Question:
Does anybody have a simple schematic drawing of a simple photo-electric smoke detector? If you have it, or know how to get it, please respond.

Answer: I don't have a schematic, but I can explain how one works if that's what you're after. Most of the commercial units use a special IC for the bulk of the circuit, so having a schematic doesn't help much. If you would like to learn more, find a junked one and take it apart. Be sure it is the photoelectric type as the 'ionization' ones contain a tiny bit of radioactive material that is best not to mess with. The heart of the device is a 'smoke chamber', usually made from black plastic and built so smoke can get in but light can't. At one end is a light source (bulb or LED), and at the other end is a light detector such as a phototransistor. Inbetween, there is a baffle system so the light from the source is blocked by the walls and baffles of the chamber before it can get to the detector.

However, should smoke come in, the smoke particles will reflect some of the light past the baffles and illuminate the detector. Intuitively, you would think that it would work better detecting when smoke *blocks* the light, but this would work reliably only for very heavy smoke. It is also hard to compensate for natural variations in the brightness of the source and the sensitivity of the detector. Detecting the slightest reflection from the smoke is more likely to give an earlier warning. And the normal condition for the detector is 'completely dark', making it immune to variations in sensitivity or source intensity. A test function is provided that simulates the presence of smoke so the system can be checked out. When the detector gets enough light, a comparator triggers the alarm buzzer or signals the building's alarm control panel. One final variation used for battery-powered detectors especially is to turn the whole circuit including the light source off almost all of the time except for a very brief sample every few seconds. This saves the battery from running down too fast. Another circuit is also there to detect when the battery is almost dead, and sound short beeps as a warning. These functions are all taken care of by the special IC.