The smoke sensor will use optical techniques to determine the concentration of airborne smoke particles. The main alternative method to achieve this, which most domestic smoke sensors employ, is to use ionisation. However this requires a radioactive source and would prove more difficult to use.

"[optical] smoke detectors may respond more quickly than ionization detectors to low energy fires, such as slow, smoldering fires. The low energy fire will generally produce large smoke particles that are easily detected by optical means."¹

The circuit below in figure 1 would be one of the simplest ways of reading the phototransistor. This implements a voltage divider between the phototransistor and the resistor. As the impedance of the phototransitor grows the voltage dropped across it also increases, leaving less to be dropped across the resistor, and therefore the output would be lower.

 

Figure 1

Another method would be to use a Wheatstone bridge arrangement.

 

¹http://www.sas.org/tcs/weeklyIssues/2004-07-16/feature1/index.html