Heat Sensors and Smoke Detectors

Heat sensors are designed to detect a rapid rise in temperature. They also
have a feature that sets off an alarm when a fixed temperature is reached.
Smoke detectors do not react to heat but use one of two common sensor
designs to detect smoke. An ionization type of detector forms an electrical
path inside a small chamber with a very small amount of radioactive material. 

When smoke enters the chamber, the particles attach themselves to the
ions and change the electrical current flow. A photoelectric type of detector
works by using a photoelectric cell and a light source. The light does not
usually reach the photoelectric cell, but when smoke is present the light is
dispersed and reaches the photoelectric cell, triggering the alarm. The main
difference between the two types is photoelectric types are more sensitive to
large particles and ionization types are more sensitive to small particles.

Modern home design should include at least one of each type. Smoke and
heat detectors should be located in each sleeping area and on each story of
the home and placed on the ceiling or on the wall 6''–12'' from the ceiling.

Remember that heat and smoke detectors operate on different types of technology. Heat detectors react to abrupt changes and go into an alarm condition when a temperature changes rapidly or reaches a fixed value. Smoke alarms do not react to heat but go into an alarm condition when smoke enters the sensor area. The two main types of smoke detectors are ionization and photoelectric.