Schmitt Trigger Video Lecture | Digital Electronics - Electrical Engineering (EE)

hello then the locus key here from auto
mechatronics comm in this tutorial we
will learn what is Schmitt trigger and
how it works the Schmitt trigger is a
logic input type that provides
hysteresis or to different threshold
voltage levels for rising and falling
edge this is useful because it can avoid
the errors when we have a 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 two 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 there are
many logic Isis 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
here we have an op-amp which inverting
input is connected to the ground or zero
volts and non-inverting input is
connected to voltage input V in so this
is actually a comparator that compares
the non-inverting input to the inverting
input or in this case the input voltage
V into the ground or the zero volts so
when the V in value is below zero volts
the output of the comparator will be
negative
see and if the input voltage is above
zero 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 V in and the non-inverting
input we will get the Schmitt trigger
now the output will switch from VCC
minus to VCC plus when the voltage at
the a node will cross the zero volts
that means that now by adjusting the
values of the resistors we can set at
that value of the V in input the switch
will occur using the following equation
we get this equation with the following
relationships the current I through this
line equals V in minus VA divided by r1
as well as VA minus V out divided by r2
so if we replace the VA with zero as we
need that value for the switch to occur
we will get the final equation for
example if the output is minus 12 volts
and V in input is negative and rises the
speech from minus 12 volts to +12 volts
will occur at 6 volts according to the
equation and the values of the resistors
and vice versa
when the V in input is high and declines
the switch from plus 12 volts to minus
12 volts will occur at minus 6 volts in
order to get two different non
symmetrical thresholds we can use this
circuit of an inverting single powered
Schmitt trigger here the V ref bolta CH
is the same as the VCC of the op amp now
because the V in input is connected to
the inverting input of the op amp when
its value will reach the upper threshold
the
output will switch off to zero volts and
when its value will decline to the lower
threshold the output will switch on to
five volts
here's an example of how we can
calculate the threshold the V ref and
the VCC will be five volts and 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 zero volts our
circuit will look like this
a simple voltage divider and the value
of the VA will be one point sixty six
volts this means that the V in input
needs to decline below that value in
order the output to switch on to five
volts now with these five volts at the
output the circuit will look like this
the value of the VA will be 3 point 33
volts this means that the V in input
needs to rises above that value in order
to output to switch off to zero volts we
can also build a Schmitt trigger using
transistors you can learn how the
transistor Schmitt trigger works in my
next tutorial click here to visit it on
my official website thanks for watching
and for more tutorials visit my official
website how to measure onyx comm