Servo - March 2014

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 gap.

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 works.

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