Schmitt Trigger

Definition

A Schmitt trigger is a comparator-based circuit with positive feedback that switches its output between two stable states when the input crosses one of two threshold voltages: an upper trigger point (UTP) and a lower trigger point (LTP). The difference between these two thresholds is called hysteresis, and it prevents unwanted switching caused by noise or slow input variations.

Main Content

1. Hysteresis

Hysteresis is the most important concept in a Schmitt trigger.

• It means the circuit does not use one single switching threshold; instead, it uses two different thresholds depending on whether the input is rising or falling. This creates a “memory” effect in the circuit.

How it works in practice

When the input voltage increases and crosses the upper threshold, the output switches to one state. Once switched, the input must fall below the lower threshold before the output can change back. This gap between thresholds eliminates rapid toggling when the input is noisy or slowly changing.

Example:
If a Schmitt trigger has:

• Upper threshold = 3 V
• Lower threshold = 1 V

Then:

• As input rises above 3 V, output changes state.
• As input falls below 1 V, output returns to the original state.

This prevents the output from oscillating when the input is around 2 V and contaminated with noise.

2. Positive Feedback

A Schmitt trigger uses positive feedback to create hysteresis.

• In a comparator, positive feedback means a portion of the output is fed back to the input in such a way that it reinforces the current output state.

Why positive feedback is useful

It makes the switching action regenerative, meaning once the output begins to change, the feedback speeds up the transition and drives the circuit quickly to the opposite stable state. This results in a crisp, fast output edge rather than a slow or uncertain change.

Typical behavior:

• If output is HIGH, positive feedback shifts the threshold upward.
• If output is LOW, positive feedback shifts the threshold downward.

This threshold shift is what creates two different switching levels.

A basic conceptual representation:

          +-------------------+
Input --->|   Comparator      |----> Output
          |      +            |
          |   Positive FB ----|
          +-------------------+

In real circuits, the feedback network is usually made of resistors and sometimes an op-amp or transistor pair.

3. Threshold Levels

The two threshold levels are the operational heart of the Schmitt trigger.

• They define exactly when the output changes state.

Upper Trigger Point (UTP)

This is the input voltage level at which the output switches when the input is rising. Once the input crosses this level, the output changes state.

Lower Trigger Point (LTP)

This is the input voltage level at which the output switches when the input is falling. Once the input drops below this level, the output changes back.

The area between UTP and LTP is the hysteresis band.

A simple voltage transfer idea:

Output
  ^
  |        ________
  |       |        |
  |_______|        |_______
  +--------------------------> Input
        LTP      UTP

This graph shows that the output remains stable in the middle region and only changes at the thresholds.

The threshold values depend on:

• Supply voltage
• Resistor network
• Reference voltage
• Circuit type (op-amp, transistor, CMOS, etc.)

Working / Process

1. Input is below the lower threshold

• Suppose the Schmitt trigger output is in one stable state, for example LOW.
• If the input is lower than the lower trigger point, the output remains unchanged.
• Even if small noise appears, it will not cause switching because the input has not reached the required threshold.

2. Input rises and crosses the upper threshold

• As the input voltage increases, it eventually reaches the upper trigger point.
• At this point, the output switches rapidly from LOW to HIGH.
• Due to positive feedback, the change is sharp and stable.
• After switching, the threshold effectively shifts because the output state has changed.

3. Input falls and crosses the lower threshold

• Once the output is HIGH, the input must drop significantly before the output changes back.
• It must fall below the lower trigger point.
• Only then does the output switch from HIGH to LOW.
• This prevents chatter or repeated switching when the input is near the boundary.

A clear switching sequence:

Input rising:   below LTP ---> no change ---> crosses UTP ---> output switches
Input falling:   above UTP  ---> no change ---> crosses LTP ---> output switches back

This process is why Schmitt triggers are widely used for cleaning noisy waveforms and converting analog signals into digital pulses.

Advantages / Applications

Removes noise and prevents false triggering

Because of hysteresis, the circuit ignores small fluctuations around the threshold. This is especially useful for mechanical switches, sensor outputs, and weak analog signals.

Produces clean digital output from slow input signals

A slowly varying sine wave, triangle wave, or ramp can be converted into a sharp square wave with well-defined transitions.

Widely used in real electronic systems

Common applications include:

• Switch debouncing in keyboards and push buttons
• Wave shaping circuits
• Level detectors
• Pulse generation
• Signal conditioning in sensors
• Square-wave generation
• Noise immunity improvement in digital systems

Example application:
When a push button is pressed, the contacts may bounce several times before settling. A Schmitt trigger ensures that these bounces do not create multiple unwanted output transitions.

Summary

• A Schmitt trigger is a comparator circuit with hysteresis that gives two switching thresholds.
• It produces a stable output even when the input is noisy or slowly changing.
• Important terms to remember: hysteresis, upper trigger point, lower trigger point, positive feedback, threshold voltage