A Schmitt trigger circuit is an electronic circuit that functions as a comparator with hysteresis. In simple terms, it is designed to convert an analog input signal into a digital output signal that switches between two voltage levels based on a certain threshold. The hysteresis feature helps prevent rapid switching or noise-induced oscillations around the threshold.
Here's how a Schmitt trigger circuit works:
Voltage Thresholds: The Schmitt trigger circuit has two voltage threshold levels: a higher threshold (V_high) and a lower threshold (V_low). These thresholds define the points at which the circuit changes its output state.
Comparator Action: The heart of the Schmitt trigger is a voltage comparator. This comparator compares the input voltage (Vin) with the voltage thresholds (V_high and V_low). When the input voltage crosses the higher threshold (V_high) while increasing, the output of the comparator changes from a low voltage level (usually 0V or ground) to a high voltage level (usually the supply voltage, Vcc). Conversely, when the input voltage crosses the lower threshold (V_low) while decreasing, the output switches back from high to low.
Hysteresis: The key feature of the Schmitt trigger is hysteresis. Hysteresis introduces a small voltage difference between the two threshold levels (V_high and V_low). This means that the output state does not change as soon as the input voltage crosses the threshold; it only changes when the input voltage crosses the threshold plus (for rising edge) or minus (for falling edge) the hysteresis voltage. This prevents rapid, unwanted switching around the threshold caused by noise or small fluctuations.
Feedback: The hysteresis is achieved through positive feedback. The output of the comparator is connected to a feedback network that influences the input voltage to the comparator. This feedback network consists of resistors or other components that create a voltage divider or similar configuration. The feedback helps establish the hysteresis window.
In summary, a Schmitt trigger circuit is commonly used in digital systems to stabilize and condition analog input signals, converting them into well-defined digital output signals. It ensures that the output transitions occur at clearly defined voltage levels, reducing the impact of noise and improving the robustness of the circuit's behavior. This makes Schmitt trigger circuits useful in applications such as signal debouncing, waveform shaping, and noise filtering.