Schmitt Trigger

Introduction to the Schmitt Trigger

Generally, We use the schmitt trigger to “clean up” a digital signal.    When digitial data is passed through a cable, you will have capacitance.   Furthermore, the longer the cable is, the more capacitance you will have.  We know that capacitance opposes a change in voltage.

When transmitting data at high speeds, the voltage needs to rise and fall very quickly to transmit 1’s and 0’s across a network.   When transmitting data over a long distance, this becomes very difficult.   Because of the capacitance in the cable, it takes longer for the voltage to rise and fall in the cable to transmit data.

Cleaning up the signal

In this case, we have a 74HC14 Schmitt Trigger.    This is an inverting trigger.   That means that if I send a high voltage into the trigger, I will get a low voltage out, and vice versa.

As an illustration, let’s look at the following diagram that I made with Tinkercad.

No Schmitt Trigger

Here, we have a function generator feeding a square wave into the breadboard.   I’ve used a 200 Ohm resistor to simulate a load.   Remember that when you have a long cable, there will be capacitance in the cable.   I’ve simulated the capacitance with a 200 MicroFarad capacitor.  If you look at the output on the scope, you will see that it takes time for the signal to rise from 0 to 5 volts.    Similarly, it takes time for the signal to drop to 0 volts.

Loosing signal

As the frequency increases, there is simply not enough time for the voltage to fully rise or fall, so our signal begins to degrade.

Square Degraded

At this point, we have just threee choices.   First, we can clean up the signal with a schmitt trigger.  Secondly, the communication rate can be slowed down.   Thirdly, we can shorten the cable (less capacitance).

In this instance, we want a high communication speed, and cannot shorten the cable, so we will clean up the signal.

Using the Schmitt Trigger

Consider the following diagram.   In essense, our degraded circuit is still in place.  Using the 74HC14 chip, we will clean up this signal.   Once again we will have a true square wave.  A stable power supply will be placed on the chip.   We will feed the noisy signal into one of the chip’s inputs.    Because this input is inverting, we will run the output of the first inverting input through a second inverter.  The result is a pure square wave once again.

The scope on the left shows the “dirty” input.   The scope on the right shows the “clean” output!

Schmitt Trigger clean

As the input to the 74HC14 rises, the output remains low until the input reaches 2/3 (66%).   At this point, the output is switched fully on, and will stay on.    In contrast, when the input begins to fall, and reaches 33%, the output will be low.  By placing the Schmitt trigger at various points in the communication line, we can transmit high speed data for long distances.



— Ricky Bryce


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