Schmitt Trigger

When the input voltage exceeds a few hundred millivolts Q1 just starts to conduct and the current goes straight into the base of Q2 which then starts to strongly conduct. The voltage at collector of Q2 is pulled up causing current to flow through R3 into the base of Q1. Resulting in positive feedback. The circuit snaps on. To snap the circuit off the voltage at the input has to fall close to ground, such that the voltage at the input falls a few 10s of millivolts below that needed to trigger the circuit initally. D1 protects the base of Q1 from very negative voltages.

Schmitt triggers are versatile circuits with a wide range of applications, including:

Signal conditioning: Schmitt triggers can be used to clean up noisy signals, remove jitter, and sharpen edges. This is useful in applications such as digital communications, industrial control, and sensor interfaces.
Switch debouncing: Schmitt triggers can be used to remove the electrical noise (bounce) that is generated when a mechanical switch is closed or opened. This is important in digital circuits to prevent false triggering.
Oscillators: Schmitt triggers can be used to generate square waves, which are used in many electronic circuits, such as clocks, timers, and pulse-width modulation (PWM) controllers.
Comparator: Schmitt triggers can be used as comparators to compare two voltages and generate a digital output based on the result. This is useful in applications such as level detection, thresholding, and windowing.

Here are some specific examples of interesting applications of Schmitt triggers:

Metal detector: A Schmitt trigger can be used to detect the presence of metal by sensing the change in inductance of a coil when metal is nearby.
Heart rate monitor: A Schmitt trigger can be used to detect the heartbeat by sensing the small electrical signals generated by the heart.
Rain sensor: A Schmitt trigger can be used to detect rain by sensing the change in conductivity of a raindrop sensor.
Touch sensor: A Schmitt trigger can be used to detect touch by sensing the change in capacitance of a human body when it touches a sensor.