One of the smoke detectors in the house has started raising a lot of false alarms and was replaced after a 3:30AM episode. Naturally, we’re going to take it apart. Today’s home smoke detectors come in two types: ionization vs. photo-electric. This particular detector, a First Alert P1210, is a photo-electric detector.
The cover was held on by four screws, only one of which was immediately accessible. The remaining three were blocked by the paper label which was easily punctured for access with a screwdriver.
Inside the plastic housing is a surprisingly large circuit board, its battery, and piezoelectric alarm buzzer. A cursory examination of the circuit board revealed some pads for absent components, which likely meant this board is shared with higher-end models with more features. Beyond those empty pads, almost a third of the circuit board real estate serves no apparent purpose.
The brains of the operation is a single chip printed with the Microchip logo which made it easy to look up its datasheet. A search on its numbers identified it as a Microchip RE46C190 which is apparently a turnkey solution for anybody to build a smoke detector around the chip.
The nose of the operation is an infrared LED paired with a detector, kept in a housing that keeps the detector out of direct line of sight to the LED. When smoke particles enter the detection chamber, it will be illuminated by the LED and reflect light into the detector.
There is no obvious indication of why this smoke detector started sounding false alarms. Perhaps some dust entered the detection chamber? A smoke detector chip shouldn’t sound the alarm for just any reflection, it should look for specific characteristics of smoke particles. But there’s a chance we are expecting too much of this little detector.
And even if it does panic at any reflection regardless of source, the knowledge is not particularly helpful. The detection chamber and the baffles surrounding it is not accessible for cleaning without taking the detector apart.
For the immediate future, these parts will sit in a plastic bag. Added to the stockpile of electronic parts for potential future projects.