Look at Figure 1 and you’ll see a simple Schmitt trigger circuit. The nice thing about Schmitt trigger circuits is the
fact that you can set up two voltage
level trigger points — each of which
can be selected simply by adjusting a
few resistor values. In Figure 1, it’s
the combination of the resistors R1,
R2, and R3 which sets up the upper
trip point (UTP) and lower trip point
(LTP). Notice also in Figure 1 that
the combination of the R1 and R2
resistors forms a simple voltage
divider circuit.
Let’s assume the input voltage
level on the inverting terminal (pin 4)
in Figure 1 is 0V. That means the
output voltage (pin 2) is + 5 volts. Remember I said that
resistors R1 and R2 in Figure 1 form a voltage divider
circuit? Well, with the output at +5V, R3 (at pin 2) is
“seeing” 5V, and is technically in parallel with R1.
If we use the voltage divider formula, we can calculate
the UTP as shown.
Now that we have the UTP set to 1.88V, the input
signal voltage must rise above this point in order for the
output voltage of the comparator to swing to the opposite
level (zero volts). Okay, what happens when the input
The Schmitt trigger is one of the
most widely used circuits in the
electronics industry. If you have
to clean up a noisy signal, catch
a slow rising voltage at specific
points, or need to eliminate
unwanted triggering in your
robot line follower circuit, then
the Schmitt trigger is the right
tool for the job.
The Schmitt Trigger
and How It Works
By Roger D. Secura
60 SERVO 03.2014
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article/march2014_Secura.
R2 UTP = x Vcc
R1//R3 + R2
10 UTP = x 5
26. 50
10 UTP = x 5
33//33 + 10
10 UTP = x 5
1 + 10 1 1
33
+
33
10 UTP = x 5
1 + 10.0303+.0303
FIND UTP
UTP = 1.88 volts
FIGURE 1.
