UTP stands for upper trigger point, whereas LTP stands for the lower trigger point. Hysteresis can be defined as when the input is higher than a certain chosen threshold (UTP), the output is low. When the input is below a threshold (LTP), the output is high; when the input is between the two, the output retains its current value. This dual threshold action is called hysteresis.

V Hysteresis = UTP-LTP in our example

Upper threshold (Trigger) point, Lower Threshold (Trigger) points – these are the points where the input signal is compared. The values of UTP and

LTP for the above circuit includes the following

UTP = +V * R2 / (R1 + R2)

LTP = -V* R2 /(R1 + R2)

When two levels are to be compared there may be oscillation (or hunting) at the border. Having hysteresis prevents this oscillation problem is solved. The comparator compares always with a fixed reference voltage (single reference) whereas Schmitt trigger compares with two different voltages called UTP and LTP.

The UTP and LTP values for the above Schmitt trigger using the op-amp 741 circuit can be calculated using the following equations.

We know that,

UTP = +V * R2 / (R1 + R2)

LTP = -V* R2 / (R1 + R2)

UTP = +10V *5𝐾/5𝐾+10𝐾= + 3.33 V

LTP = -10V *5𝐾/5𝐾+10𝐾= – 3.33 V

Schmitt Trigger using IC 555

The circuit diagram of the Schmitt trigger using IC555 is shown below. The following circuit can be built with basic electronic components, but IC555 is an essential component in this circuit. Both pins of the IC such as pin-4 & pin-8 are connected with the Vcc supply. The two pins like 2 & 6 are shorted, and the input is mutually given to these pins with the help of a capacitor.

The mutual point of the two pins can be supplied with an external bias voltage (Vcc/2) using the voltage divider rule that can be formed by two resistors namely R1 & R2. The output keeps its values while the input is among the two threshold values which are called Hysteresis. This circuit can perform like a memory element.

The threshold values are 2/3Vcc &1/3Vcc. The superior comparator tours at the 2/3Vcc while the minor comparator tours at the supply of 1/3Vcc.
The key voltage is contrasted with the two threshold values using individual comparators. The flip-flop (FF) is arranged or rearranged consequently. The output will become high or low depending on this.

Schmitt Trigger using Transistors

The Schmitt trigger circuit using a transistor is shown below. The following circuit can be built with basic electronic components, but two transistors are essential components for this circuit.

When the input voltage (Vin) is 0 V, then the T1 transistor will not conduct, whereas the T2 transistor will conduct due to the voltage reference (Vref) with the voltage1.98. At node B, the circuit can be treated as a voltage divider to calculate the voltage with the help of following expressions.

Vin = 0V, Vref = 5V

Va = (Ra + Rb/ Ra + Rb + R1) * Vref

Vb = (Rb/Rb + R1 + Ra) * Vref

The conducting voltage of the T2 Transistor is low & the transistor emitter terminal voltage will be 0.7 V is lesser than the base terminal of the transistor that will be 1.28 V.

Therefore, when we increase the input voltage, the T1 transistor value can be crossed so the transistor will conduct. This will be the reason to drop the base terminal voltage of the transistor T2. When the T2 transistor is not conducting longer then the output voltage will be increased.
Subsequently, the Vin (input voltage) at the T1 transistor base terminal will begin refusing & it will deactivate the transistor as the transistor base terminal voltage will be above 0.7 V of its emitter terminal.

This will occur when the emitter current will refuse to an end wherever the transistor will find into the mode of forward-active. So the voltage at the collector will rise, and also the base terminal of the T2 transistor. This will reason to flow little current through the T2 transistor further it will drop the voltage of the emitters of the transistor and also turn off the T1 transistor. In this case, the input voltage requires dropping 1.3V to deactivate the T1 transistor. So finally the two threshold voltages will be 1.9V & 1.3V.

Schmitt Trigger Applications

The uses of the Schmitt trigger include the following.

• Schmitt triggers are mainly used for changing a sine wave to square wave.
• They must be utilized in the switch de-bouncer circuit for a noisy otherwise slow input requirements like to be cleaned up or speed up
• These are normally utilized in applications like signal conditioning for removing signals noise in digital circuits.
• These are used to implement relaxation oscillators for closed loop negative response designs
• These are used in switching power supplies as well as function generators

Thus, this is all about the Schmitt trigger theory. These are found in several applications within analog and digital numerical circuits. The flexibility of a TTL Schmitt is disadvantaged with its narrow supply range, partial interface capacity, small input impedance & unstable characteristics of output. This can be designed with discrete devices to convince an exact parameter, however, this is cautious & takes time to design. Here is a question for you, what are the advantages of a Schmitt Trigger?