{"id":1069,"date":"2014-07-22T07:44:50","date_gmt":"2014-07-22T11:44:50","guid":{"rendered":"https:\/\/itp.nyu.edu\/physicalcomputing\/?page_id=1069"},"modified":"2022-06-23T08:00:52","modified_gmt":"2022-06-23T12:00:52","slug":"sensors-the-basics","status":"publish","type":"page","link":"https:\/\/itp.nyu.edu\/physcomp\/lessons\/sensors-the-basics\/","title":{"rendered":"Sensors: the Basics"},"content":{"rendered":"\n

<\/span>Introduction<\/span><\/h2>\n\n\n\n

Sensors<\/strong> convert various forms of physical energy into electrical energy, allowing microcontrollers to read changes in the physical world.<\/p>\n\n\n\n

The simplest sensors read changes in mechanical energy, usually by moving electrical contacts. For example the pushbutton or switch (related video)<\/a> converts mechanical energy (e.g, your finger’s press) into electrical energy by closing a connection between two metal contacts. The potentiometer (related video)<\/a> shown in Figure 1 and Figure 2 is another sensor that reads mechanical energy changes: a metal contact called a wiper slides along a resistor, effectively short circuiting the resistor (related video)<\/a> into two halves and creating a voltage divider circuit.<\/p>\n\n\n\n

\"Photo<\/a>
Figure 1. A potentiometer broken open, showing the carbon substrate and wiper.<\/figcaption><\/figure><\/div>\n\n\n\n
\"Drawing<\/a>
Figure 2. Inside a potentiometer, the wiper, attached to the center contact, slides along the carbon resistor, forming effectively two resistors in series.<\/figcaption><\/figure><\/div>\n\n\n\n

Although switches and pushbuttons typically only read an on state or an off state, most other sensors can read a wide range of possible states. In other words, they’re analog sensors<\/strong>, because they read a variable change. Whenever you use a new sensor, the first thing you need to understand is what the range is that it can read and deliver. When you’re using your sensor in an application, you need to know the range of possible values your application requires. Do you just need high, medium, and low (3 possible values), or a range from 0 to 10? Do you need a 100-point range? The range that you need depends on what your user can perceive. For example, if you’re making a lighting dimmer, and your user can only perceive about a dozen different light levels, then you don’t need to give her a sensor that can give her a 1000-point range of control.<\/p>\n\n\n\n

<\/span>Resistive Sensors<\/span><\/h2>\n\n\n\n

Related Video: Sensors – Survey 1<\/a><\/p>\n\n\n\n

\"Resistive<\/a>
Figure 3. Resistive sensors. Stretch sensor, top; flex sensor, middle and bottom; force-sensing resistor, bottom<\/figcaption><\/figure><\/div>\n\n\n\n

Many sensors work by converting the energy they read into a changing electrical resistance by using a variably resistive material at their heart. Examples of resistive sensors are shown in Figure 3. For example, force sensors and stretch sensors are made of a partially conductive rubber. When the rubber is stretched or compressed, the resistance change. Photoresistors or light dependent resistors change their resistance when exposed to a change in light energy. In order to read changes in resistance, you typically place these sensors in a voltage divider circuit , which converts the resistance change into a changing voltage.<\/p>\n\n\n\n

<\/span>Optical Sensors<\/span><\/h2>\n\n\n\n

Related video: Light Sensors<\/a><\/p>\n\n\n\n

Light is used in many sensors in a variety of ways. Light-emitting diodes, light-dependent resistors, and phototransistors  can be combined to sense dust, measure distance, or determine reflected color.  Light is also used in some ranging sensors to determine distance from a sensor to a target.<\/p>\n\n\n\n

<\/span>Ranging Sensors<\/span><\/h2>\n\n\n\n

Related video: Ranging Sensors<\/a><\/p>\n\n\n\n

\"Ranging<\/a>
Figure 4. Ranging sensors: ultrasonic ranger, top; infrared ranger, bottom.<\/figcaption><\/figure><\/div>\n\n\n\n

Many sensors measure movement or distance indirectly, by sending out a pulse of light or sound and reading the reflected signal when it bounces off a target as illustrated in Figure 5. These are called ranging sensors<\/strong> (Figure 4) because they read a range of distance.<\/p>\n\n\n\n

\"Drawing<\/a>
Figure 5. Ranging sensors work by bouncing energy off the target and reading the reflection.<\/figcaption><\/figure><\/div>\n\n\n\n

<\/span>MEMS Sensors<\/span><\/h2>\n\n\n\n

Related video: MEMS Sensors<\/a><\/p>\n\n\n\n

Still other sensors work by converting the energy they read into a change in capacitance. For example, accelerometers and other miniaturized electromechanical (MEMS) sensors typically have a tiny moving conductive mass at their core, suspended on tiny springs and surrounded on both sides by electrical contacts. Because the conductive mass is parallel to the outer contacts, a capacitance builds up between the contact. When the mass is moved, the capacitance changes between the two sides, effectively creating two variable capacitors as illustrated in Figure 6. That variable capacitor is then placed in a resistor-capacitor circuit to convert the change in capacitance into a changing voltage.<\/p>\n\n\n\n

\"Diagram<\/a>
Figure 6. MEMS sensors often form a variable capacitor by moving a capacitive plate in between two other capacitive surfaces.<\/figcaption><\/figure><\/div>\n\n\n\n

<\/span>Digital Interface Sensors<\/span><\/h2>\n\n\n\n

Related video: Sensors – Interfacing<\/a><\/p>\n\n\n\n

Other than switches and variable resistors, most sensors you buy are integrated circuits that include the resistance-to-voltage or capacitance-to-voltage circuit and provide you with an analog voltage output. Some will even include an analog-to-digital converter and provide you with a serial data interface, either I2C<\/a>, SPI<\/a>, or asynchronous serial<\/a>, so that you can connect the sensor to the serial ports of your microcontroller. Still others will provide a changing pulse output, or pulse width modulation (PWM) output<\/a>, where the width of the pulse represents the sensor value.<\/p>\n\n\n\n

\"Photo
Figure 7. A collection of sensors with digital interfaces. Clockwise, from top: AS7341 spectrometer; LIS3DH accelerometer; VL53L0X Time-of-flight distance sensor; Max3010x particle detector. These sensors are on small breakout boards which can connect to a solderless breadboard. They all connect to a microcontroller using SPI or I2C.<\/figcaption><\/figure>\n\n\n\n

<\/span>Data Sheets<\/span><\/h2>\n\n\n\n

Related Video: Datasheets<\/a><\/p>\n\n\n\n

When you shop for sensors, you need to look at the data sheet to learn how they operate. The data sheet will usually include the following essential facts:<\/p>\n\n\n\n