e-paper displays can be used to improve safety in smoke alarms

28th August 2019
Lanna Cooper

The recent news story about an 11-year-old’s Samsung tablet battery smouldering and nearly causing a house fire while charging overnight has again raised the importance of installing smoke alarms in homes. The house where the incident occurred had no working smoke detector which could have alerted the boy’s family to the incident much earlier.

By Hardy Kuo, Field Applications Engineer, Pervasive Displays

We all know smoke and carbon monoxide alarms save lives. Be it through heart-wrenching adverts from the fire service or disaster stories, to the many tales of true heroism - such as the fantastic recent news story in which a bride-to-be stopped her hen party to rescue an elderly man after hearing a smoke alarm going off; kicking down a door and running into the burning building - she did not even stop to remove her veil.

But it’s not just anecdotal evidence that shows the importance of a working detector. Data from the US highlights that the death rate more than doubles if there is no working smoke alarm, and that even when a smoke alarm is present, 46% have a dead or disconnected battery.

And in the UK, where it is recommended that smoke alarms are tested every three months, a government housing survey highlighted that a fifth (22%) of all households and a third (33%) of tenants never checked it - and being a survey with a right/wrong answer, it is likely that people lied, making the real numbers even larger and more worrying.

Smoke alarm batteries

Obviously the battery is a key consideration in both smoke and carbon monoxide detectors, and manufacturers create alerts to give peace of mind. For example, Google’s top ranked (UK-search, 26 June 2019) smoke alarm integrates both an audio alert if the battery is running low (a regular, intermittent beep), and an LED, which flashes to indicate the battery has charge and the device is working normally.

The problem with this method is twofold. Firstly, it is annoying: I for one have removed a battery to stop the intermittent beeping and then waited weeks (possibly longer) before remembering to buy and install the new battery, and the beeping is particularly annoying if it happens during the night (making it more likely for someone to just remove the battery). Secondly, it relies on someone noticing the absence of the flashing LED.

The reasons for these being implemented are simple - they are low power methods and batteries need to last as long as possible - but they are far from ideal and another method for battery alerts is needed.

A display is needed

Systems, such as the above smoke alarm, are incorporating increasing levels of sophistication. For example an MCU that allows a reduced volume for testing, and night-time modes to stop it making (what feels like) incessant beeps in the night. This would therefore allow the easy addition of a display - and because of the power constraints, this should be e-paper.

E-paper displays consume less than ten percent of the power of an LCD for a full-screen refresh, can be read in ambient light and only draw power when the image is changed. Compare this with a TFT LCD, which not only needs to be backlit constantly, but is also refreshed at rates of up to 50 times per second.

To put this in context, a 2" e-paper module refreshed at even six times per day would consume 3.29mAh over the course of a year - a smoke alarm display would actually need updating far less frequently, for example when the battery dropped from full to 75%, 50%, 25%, 10%, 5% and 2%. Over this same period a similarly-sized TFT LCD would have used 262,800 mAh of power.

And with displays from 1.44" (27.5x27.5mm), sizes are suitable for smoke and carbon monoxide detector form factors - this 1.44" display would take up just nine percent of the surface of a 104mm diameter round smoke alarm.

Of course, it will be vital to catch attention if a battery is running low; colour is an easy way to enable this.

An increasing number of e-paper displays can simultaneously show both multiple colours and e-paper manufacturer Pervasive Displays makes eight sub-4.4" displays which can simultaneously show both black and red e-inks. In May, Pervasive Displays also added 7 screens (5 sub 4.4") that could display yellow. This allows a quickly visible and eye-catching way to give alerts to home users to not just highlight when a battery needs replacing, but also when a system needs testing.


E-paper is an established, trusted technology that is ideally suited to (and indeed used for) many home devices - from smart energy meters to thermostats - to give a low power display functionality. In the case of life-saving alarms, these displays could prove vital in helping people identify when tests and battery changes will be required without frustrating beeps/LED flashes that only alert you that devices are working, not that they have failed.

E-paper displays for such devices are available for high volume manufacturing immediately from Pervasive Displays.

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