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A SILENCING TECHNIQUE FOR NUISANCE ALARMS IN SMOKE DETECTORS

IP.com Disclosure Number: IPCOM000005750D
Original Publication Date: 1988-Oct-01
Included in the Prior Art Database: 2001-Nov-01
Document File: 2 page(s) / 114K

Publishing Venue

Motorola

Related People

Jules Campbell: AUTHOR

Abstract

The evolution of low power, CMOS It's for use with smoke detector ionization chambers has allowed this technology combination to dominate this cost-sensitive, consumer market for over ten years. By strobing the detector's power-consuming analog circuits at a very low duty cycle, these It's achieve over one year of opera- tion from an inexpensive nine volt battery.

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MOTOROLA Technical Developments Volume a October 1988

A SILENCING TECHNIQUE FOR NUISANCE ALARMS IN SMOKE DETECTORS

by Jules Campbell

   The evolution of low power, CMOS It's for use with smoke detector ionization chambers has allowed this technology combination to dominate this cost-sensitive, consumer market for over ten years. By strobing the detector's power-consuming analog circuits at a very low duty cycle, these It's achieve over one year of opera- tion from an inexpensive nine volt battery.

   The greatest problem with ionization type smoke detectors has always been "nuisance" alarms due to cook- ing fumes, or other transient conditions exceeding the sensitivity required by the UL safety standard. During a non-life-threatening nuisance alarm, many consumers silence the alarm by removing power. However, con- sumers frequently forget to restore power after the condition clears, thereby losing protection. Several tech- niques, such as dual alarm thresholds having different response times, have been tried to reduce the incidence of nuisance alarms, but have not been cost competitive.

   A recent development has been the introducton of silencing circuits, known as "hush" circuits, external to the IC. When a nuisance alarm occurs, the homeowner can activate the alarm silencing circuit by the push of a momentary switch button. Most schemes use a large value timing capacitor, transistor or diode, and a few passive components to shift the alarm reference voltage. The major drawbacks of such crude circuitry include excessive variation of the silencing time interval, poorly defined transitions between sensitivity levels, or even completedisablement of the detector. A proposed ULstandard will require the unit to remain capable of detec- ting smoke at a slightly reduced sensitivity, while giving a visual or aural indication of this reduced sensitivity mode. In addition, the smoke detector must automatically return to normal sensitivity after a specified time has elapsed.

   Incorporation of the silencing circuitry into the IC itself improves the control of the sensitivity and silenc- ing time interval, while providing easy implementation of the mode indicator. A basic smoke detector is COn- netted as shown in Fig. 1. The dual ionization chamber has its center electrode connected to the VIN com- parator input. The STRB output is used to activate the reference voltage divider at the VREF comparator input. A single flip-flop is normally used to sample the comparator. Additional circuitry to implement an alarm silenc- ing feature is also shown in Fig. 1 and described below. Representative timing of the relevant signals und...