When smoke enters the space between the LED and the photodiode, the photocurrent decreases, which sets off an alarm.
The photodetectors consist of a light source, such as a light-emitting diode (LED) and a photodiode to produce a current proportional to the intensity of light from the LED. Smoke detectors are of two types: photodetectors and ionization detectors. The detector in a spectrograph is a photographic film or plate. The information processor in a visual color measuring system is the human brain. The most common input transducers convert chemical or physical information into a current, voltage, or charge, and the most common output transducers convert electrical signals into some numerical form.
A transducer is a device that converts chemical or physical information into an electrical signal or the reverse.
Skoog/Holler/Crouch Chapter 1 Principles of Instrumental Analysis, 6 th ed.
