Ionization Smoke Detectors/Sensors Principle

These devices use a small radiation source, Americium-241 or other sources, which emits alpha particles that
ionize air molecules between two electrically charged electrodes. With the application of a
DC voltage .to these electrodes, a small ionization current flows within the chamber. As smoke
enters the chamber, a decrease in ionization current results.

This current is converted into a signal voltage by a transimpedance circuit. In a detector, when this signal voltage drops below a preset level, an alarm is produced. In a sensor, this signal voltage goes into an 8-bit A to D (analog to digital) converter. A digital representation of this signal voltage is then transmitted to the fire alarm panel for further processing.