Analog Input

Originally written on July 7, 2014 by Tom Igoe
Last modified on August 27, 2016 by Tom Igoe

Introduction

This is an introduction to basic analog input on a microcontroller. In order to get the most out of it, you should know something about electrical circuitswhat a microcontroller is and what it can do, and digital input and output on a microcontroller.

These videos will help in understanding this topic:

Analog Input

While a digital input to a microcontroller can tell you about discrete changes in the physical world, such as whether the cat is on the mat, or the cat is off the mat, there are times when this is not enough. Sometimes you want to know how fat the cat on the mat is. In order to know this, you’d need to be able to measure the force the cat exerts on the mat as a variable quantity. When you want to measure variably changing conditions like this, you need analog inputs. An analog input to a microcontroller is an input that can read a variable voltage, typically from 0 volts to the maximum voltage that powers the microcontroller itself.

Many transducers are available to convert various changing conditions to changing electrical quantities. There are photocells that convert the amount of light falling on them to a varying resistance; flex sensors that change resistance as they are bent; Force-sensitive resistors (FSRs) that change resistance based on a changing force applied to the surface of the sensor; thermistors that change resistance in response to changing heat; and many more. Related video: Resistors, variable resistors, and photocells

In order to read these changing resistances, you put them in a circuit and pass a current through them, so that you can see the changing voltage that results. There are a few variations on this circuit. The simplest is called a voltage divider. Because the two resistors are in series voltage at the input to the microcontroller is proportional to the ratio of the resistors. If they are equal, then the input voltage is half the total voltage. So in the circuit below. if the variable resistor changes (for example, if it’s a flex sensor being bent), then the voltage at the input changes.  The fixed resistor’s value is generally chosen to complement the variable resistor’s range. For example, if you have a variable resistor that’s 10-20 kilohms, you might choose a 10 kilohm fixed resistor.

analog in schematic

In the diagram below, you use a potentiometer,  which is a variable resistor with three connections. The center of the potentiometer, called the wiper,  is connected to the microcontroller. The other two sides are attached to power and ground. The wiper can move from one end of the resistor to the other. In effect, it divides the resistor into two resistors and measures the resistance at the point where they meet, just like a voltage divider. Related video: Connecting the potentiometerPotentiometers’ SymbolMeasure a potentiometer’s variable resistance

potentiometer schematic

Since a microcontroller’s inputs can read only two values (typically 0 volts or 5 volts), an analog input pin needs an extra component to read this changing, or analog voltage, and convert it to a digital form. An analog-to-digital converter (ADC) is a device that does this. It reads a changing input voltage and converts it to a binary value, which a microcontroller can then store in memory.

ADCs are a common tool, and many microcontrollers have ADCs built in to them. The Arduino has an ADC built into it, attached to the analog input pins.

The ADC in the Arduino can read the input voltage at a resolution of 10 bits. That’s a range of 1024 points. If the input voltage range is 0 to 5 volts, that means that the smallest change it can read is 5/1024, or 0.048 Volts. When you take a reading with the ADC using the analogRead() command, the microcontroller stores the result in memory. It takes an int type variable to store this, because a byte is not big enough. A byte can hold only 8 bits, or a range from 0 to 255.

The resolution of an ADC changes from one model to another. However, most microcontrollers with ADCs on board typically have 10-bit resolution.

The command in Arduino is the analogRead() command, and it looks like this:

sensorReading = analogRead(pin);
  • Pin is the analog input pin you are using;
  • sensorReading is an integer variable containing the result from the ADC.

The number produced in sensorReading is will be between 0 and 1023. Its maximum may be less, depending on the circuit you use. a potentiometer will give the full range, but a voltage divider where one of the resistors is fixed will not.

Decoupling Analog Inputs

If you find the readings from your analog inputs are inconsistent (for example, you see changes on one channel when the sensor on a different channel is the one sensing action), it helps to decouple your input circuit. Decoupling means smoothing out the dips and spikes going into the circuit from the rest of your microcontroller circuit. To do this, place a small capacitor (0.1 microfarad is good) between voltage and ground as close to where the analog input connects to voltage, like so:

analog input with decoupling capacitor

A capacitor used this way is referred to as a decoupling capacitor. You’ll see them a lot in electronic circuits. Think of them as tiny surge protectors.

The analog inputs on an Arduino (and in fact, on most microcontrollers), are all connected to the same ADC circuit, so when the microcontroller has to switch the ADC’s input from one pin to another when you try to read two pins one after another. If you read them too fast, you can get unstable readings. You can also get more reliable readings by introducing a small delay after you take an analog reading. This allows the ADC time to stabilize before you take your next reading.

Here’s an example of how to read three analog inputs with minimal delay and maximum stability:

 sensorOne = analogRead(A0);
 delay(1);
 sensorTwo = analogRead(A1);
 delay(1);
 sensorOne = analogRead(A2);
 delay(1);

Analog and digital inputs are the two simplest ways that a microcontroller reads changing sensor voltage inputs. Once you’ve understood these two, you’re ready to use a variety of sensors.