Master the Basics: Compare signals

When working with analog signals in electronics, sometimes you don’t need to process every voltage level—you just need to know whether one signal is above or below a threshold. That’s where comparators come in.

What Is a Comparator?

A comparator is an operational amplifier (op-amp) used in an open-loop configuration—meaning no feedback is applied. It acts as a digital decision-maker that compares two voltages and outputs a binary result:

• If the non-inverting (+) input is higher than the inverting (-) input, the op-amp outputs its positive supply voltage.
• If the inverting (-) input is higher, it outputs its negative supply voltage.

This behavior makes comparators ideal for converting analog signals into simple high or low digital outputs.

How It Works: Voltage Comparison

Op-amps have very high open-loop gain (often over 100,000×). That means even the smallest voltage difference between inputs quickly drives the output to one extreme:

• A slightly positive input difference causes the output to rise toward the positive supply rail.
• A slightly negative difference pushes it toward the negative supply rail.

Effectively, comparators digitize the result of an analog comparison—outputting either a 1 (high) or a 0 (low).

Practical Example: Comparing a Signal to a Threshold

Let’s say you want to detect if a signal exceeds 2.5 volts:

1. Set a reference voltage at 2.5V (using a voltage divider, battery, or Zener diode).
2. Connect the reference to the inverting (-) input.
3. Feed your changing signal into the non-inverting (+) input.

As the input crosses 2.5V:

• If it's below 2.5V, the output stays low (e.g., 0V).
• If it's above 2.5V, the output jumps high (e.g., 5V).

This allows you to create a fast-acting switch or trigger system based on voltage.

Application: Using a Comparator with a Raspberry Pi

Most Arduino boards have built-in analog inputs. But Raspberry Pi boards do not—they only accept digital inputs. That’s where a comparator shines.

Let’s say you’re using a photodiode to detect light levels:

• Connect the photodiode to a circuit that outputs a voltage based on light intensity.
• Use a comparator to check if the light level (voltage) is above or below a threshold.
• Feed the comparator output into a digital input pin on the Raspberry Pi.

This setup acts as a one-bit analog-to-digital converter: the Pi sees 1 for bright and 0 for dark.

To fine-tune your threshold, use a potentiometer (adjustable resistor) to set the reference voltage.

Key Takeways

Comparators are powerful tools for bridging the gap between analog and digital. Whether you're monitoring light, temperature, or voltage, using an op-amp in comparator mode lets you make fast, logical decisions in your circuits—perfect for microcontroller integration, alarms, or switching systems.