Consider the following transducers:

1. LVDT

2. Thermocouple

3. Piezo electric

4. Photovoltaic cell

Which of the above are Active transducers?

Active transducer:

An active transducer is a self-generating type transducer that has the ability to generate electrical voltage or current as of its output without using any external source of power

e.g. Photovoltaic cell, thermocouple, Piezoelectric, tacho generators.

Passive transducer:

These are externally powered transducers, that induce variation in the parameters associated with the electrical circuits, with the variation in the applied input signal. 

e.g. potentiometer, LVDT, strain gauge, thermistor, etc.