12. Transducers

A transducer converts physical phenomena into electrical signals (or vice versa). Every measurement chain starts with a transducer; without one, your DMM or scope has nothing to measure.

12.1 Active vs passive

Active transducers generate their own electrical output. Examples: thermocouple, piezoelectric crystal, photovoltaic cell. No external excitation needed.

Passive transducers modulate an external excitation. Examples: strain gauge (resistance changes, you must apply a voltage), thermistor (resistance changes, applied current), capacitive sensor (capacitance changes, applied AC).

12.2 Resistive transducers

Strain gauge. A foil pattern bonded to a substrate; mechanical strain stretches the foil and changes its resistance. A typical strain gauge has 120 $\Omega$ nominal resistance with a "gauge factor" of 2.0: $\frac{\Delta R}{R} = GF \cdot \epsilon$ where $\epsilon$ is the strain. For typical strains (microstrain to millistrain), $\Delta R/R$ is in parts per million, so a Wheatstone bridge with high-gain amp and good shielding is needed.

Used in load cells (combine 4 strain gauges in a full bridge for force/weight), pressure sensors, accelerometers, structural health monitoring.

Thermistor. A ceramic semiconductor whose resistance changes strongly with temperature. NTC (negative coefficient) thermistors are common: resistance falls as temperature rises. Highly nonlinear (Steinhart-Hart equation gives the relationship); sensitive (5-10% per °C); cheap. Used everywhere from PCB temperature monitoring to oven controllers.

Photoresistor (LDR / CdS cell). Cadmium sulfide film whose resistance drops in light. Slow (hundreds of ms response), but cheap. Used in night-light sensors, low-end exposure meters.

RTD (Pt100, Pt1000). Platinum wire whose resistance increases linearly with temperature. Very accurate (0.1°C is achievable); slow (thermal mass); expensive. The standard for industrial temperature measurement.

12.3 Capacitive transducers

The capacitance $C = \epsilon_0 \epsilon_r A / d$ varies with area $A$, separation $d$, or dielectric $\epsilon_r$.

Pressure sensors. A diaphragm forms one plate of a capacitor; pressure flexes it, changing $d$. Used in TPMS (tire pressure), medical, industrial.

Level sensors. A vertical capacitor whose dielectric is partly liquid: as the liquid rises, the average $\epsilon_r$ rises, so $C$ rises.

Touch screens. Capacitive: your finger forms a capacitor with electrodes underneath. Self-capacitance and mutual-capacitance schemes localize touches. Why they don't work with gloves (the dielectric losses are different, breaking detection).

12.4 Inductive transducers

LVDT (Linear Variable Differential Transformer). A primary coil and two secondaries on a hollow form, with a movable ferrite core. Position of the core determines coupling; output is the difference between the two secondaries. At center, output is zero (null); off-center, output magnitude is proportional to displacement, phase indicates direction.

Used in machine tool position feedback, hydraulic actuator position, aircraft control surface position. Very rugged, contactless, and high-resolution (sub-micron with quality electronics).

Eddy-current proximity sensor. A coil drives a high-frequency current; nearby conductive material induces eddy currents that load the coil; the load is detected. Used to detect metal at a distance (proximity switches in industry), and in tachometers (gear teeth pass the sensor).

12.5 Thermocouples (Seebeck effect)

A junction of two dissimilar metals develops a small voltage when its temperature differs from a reference junction. The Seebeck coefficient $\alpha$ is the constant of proportionality: $V = \alpha (T_{hot} - T_{ref})$ For type K (chromel-alumel), $\alpha \approx 41\,\mu\text{V}/°\text{C}$. Range: -200 to +1250 °C.

The reference junction is the hidden complication: traditionally an ice bath at 0 °C; modern instruments use cold-junction compensation with a thermistor or RTD measuring the terminal block's temperature, with software correcting for the offset.

Thermocouple types: J (iron/constantan, 0-750 °C), K (chromel/alumel, -200-1250 °C), T (copper/constantan, -200-350 °C), N, S, R, B (platinum-based, very high temp, expensive).

For amplification, the Seebeck voltage is small (~40 μV/°C). A thermocouple amp like the AD594, AD595, MAX31855, or LTC2986 handles cold-junction compensation and amplification together.

12.6 Piezoelectric transducers

Crystals like quartz, tourmaline, lead zirconate titanate (PZT), and BaTiO3 generate voltage when stressed (and vice versa: apply voltage and they deform). The piezoelectric effect is bidirectional.

Used as:

• Microphones. Cheap mics for embedded use.
• Accelerometers. Mass on a piezoelectric crystal; force from acceleration produces voltage. Used in earthquake monitoring, machinery vibration analysis, industrial test.
• Ultrasonic transducers. Drive the crystal at MHz; it emits ultrasound. Reverse: incoming ultrasound makes it generate voltage. Sonar, parking sensors, medical ultrasound.
• Quartz crystal oscillators. Drive electrically; mechanical resonance gives a stable frequency. Every microcontroller's clock crystal works this way.
• Spark lighters. Mechanical impact on a piezo gives a high-voltage spark.

Output is high-impedance and AC (charge dissipates through bias resistors); needs a charge amplifier or high-impedance buffer.

12.7 Hall-effect sensors

A Hall sensor is a semiconductor through which a bias current flows; in the presence of a magnetic field perpendicular to the current, charge carriers deflect to one side, producing a transverse voltage. Output is proportional to magnetic flux density.

Used in: current sensors (detect the field around a wire), brushless DC motor commutation (rotor position), ABS wheel sensors, joysticks, magnetic encoders.

12.8 Semiconductor temperature sensors

PN junctions have a temperature-dependent forward voltage of about -2 mV/°C at 1 mA. Sensors like the LM35 (10 mV/°C, calibrated), LM75 (digital I2C output), TMP102, DS18B20 (1-Wire) use this principle. Cheap, accurate, simple to interface. Used in CPU thermal monitoring, datacenter sensors, IoT environmental sensors.

12.9 Optical encoders

A disk with alternating opaque and transparent segments rotates between an LED and a photodetector. Each transition is a pulse. Incremental encoders give pulses; you count them to get position (relative). Absolute encoders have a code pattern (Gray code) that uniquely identifies position. Used in motor shafts, robotic joints, CNC machines.

12.10 Signal conditioning

Raw transducer signals are usually unsuitable for direct digitization. Signal conditioning between sensor and ADC includes:

• Amplification. Strain gauges produce mV; thermocouples produce µV. Amplify to a useful level (often using instrumentation amps like AD620 or INA126).
• Filtering. Low-pass to remove high-frequency noise; sometimes notch to remove 50/60 Hz line interference.
• Linearization. Some sensors (thermistors, thermocouples) have nonlinear response; linearize in hardware (precision resistor networks) or software (lookup tables, polynomial fits).
• Isolation. Optocouplers or transformer isolation amplifiers separate the sensor's ground from the digitizer's ground. Critical for high-voltage measurements, medical (patient safety), and industrial environments with ground loops.
• Excitation. Constant current or constant voltage source for resistive sensors.
• Cold-junction compensation. For thermocouples, embedded RTD or thermistor at the connector measures the reference junction's temperature.

The Texas Instruments and Analog Devices catalogs are full of integrated signal-conditioning chips: dedicated load-cell amps, RTD converters, thermocouple front-ends.