A simple way to demonstrate this concept is to place a light sensor on a breadboard and cover the sensor with your hand. Although the environment remains relatively unchanged, the output voltage from the sensor could change enough that it could be detected by a display or control system. Thus, an instrumentation system is one that translates a physical parameter (measurand) into an electrical signal.
The measurand could be temperature, pressure, light, position, force, etc. The sensor responds to the measurand. The transducer converts the sensor response into some useful form. In the basic circuits discussed here, the response is some electrical parameter such as voltage, current, resistance, capacitance, or pulse frequency. The value shown on the display is not temperature or light. It is the interpretation of the electrical signal produced by the sensor.
For example, consider a temperature-sensitive resistor. The resistance of the resistor changes with temperature, but you cannot easily interpret the resistance as a temperature unless you have a suitable display circuit. If you connect the resistor to a circuit that converts its resistance into a voltage, then you can use that voltage as the basis for displaying a temperature value. The sensor output may need signal conditioning such as amplification to increase the strength of a very small sensor output or filtering to reduce unwanted electrical noise. An analog-to-digital converter samples the signal after signal conditioning to produce digital values that a controller can transmit to a display.
If you draw out this signal path from the measurand to the display, then each part of the chain will be easier to visualize. On a piece of paper, draw five boxes labeled: Physical Input, Sensor, Signal Conditioning, Conversion, and Display. Draw arrows between the boxes to indicate the flow of the signal. Choose a device you know how to operate, such as a digital thermometer. Label each box by describing the process that occurs in each part of the signal chain.
In the case of the digital thermometer, the physical input is temperature, the sensor is some component that changes its electrical characteristics with temperature, signal conditioning is a circuit that manipulates the sensor output, the conversion is an analog-to-digital converter that converts a voltage into a number, and the display is a device that shows a number in degrees. Working through this process will help ensure that each individual wire and component is understood in terms of the function it serves.
One reason for using the term transducer is because you don’t always measure the measurand directly. For example, if a sensor is producing 0.8 V, that doesn’t mean it’s measuring 0.8 degrees, 0.8 lux, or 0.8 mm. You can see in the specification sheets for some sensors that they provide an equation that relates their output to the measurand (e.g., 10 mV/°C). The sensor output is also subject to gain, zero offset, and scaling from the rest of the circuit. When troubleshooting an electronic system, you should verify the supply voltage, ground, pinout, output type, and measurement range before assuming the sensor itself is defective.
It is easy to observe the above procedure if you have a low-voltage sensor module with known limits and a digital multimeter. Be sure to read the limits of the sensor module and verify the supply polarity. Connect the multimeter to the appropriate DC voltage scale, connect the black lead to the ground terminal, and connect the red lead to the sensor output. Record the output under a set of fixed conditions. Then, change only the physical input (place a light sensor in a bright environment, for example). Warm a temperature sensor slightly (not exceeding its temperature limit). Perform several measurements and record whether the output changes in the same direction and by the same magnitude.
You will know the above has been useful if you can articulate all of the quantities involved in the above experiment: what is being measured, what electrical property changes, where the output is checked, and why the display value may differ from the raw sensor output. You should keep a neat notebook with units, multimeter scale, supply voltage, conditions, and output recorded. Under these circumstances, an erroneous reading is not an unknown value. It is a value that you can investigate and correct.