Analog sensors work by producing a continuous output signal, usually voltage or current, that's directly proportional to the quantity being measured. In simpler terms, as the measured property changes (like temperature, pressure, or light), the sensor's output signal changes smoothly and proportionally.
Here's a breakdown of the process:
1. Sensing the Physical Quantity
The analog sensor contains a sensing element that reacts to the physical quantity it's designed to measure. Examples include:
- Thermistor: A resistor whose resistance changes with temperature.
- Strain Gauge: A resistor that changes resistance with applied strain (used for measuring force or pressure).
- Photodiode: A semiconductor diode that generates current when exposed to light.
2. Conversion to an Electrical Signal
The change in the sensing element's physical property (e.g., resistance, capacitance, etc.) is converted into an electrical signal. This conversion often involves:
- Wheatstone Bridge: A circuit used to measure small changes in resistance, commonly used with strain gauges and thermistors. The change in resistance unbalances the bridge, creating a voltage difference that is proportional to the change.
- Operational Amplifiers (Op-Amps): Used to amplify and condition the electrical signal, making it strong enough to be processed by other circuits. Op-amps can also convert current signals to voltage signals, or vice-versa.
3. Outputting a Continuous Signal
The amplified and conditioned electrical signal is then output as a continuous voltage or current. The magnitude of this output signal directly corresponds to the magnitude of the measured physical quantity. For example:
- If the sensor is measuring temperature, a higher temperature will result in a higher output voltage.
- If the sensor is measuring pressure, a greater pressure will result in a higher output current.
Key Characteristics of Analog Sensors
- Continuous Output: They provide a continuously varying signal.
- Direct Proportionality: The output signal is directly proportional to the measured quantity.
- Simple to Interface: While sometimes requiring signal conditioning, they are generally straightforward to connect to analog data acquisition systems.
Example: Temperature Measurement with a Thermistor
- A thermistor is used as the sensing element.
- The thermistor's resistance changes as the temperature changes.
- The thermistor is placed in a voltage divider circuit. As its resistance changes, the voltage at the midpoint of the voltage divider also changes.
- An op-amp amplifies this voltage change, providing a higher voltage signal that's easier to read.
- The output voltage is then a representation of the temperature.
Advantages of Analog Sensors
- High Resolution: They can detect very small changes in the measured quantity.
- Real-time Response: Changes in the measured quantity are reflected almost instantly in the output signal.
Disadvantages of Analog Sensors
- Susceptible to Noise: Analog signals are vulnerable to electrical noise, which can affect the accuracy of the measurement.
- Drift: Their calibration can drift over time, leading to inaccuracies.
- Signal Conditioning Required: Often require additional circuitry (amplifiers, filters) to produce a usable signal.
In conclusion, analog sensors provide a continuous electrical signal representing a physical quantity. They are fundamental components in many measurement and control systems, offering high resolution and real-time response, but require careful consideration of noise and drift.
