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  • Photoelectric Smoke Detector using RE46C140

  • Created: Sep 20, 2016

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Summary

Fire is a very hazardous event that could happen to anyone. It could cause deaths, injuries and damages to property. We need precautionary measures to avoid fire and knowledge on handling this kind of events to prevent extensive damages. One of the devices that was developed to alert people when fire happens is the smoke detector. This device detects smoke from fire and alerts the people around by sounding its alarm.


This schematic is a photoelectric smoke detector. It is a type of a smoke detector that uses optical electronic devices to operate. Usually, an LED and a photodiode are used. They are placed in the smoke chamber of the device normally positioned in an angle where the light of the LED does not hit the photodiode. In no-fire situations, the photodiode does not catch any light from the LED, thus no detect signal is transmitted to the smoke detector IC. When a fire is present, the smoke goes into the smoke chamber and disrupts the light from the LED. The smoke particles redirects the light making it hit the photodiode sending a detect signal to the smoke detector IC. The circuit uses the RE46C140, a low power CMOS photoelectric smoke detector IC from Microchip. It can be operated with minimal external components and complies with the Underwriters Laboratory Specification UL217 and UL268. The signal from the photodiode goes into the DETECT pin of the IC which is an input to an internal photo amplifier. A comparator compares the photo amp output to an internal reference voltage. If the required number of consecutive smoke conditions is met the device will go into local alarm and the horn will be active. The temporal horn pattern of this device supports the NFPA 72 emergency evacuation signal.


A 9V battery powers the circuit and once every 40 seconds the status of the battery voltage is checked. This status is checked and latched at the conclusion of the LED pulse. The device also features an interconnect operation that allows multiple detectors to be connected through the IO pin of RE46C140.

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