Schmitt Trigger: Everything you need to know about this component

Schmitt Trigger

Today we will explain another new element that has been added to our list. Schmitt TriggerA story that many people don’t know, which will now no longer be a mystery. And we’ll explain everything you need to know about it, like what it is and what it’s for. How does this electronic device work? And even then You can integrate it into your projects with Arduino etc.

Let’s see what this element can help us with…


necessary previous concepts

Before starting with the Schmitt Trigger, it is necessary to have Define a few concepts. Which will be helpful in understanding better what it is and how it works. I mean:

  • comparator: in electronic equipment A comparator is a device that compares two voltages or currents. Then send out a digital signal to indicate which value is greater. There are two analog input terminals and one binary digital output. This is important to note. This is because the Schmitt trigger is a type of comparator. Additionally, this comparator incorporates a special high gain differential amplifier.
  • HysteresisHysteresis: Hysteresis is a property in which the state of a system depends on its history. For example, a magnet can have different magnetic moments in a magnetic field. It depends on past changes in the magnetic field. This causes a hysteresis curve. This property is found in ferromagnetic and ferroelectric materials. and natural phenomena such as the deformation of rubber and shape memory alloys. Hysteresis involves irreversible changes, such as phase transitions. and is normal in natural systems Why should you know this? Because the Schmitt Trigger is a comparator circuit with hysteresis.

What is a Schmitt Trigger?

Schmitt trigger chip DIP

g Schmitt TriggerAlso known as a Schmitt trigger in Spanish, it is an electronic comparator circuit that converts an analog input signal into a digital output signal. This is done by using positive feedback to create a hysteria tipping point. This means that the threshold for switching between “high” and “low” states is different for the up and down of the input signal. This hysterical behavior prevents unwanted fluctuations. and make allowance for slightly noisy or jittery input signals.

The design was created for the first time. Otto H. Schmitt in 1934Therefore is the origin of the name. Since then This electronic component is also widely used in a variety of applications. As we will see later, you should also know that chips are usually encapsulated in an integrated circuit or chip. This is usually a DIP and typically consists of an operational amplifier (op-amp) with positive feedback through a resistor. The op-amp’s non-inverting (+) and inverting (-) inputs are connected via a resistor chain and an additional resistor is included for positive feedback from the output to the inverting input.

for Hysterical behaviorHaving said that, when the input signal exceeds a certain upper threshold, the output of the Schmitt Trigger turns “high” and if the input signal drops below another lower threshold. The output turns “low”. The difference between the two thresholds is called the hysteresis window. and is required for hysteresis behavior. This has the advantage of being able to avoid undesired fast responses due to small fluctuations or noise in the input signal. Therefore, it is immune to noise.

A Schmitt trigger is used. Improve noise immunity In a circuit with a single input threshold, in this case, noise close to the threshold can cause rapid changes in the output due to the noise. Schmitt triggers have two thresholds to avoid unwanted changes. This is because noise close to the threshold only causes changes in the output. If you want to make changes again The signal must travel beyond another threshold.

g Practical example It involves an amplified infrared photodiode that creates a signal that fluctuates between very high values. This signal is smoothed with a low-pass filter. And the filtered output is connected to a Schmitt trigger. This device ensures that the output changes from low to high only after the infrared signal excites the photodiode for a longer than a known time period. When the Schmitt trigger is high, it returns to low after the infrared signal stops exciting the photodiode for longer than a similar known time period. This avoids spurious shifts caused by environmental noise. Schmitt triggers are common in switching circuits, such as de-bouncing switches.

How does a Schmitt Trigger work?

Circuits with hysteresis are based on positive feedbackThis makes it possible to convert any active circuit. as a Schmitt trigger using positive feedback with a loop gain greater than one. Positive feedback involves adding some output voltage to the input voltage. These circuits, which include attenuators, adders, and amplifiers, act as comparators. It can be implemented using three specific techniques.

The first two techniques are Various models A pair (series and parallel) of a common positive feedback system. To configure these values The output voltage corrects for the effective difference in the comparator’s input voltage, either by ‘lowering the threshold’ or by ‘raising the circuit’s input voltage’. These configurations incorporate the property Limits and memory in a single component The third technique separates the threshold and memory properties. Makes it more flexible in using the circuit.

Utilities and applications


Schmitt Triggers can be used for many practical applications. Depends on configuration, such as:

  • analog to digital conversion– This component is an efficient single-bit analog-to-digital converter. When the signal reaches the specified level The signal will change from a low state to a high state.
  • Level detection– Can detect level When making this application It is necessary to take into account the hysteresis voltage. So that the circuit changes according to the desired voltage.
  • Answering a call– When running data lines that may collect noise into a logic gate. It is necessary to ensure that the logic output level changes only when the data changes. And it is not the result of picking up fake noise. Using a Schmitt trigger allows peak-to-peak noise to reach hysteresis levels. Before false triggers occur

In more specific cases You can see this in a circuit where you want to eliminate bounce in mechanical keys. in a square wave generator in level detector In the circuit to prevent data line interference Pulse generator and the famous converter ADC

Use it as an oscillator.

The Schmitt trigger is a bistable multivibrator. Can be used with other types of multivibrators., relaxation oscillator This is achieved by connecting a single RC integrated circuit between the output and the input of an inverting Schmitt trigger. The output is a continuous square wave whose frequency depends on the values ​​of R and C as well as the Schmitt trigger’s threshold point, since A single IC can provide multiple Schmitt triggers (for example, a 40106 4000 series CMOS device has six), so additional ICs can quickly be used as simple and reliable oscillators with only external components. 2 pieces

In this case, a comparator-based Schmitt trigger is used in an inverted configuration. In addition, a slow negative response is added to the RC integration network. The result is that The output will automatically range from VSS to VDD. This is because the capacitor charges from one Schmitt trigger threshold to another threshold.

Pin out


You must remember that according to Pinout model It can change Therefore, I recommend that you always refer to the manufacturer’s datasheet that corresponds to the model you purchase. However, as an example, we have a 74LS14 TTL chip with 6 triggers inside. So we have one DIP pin for powering Vcc and another for ground or GND. This is how the entire trigger is driven. Then it’s a matter of using the inputs and outputs that work for you.

Where to buy?

Finally, if you want Buy One of These Schmitt Triggers You can find it in specialized stores or on online sales platforms such as Amazon:

Leave a Comment