Schmitt Trigger

The Schmitt Trigger is a logic input type that provides hysteresis or two different threshold voltage levels for rising and falling edge. This is useful because it can avoid the errors when we have noisy input signals from which we want to get square wave signals.


So for example, if we have a noisy input signal like this, that is meant to have 2 pulses, a device that has only one set point, or threshold, could get incorrect input and it could register more than two pulses as shown in this illustration. And if we use the Schmitt Trigger for the same input signal we will get a correct input of two pulses because of the two different thresholds. So that’s the primal function of the Schmitt Trigger, to convert noisy square waves, sine waves or slow edges inputs into clean square waves.


Types of Schmitt Trigers

There are many logic ICs that have built-in Schmitt Triggers on their inputs, but also it can be built using transistors or easier using an Operational Amplifier, or comparator and just adding some resistors to it and a positive feedback.


Operational Amplifier based Schmitt Trigger

Here we have an op-amp which inverting input is connected to the ground or zero volts and the non-inverting input is connected to a voltage input, VIN. So this is actually a comparator and compares the non-inverting input to the inverting input or in this case the input voltage VIN to 0 V. So when the VIN value is below 0 volts the output of the comparator will be the negative VCC and if the input voltage is above 0 volts the output will be positive VCC.


Now if we add a positive feedback by connecting the output voltage to the non-inverting input with a resistor between them and another resistor between the VIN and the non-inverting input we will get the Schmitt Trigger. Now the output will switch from VCC– to VCC+ when the voltage at the A node will cross 0 volts.


That means that now by adjusting the values of the resistors we can set at what value of the VIN input the switch will occur using the following equations. We get these equations with the following relationships. The current “i” through this line equals VIN – VA divided by R1 as well as VA – VOUT divided by R2. So if we replace the VA with zero, as we need that value for the switch to occur, we will get that final equation. For example if the output is -12 volts and the VIN input is negative and rises, the switch from -12 V to +12 V will occur at 6 volts according to the equation and the values of the resistors and vice versa when the VIN input is high and declines the switch from +12 V to – 12V will occur at -6 volts.


Non-Symmetrical Schmitt Trigger

In order to get two different non-symmetrical thresholds, we can use this circuit of an inverting single powered Schmitt Trigger. Here the VREF voltage is the same as the VCC of the op-amp. Now because the VIN input is connected to the inverting input of the op-amp when its values will reach the upper threshold, the output will switch off to 0 volts, and then when its values will decline to the lower threshold, the output will switch on to 5 volts.

Non Symmetrical Schmitt Trigger

Here’s an example of how we can calculate the thresholds. The VREF and the VCC will be 5 volts and the three resistors will be the same 10k ohms. So what we need to calculate now is the voltage at the A node. In the first case when the output is 0 V our circuit will look like this, a simple voltage divider and the value of the Vwill be 1.66 V. This means that the VIN input needs to decline below that value in order the output to switch on to 5 volts. Now with this 5 volts at the output the circuit will look like this. The value of the Vwill be 3.33 V. This means that the VIN input needs to rise above that value in order the output to switch off to 0 volts.