What Is Schmitt Trigger and How It Works

Summary

This tutorial from HowToMechatronics explains the Schmitt trigger, a logic circuit that helps convert noisy input signals into clean square waves using hysteresis with two distinct voltage thresholds for rising and falling edges. It demonstrates the advantages of using a Schmitt trigger compared to regular comparators in handling noisy signals, ensuring accurate pulse detection. The video covers the design using an operational amplifier and resistors to set specific switching thresholds. It also explores variations like inverting and single-powered Schmitt triggers and provides practical calculations for determining threshold values.

Takeaways

• 😀 The Schmitt trigger is a logic input type that provides hysteresis, or two different threshold voltage levels, for rising and falling edges.
• 👍 It helps avoid errors in noisy input signals by converting them into clean square wave outputs.
• 💡 Devices with a single set threshold can misinterpret noisy signals, whereas a Schmitt trigger ensures correct pulse detection.
• 🔧 The Schmitt trigger uses two different thresholds, which can be adjusted to convert slow edges, noisy square waves, or sine waves into cleaner outputs.
• ⚙️ Schmitt triggers can be built using operational amplifiers (op-amps) or comparators, with positive feedback and resistors to set thresholds.
• 🔄 The trigger switches output between two voltage levels when the input crosses specific threshold values, which can be controlled by adjusting resistor values.
• 📊 The formula for the threshold switching points depends on resistor values, and an example is given where the switch occurs at specific voltages (e.g., -6V and +6V).
• 🔗 Non-symmetrical thresholds can be achieved using a single-powered inverting Schmitt trigger with different voltage reference points.
• 🛠 A practical example shows how a voltage divider can be used to calculate thresholds when the Schmitt trigger switches between 0V and 5V.
• 📚 The Schmitt trigger can also be built using transistors, which will be covered in a follow-up tutorial.

Q & A

What is the primary function of a Schmitt trigger?

The primary function of a Schmitt trigger is to provide hysteresis, meaning it uses two different threshold voltage levels for rising and falling edges. This helps in converting noisy or slow input signals into clean square wave signals.

Why is hysteresis important in a Schmitt trigger?

Hysteresis is important because it helps prevent errors caused by noisy input signals. By using two different thresholds for rising and falling edges, the Schmitt trigger avoids incorrect readings that could occur if only one threshold was used.

How does a noisy signal affect a standard logic input without hysteresis?

Without hysteresis, a noisy input signal can cause incorrect readings, as the logic input could register more pulses than actually exist. For example, a signal intended to have two pulses might be incorrectly interpreted as having more pulses.

What are the two different threshold voltage levels used in a Schmitt trigger for?

The two different thresholds are used to ensure that the output switches only when the input crosses a certain point. One threshold is for the rising edge and the other for the falling edge, creating a 'dead zone' where the output remains stable even if the input fluctuates.

How can a Schmitt trigger be implemented using an operational amplifier?

A Schmitt trigger can be built using an operational amplifier (op-amp) by connecting the output to the non-inverting input with a resistor (positive feedback) and adjusting the resistor values to control when the switch occurs.

What happens when the input voltage (Vin) is below 0 volts in a Schmitt trigger circuit with an op-amp?

When the input voltage (Vin) is below 0 volts, the output of the op-amp comparator will be negative, typically at the negative supply voltage (VCC-).

What role do resistors play in a Schmitt trigger circuit?

Resistors in a Schmitt trigger circuit determine the threshold voltage levels. By adjusting the values of these resistors, you can set the specific input voltage at which the output will switch states.

How can a Schmitt trigger handle two different non-symmetrical thresholds?

A Schmitt trigger can handle two non-symmetrical thresholds by using an inverting single-powered configuration. In this setup, when the input reaches the upper threshold, the output switches to 0 volts, and when it declines to the lower threshold, the output switches to 5 volts.

What is the purpose of the positive feedback in a Schmitt trigger circuit?

The positive feedback in a Schmitt trigger circuit helps maintain stability by ensuring that the output only switches when the input crosses the designated thresholds, thus preventing noise from causing unwanted switching.

Can a Schmitt trigger be built using components other than op-amps?

Yes, a Schmitt trigger can also be built using transistors. This alternative method can achieve similar functionality in creating clean square wave outputs from noisy signals.