Smoke Detectors

Photoelectric Detectors


Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photodetector that can “see” the light.

When you cross the beam of light, you block it. The photodetector senses the lack of light and triggers a bell. You can imagine how this same type of sensor could act as a smoke detector. If it ever got smoky enough in the store to block the light beam sufficiently, the bell would go off. But there are two problems here:

  1. It’s a pretty big smoke detector. 
  2. Its is not very sensitive.

There would have to be a LOT of smoke before the alarm would go off — the smoke would have to be thick enough to completely block out the light. It takes quite a bit of smoke to do that. </p>

Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are positioned at 90-degree angles to one another, like this:

In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor:

The sensor then sets off the horn in the smoke detector.

Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.

Carbon monoxide detectors to become mandatory


Carbon monoxide detectors will soon be compulsory in all homes.

A private member’s bill to protect against carbon monoxide poisoning was passed in Federal Parliament with bipartisan support this morning.

The Federal MP for Murray, Sharman Stone, introduced the bill after Mooroopna brothers Chase and Tyler Robinson died of carbon monoxide poisoning last May.

Doctor Stone says before the regulations become law, the detectors need to be approved in Australia.

The new regulations will come into force in eight to 12 months ( from 2012 ) when legislation in all states and territories complies with the new laws.

All residential properties with gas appliances will have to be fitted with the detectors.

“Landlords will have to have these detectors in place and have regular inspections by qualified contractors,” she said.

She says there will also be training programs to ensure there are enough gas fitters to do the safety checks.