The high and low transitions in the inputs of most CMOS devices must be fast edges. If the edges are not fast enough, they tend to provide more current and this could damage the device. Analog signals are generally not perfect and may not have clean edges every time. Schmitt Trigger is a special type of comparator that is used to avoid such signals.
A comparator is a device that compares two voltages and the result is the indication of whether one voltage is greater than the other or not. The Schmitt trigger, also called a regenerative comparator, compares the input voltage to two reference voltages and produces an equivalent output. The output of a Schmitt trigger is always a square or rectangular wave regardless of the shape of the input. It is often used when we have to do the following:
• Convert sine wave to square wave
• To clean noisy signals
• To convert slow edges (such as a triangular wave) to fast edges (such as a square wave)
Schmitt can be constructed from a 555 timer. Some of the other functions of the 555 timer, other than the timer operation, is to use the two internal comparators as separate units to form a Schmitt Trigger. The general Schmitt trigger performance built from a 555 timer is investing but the discussion will be not to reverse.
Timer Circuit 555 as Schmitt Trigger
The next circuit shows the structure of a 555 timer used as a Schmitt trigger.