signal goes above 1.88V and output
swings low? Well, R3 now sees 0V
at pin 2 and it is essentially tied to
ground. This, in turn, makes R2 in
parallel with R3. Again, we use the
voltage divider circuit formula to
determine the LTP.
The LTP is set at .94V. Now, any
signal voltage applied to the
inverting input terminal (pin 4) will
have to go below .94V before the output of the
comparator will swing in the opposite direction (+5V).
Notice — and this is important — how the output voltage
level of the comparator (0V or 5V) determines which
resistor (R1 or R2) is in parallel with R3. This gives the
Schmitt trigger circuit the ability to lock in either the UTP
or the LTP simply by having R3 in parallel with R1 or in
parallel with R2.
This means that any input signal into the comparator
will not affect the output voltage of the comparator until
it has risen above the UTP or fallen below the LTP. Figure
2 shows just how this works.
Again, let’s assume we have 0V at the negative input
terminal (pin 4) and +5V at the output terminal (pin 2).
Now, we apply a slow rising signal (0V to 5V) into the
inverting terminal. Once the voltage rises a little above
1.88V, the output voltage of the comparator will swing
negative (0V). This causes R2 and R3 to be in parallel and
this, in turn, sets up the LTP.
Now, the comparator’s output will not swing high
again until the input signal voltage goes a little below
.94V. During this time, the input signal voltage can
fluctuate up or down all it wants and it still won’t affect
the output of the comparator.
You’ll notice in Figure 2 a “voltage gap” between the
1.88 volts and .94 volts. This narrow
channel is commonly referred to as
a “hysteresis” gap (i.e., UTP minus
LTP). It serves as a “band gap” or
dead zone for ignoring any input
signals which fall into its constraints.
The selection of resistors R1, R2,
and R3 determines the hysteresis
I set up a test circuit (refer to
Figure 1 again) using an LM339N
IC and made a chart of the inputs
and outputs of the comparator in
order to show you how the UTP
and LTP actually work. Figure 3
shows the results of the test. You’ll
notice in Figure 3 that the input
signal (pin 4) rises from 0V to 1.2V
to 1.4V to 1.6V, and yet the output
voltage of the comparator (pin 2)
stays high ( 4.77V). It’s not until the
input signal reaches 1.89V that the
output (pin 2) swings to 0V. The
LTP (.98V) is now activated (i.e.,
R2//R3) and the output voltage is
locked in at 0V.
As you can see, the chart
reveals a simple fact about how the
Schmitt trigger comparator circuit
The input signal voltage has
absolutely no effect on the output
voltage of the comparator — at
least not until the signal reaches one or the other
predetermined set points (UTP or LTP).
So, to sum this up, there are two key points to
remember about the Schmitt trigger circuit:
1. The UTP and LTP of a Schmitt trigger circuit can be
determined by using a simple voltage divider network.
2. The output voltage level (high or low) of the
comparator is what determines which resistor (R1 or R2)
is currently in parallel with R3 and that, in turn,
determines which one of the two trip points is active at
any one time.
I hope this helps to clear up any confusion you might
have about the Schmitt trigger comparator circuit.
Real World Application
Okay, now that you’ve got a handle on how the
Schmitt trigger works, let’s take a look at a real world
application. Figure 4 is a simple circuit you can wire
together to control a single axis (east to west) motorized
solar panel that tracks the sun as it moves across the sky.
What the circuit will do is detect the amount of
sunlight shining onto two small ( 14 mm x 35 mm) solar
cells. The solar cell (i.e., sensor) receiving the most light
SERVO 03.2014 61
R2//R3 LTP = x Vcc
R1 + R2//R3
7. 67 LTP = x 5
33 + 7. 67
10//33 LTP = x 5
33 + 10//33
1 + 1
LTP = x 5 1
33 + 1 1
10 + 33
LTP = x 5 1
33 + .1303
LTP = .94 volts