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Soft on/off is designer's dilemma, user's headache

-September 23, 2013

I'm old fashioned when it comes to switches: when a switch says "off," it should really mean off, via a "hard" switch which disconnects the electronics from the power source. But reality is that most devices these days use "soft" switching, where the so-called on/off switch is really just another contact closure sensed by the electronics. It must be constantly monitored by the circuitry even when purportedly off, using low-quiescent drain circuitry. Adding to the issue, many devices don't even have the soft switch, they simply put themselves into a quiescent state after a few minutes of inactivity.

I was reminded of this when I went to use a small home gadget for the first time in several months. Surprise: it didn't turn on. Quick investigation showed the obvious, that the batteries were dead. The solution was also obvious: change them.

Then I found a tiny note I had placed inside the battery pack, indicating that I had changed these same batteries about a year ago. A little detective work revealed the true problem. The coin cells in this device have only modest capacity (mAhr) and the small but definite drain of the unit even in quiescent mode trickle-discharges them in a few months.

It's especially frustrating when this is a device that you may not use for a long period, then need unexpectedly. I have several such devices (see figure below, from top to bottom): a digital vernier caliper; a pocket voice recorder (yes, I could use my phone, but this is actually quicker to grab and go with); an ultrasonic distance-measuring unit from many years ago; a tire-pressure gauge; and a GPS unit, (again, I could use the phone, but I prefer the standalone unit for various reasons). The GPS unit is especially aggravating, since it seems to have only about 7-10 days of battery life when "off" and unplugged from the car's outlet, then needs 5-10 minutes of charge to get it operational.

I understand why designers use soft switching or no tangible switch at all for on/off. It's simpler in many cases for the mechanical design and packaging; it's often cheaper; or there's a need in some cases to provide keep-alive current to the product even when not in use. For the calipers, for example, it would be a challenge to fit a tiny on/off switch into the package. For that unit, I actually take the batteries out when it is not in use, but that's not practical for many products, which have hard-to-reach battery compartments or no user-accessible batteries at all.

Sometimes, though, I can solve the problem in a more direct manner. For the ultrasonic distance unit I was able to actually install a tiny on/off pushbutton switch in series with the battery lead, totally solving the problem. For the other unites, though, it's impossible, impractical, or simply undesirable to have a switch in series with the battery.

For designers, use of soft or virtual on/off switches raises challenges and adds to headaches. In addition to getting the unit's design, functionality, software, and more done properly, they have to be very diligent in using low-power parts with super-low and suitable quiescent-mode performance. As a result, the BOM may be dictated more by power specs than by performance or functional specs - a case of the power-drain tail wagging the product-performance dog, perhaps. They also have to make sure the operating code and mini-OS can deal with the issues of soft-switching, quiescent modes, wake-up, and related topics.

Have you found soft on/off switches or no on-off switches to be a user headache? What about these being designer's headache as well?

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