How Does a Test Button on a Smoke Detector Work?

Optical Detectors

• Optical smoke detectors consist of an LED and an optical sensor. The LED generates a beam of light that is received by the sensor. In the presence of smoke, this beam from the LED is disrupted, and the sensor detects either a degraded beam or no beam at all, thus triggering the alarm. Detectors can be configured for different levels of sensitivity using this approach, tolerating slight drops in the intensity of the LED beam without triggering an alarm. These detectors work best on flaming fires, which may not generate a great deal of smoke compared with other types.

Ionization Detectors

• Ionization detectors use a very small amount of radioactive material passed between two electrodes to complete a circuit. Smoke will cause these particles to dissipate, ultimately breaking the circuit and triggering the alarm. This method of detection works best on smoldering fires, which generate a lot of smoke.

The Goal of Testing

• An effective detector test is one which most closely simulates a real smoke event, but which can also be reproduced multiple times in a quick way. To accomplish this, it is generally assumed that the physical properties and location of the detector would allow smoke to reach it in a fire. From there, a test can be performed by simply breaking the circuit. In an optical detector, this is done by shutting off power to the LED when the test button is pressed. In an ionization detector, the electrodes are either powered off or overridden to cause the circuit to break. In either case, this broken circuit will trigger the alarm.

Testing Details

• The test button on a smoke detector makes use of a timed relay. This allows the tester to simply press the button one time and step away, while the timed relay continues to simulate the action of pressing the button for a configured length of time (15 to 20 seconds). This allows the test to complete without the tester having to stand next to a loud alarm.