Optical smoke alarms, also known as photoelectric detectors, are less prone to false alarms than ionisation models and are slightly quicker at detecting slow smouldering fires that tend to produce a lot of smoke.

The true duct smoke detector analogue output status levels is 2-10 Vdc. 2 Vdc means sensor fault, 3-7 Vdc means normal operation, 7-9 Vdc means optics needs cleaning and 9.5 V dc means smoke alarm.

An optical smoke alarm (also called photo-electric smoke alarm) works using the light scatter principle. The alarm contains a pulsed infrared LED which pulses a beam of light into the sensor chamber every 10 seconds to check for smoke particles.

Ionisation alarms are very sensitive to cooking vapours and if placed near a cooker or toaster are likely to give frequent false alarms. Optical alarms detect larger particles of smoke, the kind that are produced by a slow smouldering fire before it bursts into flames. They are more sensitive to smoke from cigarettes.

Essentially, a smoke detector is a device that senses when there’s smoke around. A fire alarm is triggered by the smoke detector to alert residents to a potential fire. The smoke detector will sense the smoke, and may or may not make noise when it does.

If you are unaware which type of smoke detector you have in your home, now is the time to check. You can do so by taking the smoke alarm down and look at the back for either “Photoelectric” or “Ionization,” or a symbol with the letter “P” or “I” on the back.

Most hard-wired smoke detectors have a connection from the electrical wires to the smoke detectors that engages and disengages. It’s like a plug. In fact, some smoke detector companies will give you an adapter you can use to connect both commonly used connectors to a new smoke detector.

An isotope of americium which emits alpha particles is used in smoke alarms. Alpha radiation ionises the air and this allows a small current to flow between two electrodes. Alpha is weakly penetrating so smoke stops it, the current drops and the alarm goes off.

The National Fire Protection Association (NFPA) recommends changing the batteries in your smoke alarm every 6 months. At First Alert, we offer 10-year smoke alarms that have a built-in 10-year battery, which eliminates the need for battery replacements for the life of the alarms.

If a smoke alarm is chirping consistently, one of the following may be the reason: The battery may need to be replaced. An alarm will chirp every 30 to 60 seconds for a minimum of seven days. With a “low battery” announcement, disconnect the unit and replace the batteries.

As a smoke alarm’s battery nears the end of its life, the amount of power it produces causes an internal resistance. Most homes are the coolest between 2 a.m. and 6 a.m. That’s why the alarm may sound a low-battery chirp in the middle of the night, and then stop when the home warms up a few degrees.

Resetting the Alarm

1. Turn off the power to the smoke alarm at the circuit breaker.
2. Remove the smoke alarm from the mounting bracket and disconnect the power.
3. Remove the battery.
4. Press and hold the test button for at least 15 seconds.
5. Reconnect the power and reinstall the battery.

Will a smoke alarm eventually stop chirping? A smoke alarm will eventually stop chirping if you do nothing. Once the battery has run out completely, the device will switch to residual power. Eventually, this will also drain and the device won’t have enough power to beep and let you know it’s out of power.