What is a Schmitt trigger?
A Schmitt trigger is a type of electronic circuit with hysteresis, primarily used to convert non-linear input signals into digital output signals. It helps in cleaning up noisy signals, squaring up waveforms, and providing a more stable output.
Poperties of a schmitt trigger
Hysterisis
The defining feature of a Schmitt trigger is its hysteresis behavior. It has two threshold voltage levels: one for the rising input signal (upper threshold) and another for the falling input signal (lower threshold).
This hysteresis prevents false triggering or oscillation around the input signal’s transition point, ensuring stable output transitions.
Thresholds
The upper and lower threshold voltages define the range within which the input signal must cross to trigger a change in the output.
When the input signal rises above the upper threshold, the output switches to a high state.
When the input signal falls below the lower threshold, the output switches to a low state.
Working of Schmitt trigger
If the input voltage is below the lower threshold, the output remains at its low state.
When the input voltage exceeds the upper threshold, the output switches to its high state.
The output remains in this high state until the input voltage drops below the lower threshold, causing the output to switch back to its low state.
Applications of Schmitt trigger
- Noise Immunity: Schmitt triggers are effective in eliminating noise and signal distortions in digital circuits, ensuring cleaner transitions between logic states by providing noise immunity.
- Signal Conditioning: They’re used to condition analog signals into digital signals or square waveforms, making them useful in signal processing and sensor applications.
- Debouncing: In digital systems, Schmitt triggers help in debouncing switches or mechanical contacts, ensuring that only clean and stable signals are considered.
Different circuit construction of Schmitt trigger
Opamp based schmitt trigger
How to configure an op-amp as a Schmitt trigger? Steps:
- Connect the non-inverting input (+) of the op-amp to output using R2.
- Also, connect R1 from the non-inverting pin to a reference voltage (Vref, could be connected to mid-supply as well).
- Connect the inverting input (-) of the op-amp to the input voltage.
- Optionally, we can add capacitors or diodes for filtering or clamping if needed.
The operation of the opamp in the positive feedback loop is as follows:
- When the input voltage (voltage at the inverting pin) is below the lower threshold point (VLTP, determined by the voltage at the non-inverting input), the output of the op-amp will be at the positive supply voltage (VCC). As soon as the output goes to VCC, notice the threshold changing to VUTP because the threshold point (VU/LTP) depends on the output voltage level.
$$V_{U/LTP}=\cfrac{V_{ref}R_2+V_{out}R_1}{R_1+R_2}$$
$$V_{LTP}=\cfrac{V_{ref}R_2+V_{CC}R_1}{R_1+R_2}$$ - When the input voltage exceeds the upper threshold (higher than the voltage at the non-inverting input), the output of the op-amp will switch to the negative supply voltage (VEE).
$$V_{UTP}=\cfrac{V_{ref}R_2+V_{EE}R_1}{R_1+R_2}$$ - The output remains in its current state until the input voltage crosses the other threshold in the opposite direction, creating hysteresis.
Transistor based Schmitt-trigger
Uses transistors and other discrete components to achieve the hysteresis effect.
Off-the-shelf IC Schmitt trigger
Integrated circuit-based Schmitt triggers are available in various IC packages, providing a convenient solution for implementing Schmitt triggers in circuits.
Conclusion
Schmitt triggers are essential components in electronics, particularly in digital systems and signal conditioning circuits, due to their ability to provide noise immunity, stabilize input signals, and convert non-linear input signals into clean digital outputs with well-defined transitions. They are versatile and find applications in various fields of electronics, such as communication, automation, and signal processing.