555 Timer As Schmitt Trigger

The high and low transitions on the inputs of most of the CMOS devices should be fast edges. If the edges are not fast enough, they tend to provide more current and this might damage the device. Analog signals are generally not perfect and might not have clean edges all the times. 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 outcome is the indication of whether one voltage is higher than the other or not. Schmitt trigger, also called as 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 irrespective of the shape of the input. It is often used when we need to do the following:

• Convert sine wave to square wave
• To clean up the noisy signals
• To convert slow edges (like in a triangular wave) into fast edges (like a square wave)

Schmitt can be constructed from a 555 timer. Some of the other function of the 555 timer, apart from the timer operation, is to use the two internal comparators as independent units to form a Schmitt Trigger. The general operation of the Schmitt trigger built from a 555 timer is inverting but the discussion will be for non-inverting.

Circuit of 555 timer as Schmitt Trigger

The following circuit shows the structure of a 555 timer used as a Schmitt trigger.

Pins 4 and 8 are connected to the supply (VCC). The pins 2 and 6 are tied together and the input is given to this common point through a capacitor C. this common point is supplied with an external bias voltage of VCC / 2 with the help of the voltage divider circuit formed by the resistors R1 and R2.

The important characteristic of the Schmitt trigger is Hysteresis. The output of the Schmitt trigger is high if the input voltage is greater than the upper threshold value and the output of the Schmitt trigger is low if the input voltage is lower than the lower threshold value.

The output retains its value when the input is between the two threshold values. The usage of two threshold values is called Hysteresis and the Schmitt trigger acts as a memory element (a bistable multivibrator or a flip-flop).

The threshold values in this case are 2/3 VCC and 1/3 VCC i.e. the upper comparator trips at 2/3 VCC and the lower comparator trips at 1/3 VCC. The input voltage is compared to these threshold values by the individual comparators and the flip-flop is SET or RESET accordingly. Based on this the output becomes high or low.

When a sine wave of amplitude greater than VCC / 6 is applied at the input, the flip-flop is set and reset alternately for the positive cycle and the negative cycle. The output is a square wave and the waveforms for input sine wave and output square wave are shown below.

Inverting Schmitt Trigger

The normal operation of the 555 timer as a Schmitt trigger is inverting in nature. When the trigger input, which is same as the external input, falls below the threshold value of 1/3 VCC, the output of the lower comparator goes high and the flip-flop is SET and the output at pin 3 goes high.

Similarly, when the threshold input, which is same as the external input, rises above the threshold value of 2/3 VCC, the output of the upper comparator goes high and the flip-flop is RESET and the output at pin 3 goes low.

The waveform of the inverting Schmitt trigger is shown below.