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Sensors convert physical quantities (temperature, pressure, light, motion, force) into electrical signals that engineering systems can measure and respond to. Feedback is the process of using sensor measurements to adjust a system's behavior -- comparing the actual output to the desired output and making corrections. A thermostat is a classic feedback system: a temperature sensor measures the room temperature, compares it to the set point, and turns the heater on or off to reduce the difference. Without sensors and feedback, engineering systems operate "blindly," unable to adapt to changing conditions or correct errors.
Build a simple feedback system: a light sensor (photoresistor) connected to an LED brightness controller. As ambient light increases, the LED should get brighter to compensate (or dimmer to save energy, depending on the design goal). Students experience the feedback loop directly: sensor reads environment, controller compares to target, actuator adjusts output. Then disrupt the system (cover the sensor, change the target) and observe how it responds.
Imagine driving a car with your eyes closed. You set the steering wheel straight ahead and press the gas pedal, hoping you stay in your lane. Of course, this is absurd -- without visual feedback, you would crash almost immediately. Your eyes are sensors (measuring the car's position relative to the lane), and your hands on the steering wheel are the actuator (correcting deviations). The entire system works because of feedback: continuously measuring the actual state, comparing it to the desired state, and adjusting.
Sensors are the eyes and ears of engineering systems. A temperature sensor (thermistor, thermocouple, or RTD) converts heat into an electrical signal. A pressure sensor converts mechanical pressure into voltage. A light sensor (photodiode or photoresistor) converts brightness into a measurable signal. An accelerometer detects motion and vibration. Each sensor translates a physical phenomenon into an electrical signal that a controller can process.
Every sensor has limitations that engineers must understand. Accuracy is how close the reading is to the true value -- a cheap thermometer might be accurate to plus or minus 2 degrees. Precision is how repeatable readings are -- the same temperature measured ten times should give ten similar readings. Range defines the minimum and maximum the sensor can detect. Response time is how quickly the sensor reacts to changes. Choosing the right sensor means matching these characteristics to the application's requirements.
Feedback connects sensors to action. In a closed-loop system, the sensor measurement is continuously compared to a set point (the desired value), and the difference (called the error) drives a corrective action. If the room is too cold (error is negative), turn on the heater. If it is too warm (error is positive), turn off the heater. The heater's action changes the temperature, which the sensor detects, which changes the error, which changes the heater's action -- this circular flow of information is the feedback loop.
The alternative is an open-loop system, where the output is not measured or compared to anything. A microwave oven runs for a set time at a set power with no feedback about the food's actual temperature. This works adequately because the process is predictable and the consequences of imprecision are low (slightly overcooked leftovers). But for applications where precision matters -- industrial manufacturing, climate control, autonomous vehicles, medical devices -- feedback is essential. The ability to measure, compare, and correct is what transforms a blind process into an intelligent one.