Schmitt Triggers: The Solution to Noise-Related Problems

As you might know, when we use an operational amplifier in a circuit by connecting a voltage reference of, for example, 2.5 volts to its inverting inputs and a triangle voltage between 0 and 5 volts to its non-inverting inputs, then the op-amp would create a square wave on its outputs.

The reason for this behavior can be explained by the first golden rule of op amps. The rule states that an op-amp will do anything to achieve a zero-volt difference between its inputs.

But since our op-amp configuration got no feedback system, the output either swings up to the positive supply voltage if the non-inverting input has a higher voltage potential than the inverting inputs or swings down to zero volts if the inverting input voltage is higher than the one on the non-inverting inputs.

This way, the op-amp acts as a comparator which is an important circuit when it comes to monitoring voltages and for example activating an alarm if they fall underneath a certain threshold value.

Introducing Schmitt Triggers

But of course, comparators are not perfect. If we observe the output voltage while the monitored voltage crosses the reference voltage, then we can see that there is not one definite transition.

This is where Schmitt triggers come in. A Schmitt trigger would be the solution to our noise-related problem. All we need to turn our comparator into a Schmitt trigger is a couple of resistors and depending on how we connect them to the comparator, we can create a non-inverting or an inverting Schmitt trigger.

The function of such a Schmitt trigger is to offer two threshold values, a high one and a low one. So only if the to be monitored voltage passes the high threshold value, the output gets pulled high and only if the low threshold value gets undershot, the output gets pulled low.

How Schmitt Triggers Work

This way, we can avoid noise cost oscillation on the output because in this so-called hysteresis voltage between the two thresholds no switching of the output is possible.

The functional principle of a Schmitt trigger would equal that of a non-inverting one. While an inverting Schmitt trigger would basically work the same but reverses the output states for its high and low threshold values.

And of course, we can calculate the hysteresis and threshold voltages for both Schmitt trigger types with a few different formulas.

The 74HC14 Hex Inverting Schmitt Trigger IC

But since you rarely build up a Schmitt trigger with an op-amp, let's take a look at the 74HC14 hex inverting Schmitt trigger IC.

This IC is very useful for creating simple relaxation oscillators. Because of the hysteresis voltage, the capacitor gets charged/discharge continuously which results in a square wave on the outputs.

By utilizing a potentiometer as a variable resistor, we can easily reach frequencies in the kilohertz range.

Practical Applications of Schmitt Triggers

A Schmitt trigger is very useful if you got a noisy or worn out data signal that you want to freshen up a bit.

Adding a capacitor and resistor to a Schmitt trigger, we can create a simple relaxation oscillator.

Last but not least, a Schmitt trigger is often used in debouncing circuits. For example, when adding a push button to an Arduino circuit, we would only need the RC network for debouncing since the digital inputs of the microcontroller already offer a high and low threshold voltage which is just like a Schmitt trigger.

1. Input Stage: The analog input signal is applied to the non-inverting input of the first op-amp. This stage amplifies the input signal and provides a high gain.
2. Comparator Stage: The output from the input stage is then compared with a reference voltage (Vref) using a second op-amp. If the input voltage exceeds Vref, the comparator switches to a high state; otherwise, it remains in a low state.
3. Hysteresis Stage: The output from the comparator is fed back into the input stage through a feedback resistor (Rf). This creates a hysteresis loop, where the circuit's output affects its own input. As the input voltage changes, the output switches between high and low states at specific threshold voltages.

1. Hysteresis: Schmitt triggers exhibit hysteresis, meaning the output switches at different threshold voltages depending on whether the input is rising or falling. This prevents noise-induced oscillations and ensures a stable output.
2. Noise Immunity: The hysteresis loop also provides immunity to noise on the input signal, allowing the circuit to reject small voltage fluctuations.
3. High Gain: Schmitt triggers provide high gain, which allows them to amplify weak input signals and create a clean digital output.

1. Signal Conditioning: Schmitt triggers can be used to clean up noisy signals and create a stable digital output.
2. Threshold Detection: The hysteresis loop allows Schmitt triggers to detect specific threshold voltages, making them useful for applications like level detection and alarm systems.
3. Digital Circuitry: Schmitt triggers are commonly used in digital circuits as a "clean-up" stage to remove noise from analog inputs before they are processed by digital logic gates.