A Schmitt trigger circuit is a type of electronic circuit that serves as a comparator with hysteresis. Its primary purpose is to convert an analog input signal into a digital output signal with well-defined switching thresholds and noise tolerance. The circuit's unique characteristic is its ability to provide different threshold levels for rising and falling edges of the input signal, which helps in preventing rapid and unintended switching due to noise or fluctuations near the threshold.
Here's how a Schmitt trigger works:
Input Signal Monitoring: The Schmitt trigger circuit takes an analog input signal and compares it to two threshold voltage levels: a higher threshold voltage called the "upper threshold" (Vhigh) and a lower threshold voltage called the "lower threshold" (Vlow).
Hysteresis: The key feature of the Schmitt trigger is the concept of hysteresis. Hysteresis means that the output state of the circuit depends not only on the current input voltage but also on the recent history of the input. This helps in preventing rapid toggling of the output when the input signal is near the threshold due to noise or minor fluctuations.
Output Switching: When the input voltage rises above the upper threshold (Vhigh), the output of the Schmitt trigger switches to a high state (digital "1"). However, the output will remain high until the input voltage drops below the lower threshold (Vlow).
Reset Condition: Once the input voltage falls below the lower threshold (Vlow), the output switches to a low state (digital "0"). The output will stay low until the input voltage rises above the upper threshold (Vhigh) again.
Noise Immunity: The hysteresis property of the Schmitt trigger helps improve noise immunity. When the input voltage is near a threshold, the circuit doesn't immediately switch states; it requires a more substantial change in input voltage to cause a transition. This prevents false triggering caused by noise or small fluctuations around the threshold voltage levels.
Schmitt trigger circuits find applications in various fields, such as signal conditioning, noise filtering, level detection, and waveform shaping. They can be used to stabilize and clean up noisy input signals, ensure accurate switching in digital circuits, and eliminate bouncing effects in mechanical switches.
In summary, the purpose of a Schmitt trigger circuit is to provide stable and noise-tolerant digital output signals by using hysteresis to control the switching thresholds of the circuit. This helps in reducing the impact of noise and minor signal fluctuations on the accuracy and reliability of the output signal.