In basic transducer theory and instrumentation, which device directly converts applied mechanical pressure or stress into measurable electrical energy (voltage or charge) suitable for sensing and control?

Introduction / Context:
Engineers frequently need to convert a physical quantity into an electrical signal that can be measured, recorded, or used for feedback control. Pressure, force, and acceleration are common mechanical stimuli. The device class that turns mechanical stress directly into electrical energy is crucial in mechatronics, structural health monitoring, automotive knock sensing, and medical devices such as ultrasound transducers. This question tests recognition of the correct transduction principle and the appropriate example device.

Given Data / Assumptions:

• We are interested in conversion from mechanical pressure/stress to an electrical output.
• The options include devices known for other energy conversions (light to electricity, chemical to electricity).
• Room-temperature, passive sensing behavior is implied (no exotic excitation required to create the signal).

Concept / Approach:
The piezoelectric effect produces charge separation in certain crystals and ceramics when they are mechanically stressed. The generated charge Q is approximately proportional to applied force F (Q = d * F, where d is the piezoelectric charge constant). With appropriate circuitry, this charge appears as a measurable voltage across the element. By contrast, a solar cell relies on the photovoltaic effect (light to electricity), a photodiode converts light intensity to current, and a nickel–cadmium (Ni-Cd) cell is an electrochemical energy storage device (chemical to electrical). Only the piezoelectric element natively converts mechanical stress into electrical energy.

Step-by-Step Solution:
Identify the required conversion: mechanical pressure → electrical energy.Recall device principles: piezoelectric (stress→charge), photovoltaic (light→current), photodiode (light→current), Ni-Cd (chemical→electric).Match the only device with stress-to-electric conversion: piezoelectric device.Select option “piezoelectric device.”

Verification / Alternative check:
Common pressure sensors and accelerometers use piezoelectric disks or beams. When you tap a piezo buzzer element, it produces a voltage spike measurable on a voltmeter or oscilloscope, confirming mechanical-to-electrical conversion without illumination or chemical reaction.

Why Other Options Are Wrong:
Solar cell: converts photons to electrical energy, not pressure. Photodiode: detects light, not stress. Ni-Cd cell: provides electrical energy from electrochemical reactions; pressure does not generate its output. “None of the above” cannot be correct because a valid device exists (piezoelectric).

Common Pitfalls:
Confusing piezoelectric sensing with strain gauges (resistance change, not direct charge generation). Forgetting that photonic devices require light. Assuming batteries convert any stimulus to electricity; they only store/release chemical energy.

Final Answer:
piezoelectric device