How long has the power been out?

Article By : Bill Schweber

Simple questions can have correspondingly simple low-tech answers, as long as you are not asking for too much.

We recently lost power at the house due to a tree knocked over by a windstorm. Since we were there, we heard the huge thud as the tree hit the street and also very tangibly realized the power had gone out. Of course, there have been other times over the years when we were not sat home at the time of power failure but instead came back to a dark house and wondered, “how long has the power been out?”

I thought about this simple question and what sort of technology could be used to answer it. In our case, we don’t have anything that would “die” due to a power failure (such as a fish in a tank), so it’s mainly a matter of dealing with safety and status of food in the refrigerator and freezer, and pipes freezing and bursting in winter; the U.S. Department of Health & Human Services has some detailed food safety guidelines. While it might be nice to be notified remotely that power had failed, that’s actually not my top requirement, as I am rarely out for more than 24 hours.

To deal with the concern, I started to look for off-the-shelf units to monitor the power situation. Some were costly, advanced units which not only monitor the AC line for outright failure, but also time-stamped brownouts and spikes. That was more than I needed, especially at $500 to $1000. Other units were more modest such as this iSocket 3G (Figure 1), which notified you via cellular link (3G, as the name implies) that power has failed, and also has temperature alarms (a nice feature). Still, at around $200, I was hesitant.

smart socketFigure 1 The iSocket 3G was one of the first units I considered, but it was more than I really needed. Source: iSocket Systems Ltd

Another unit with the misleadingly restrictive name PumpAlarm also looked interesting, as it sends text messages to up to three numbers. The website makes a very clear and strong case for basic power-line and temperature monitoring, calling out the many reasons you might not realize you need it. It also accepts external sensors, such as water level switch closures. It not only notifies you when power returns, but also has an adjustable delay from 1 to 999 seconds for sending out that initial alarm to avoid nuisance tries. Again though, this was a $200+ unit, with a $49 annual fee, so I hesitated.

Oddly, the website gave its dimensions (6.7 inches high, 4.9 inches wide, and 1.9 inches deep), but I could not find a picture of the unit there. However, they did have a handy chart that explains what is undoubtedly obvious to the EDN audience but perhaps not to the less-technical customer: why a Wi-Fi connection is not a good idea for alarming of power failures compared to cellular.

pump alarm featuresFigure 2 The PumpAlarm looked attractive, but the cost and annual fee were more than I wanted to spend. Source:

Of course, there were many other units available spanning many feature and price ranges, but I decided to take a break rather than get trapped in that “black hole” of near-eternal browsing and Google searching. Instead, I thought about building a small microcontroller-based unit to do what I wanted, but as I looked at the project requirements closely, it got very complicated. I’d need a way to safely sense the AC line, an AC/DC power supply, plus seamless battery or supercap backup, a display to tell me when power failed, maybe a scrolling feature to display various messages, a way to enter time-of-day at initialization, and all sorts of other pieces of circuitry all tied together with software, plus an enclosure.

Clearly the solution was getting bigger than the problem, so I stepped back and asked myself, “What do I really need here?” The answer, again, was very basic: “How long has the power been out?” So I opted for a two-part, very low-cost solution: an old-fashioned AC-power, motor-driven clock with hands ($10), and an ice cube in a cup placed in the freezer (Figure 3).

The former would unambiguously allow me to calculate the length of outage up to 12 hours with some basic clock math and with a little smart thinking I could figure it out to 24 hours; the latter would tell me the status of the freezer. Hey, it’s very low cost, has trivial “initialization,” and is truly hassle-free, so it’s worth a try. (Recall that many older detective and police stories use the “hook” of when a mechanical or crystal-based watch with hands stopped to determine time of death for someone in a crash.)

clock ice solutionFigure 3 A basic line-operated clock with motor and hands can show you how long the power had been out, by the difference between what it shows and the actual time. And Ice cubes in a cup can show the thawing status of a freezer; even if power comes back and things refreeze, there will be a frozen mini-puddle at the bottom.

Have you ever gotten so entwined in solving a problem that you were overthinking its solution, but then rebooted your thinking about them both? Did you get a renewed sense of mental clarity as you went through the process?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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