Infrared-sensing Cardiac Monitor
Schmitt trigger: circuit used when an input signal to a digital circuit doesn't directly fit the description of a digital signal. For various reasons the input signal may have slow rise and/or fall times, may have acquired some noise that could be mis-sensed by later circuitry, or could be an analog signal (such as in this heart monitor) whose frequency we want to measure. All of these conditions, and many others, require a specialized circuit that will "clean up" a signal and force it to true digital shape. The required circuit is called a Schmitt Trigger. It has two possible states just like other multivibrator circuits. However, the trigger for this circuit to change states is the input voltage level, rather than a digital pulse. That is, the output state depends on the input level, and will change only as the input crosses a pre-defined threshold.