Comparator Circuits: Threshold Detection and Schmitt Hysteresis

A comparator answers a single question: is voltage A higher than voltage B? Its output is a digital-style high or low. Comparators are used in battery-low alarms, light- and temperature-activated switches, zero-crossing detectors and inside ADCs. They look like op-amps, but they are built for a different job. This tutorial explains how to use a dedicated comparator, why noisy inputs cause output chatter, and how to calculate hysteresis to cure it.

How a Comparator Works

If V+ > V-  : output HIGH
If V+ < V-  : output LOW
(no feedback network: the device runs open-loop)

One input carries the signal and the other a reference voltage, often from a resistor divider. Connecting the signal to the + input makes the output go high when the signal rises above the reference; connecting it to the - input inverts that behaviour.

Comparator or op-amp?

Feature	Op-amp (e.g. LM358)	Comparator (e.g. LM393)
Designed for	Linear operation with negative feedback	Open-loop switching
Output stage	Push-pull	Often open-collector, needs pull-up
Recovery from saturation	Slow	Fast
Speed	Limited by slew rate	Response time in the order of a microsecond for the LM393
Output logic level	Set by its own supply	Set by the pull-up voltage, can differ from supply

An op-amp can be used as a slow comparator in simple projects, but a real comparator switches faster and cleaner. The LM393 is a dual comparator that runs from a single supply between about 2 V and 36 V, and its input range includes ground.

The Open-Collector Output

The LM393 output is a transistor that can only pull the output low. When the output should be high, the transistor turns off and the output floats, so you must add a pull-up resistor to the voltage you want as the logic high. This is useful: a comparator powered from 12 V can produce a clean 3.3 V or 5 V logic signal simply by connecting its pull-up to that rail. Several open-collector outputs can also share one pull-up to form a wired-AND.

A pull-up of 1 kΩ to 10 kΩ is typical. Lower values give faster rising edges but more current when the output is low: with 1 kΩ to 5 V, the output transistor sinks 5 mA.

Chatter and the Schmitt Trigger

Real signals contain noise. If a slowly rising signal sits near the threshold, noise makes it cross back and forth many times, and the output switches rapidly. A relay connected to such a comparator buzzes. The fix is hysteresis: two thresholds instead of one. The output switches high-to-low at an upper threshold and low-to-high at a lower one, so small noise cannot flip it back. A comparator with hysteresis is called a Schmitt trigger.

Hysteresis is created with positive feedback: a resistor from the output to the + input, so that the output state shifts the reference slightly.

        Vcc                Vcc
         |                  |
       [R1]               [Rpu] pull-up
         |                  |
  +------+--------(+)       |
  |      |           LM393 -+---- Vout
[R3]   [R2]  Vin---(-)      |
  |      |                  |
  |     GND                 |
  +-------------------------+

Output HIGH (R3 effectively in parallel with R1):
   VTH = Vcc x R2 / (R2 + R1||R3)
Output LOW (R3 effectively in parallel with R2):
   VTL = Vcc x (R2||R3) / (R1 + R2||R3)
Hysteresis = VTH - VTL

These formulas assume the pull-up resistor is much smaller than R3, so the output high level is close to Vcc.

Worked example

Vcc = 5 V, R1 = R2 = 10 kΩ, R3 = 100 kΩ, Rpu = 1 kΩ. Without R3 the threshold would be 2.5 V.

R1 || R3 = (10 x 100) / 110 = 9.09 kΩ
VTH = 5 x 10 / (10 + 9.09) = 2.62 V

R2 || R3 = 9.09 kΩ
VTL = 5 x 9.09 / (10 + 9.09) = 2.38 V

Hysteresis = 2.62 - 2.38 = 0.24 V

Because the signal goes to the - input, the output goes low when Vin rises above 2.62 V and returns high only when Vin falls below 2.38 V. Any noise smaller than 0.24 V peak-to-peak can no longer cause chatter. For more hysteresis, reduce R3; with R3 = 47 kΩ the thresholds become about 2.74 V and 2.26 V.

Practical Applications

Battery-low indicator: compare a divided battery voltage with a Zener or TL431 reference; add hysteresis so the alarm does not flicker as the battery recovers under changing load.
Dark-activated light: an LDR divider on one input and an adjustable potentiometer reference on the other.
Thermostat: an NTC thermistor divider against a set-point; hysteresis stops the heater relay cycling rapidly.
Square-wave shaping: clean up a slow or noisy sine wave into sharp edges for counting.

Troubleshooting

Output never goes high, or floats at a random level: the pull-up resistor is missing.
Output oscillates near the threshold: add hysteresis and a 100 nF decoupling capacitor near the IC.
Thresholds differ from calculation: the pull-up is not small compared with R3, or the output high level is below Vcc.
Comparator ignores inputs near Vcc: the LM393 input range only extends to about 1.5 V below Vcc; keep signals within it.

Summary

A comparator turns an analogue comparison into a clean logic level. Dedicated parts like the LM393 switch quickly but need a pull-up resistor on their open-collector output. Add positive feedback to create two thresholds, calculate them with the parallel-resistor formulas, and choose the hysteresis to exceed the noise on your signal. This single technique fixes most 'flickering relay' problems in sensor projects.