Nuts and Volts - November 2014

I’ve been working with Microchip PIC processors for quite a while, and their eXtreme Low Power (XLP) processors have some impressive low power sleep modes — with some claiming 20 nano-amps (nA). This makes them ideal for battery-powered devices. Okay, but what does that mean in real life?

We need to do a little math to get some perspective. Let’s say the design goal is to run the device off two AAA alkaline batteries for at least a year. The actual run time of a wireless doorbell transmitter, for example, is calculated as 1,000 rings per year, with each ring lasting a generous four seconds and drawing 25 mA. That comes to a total of roughly 28 milli-amp hours (mAh). The total capacity of these batteries ranges from 800 to 1,200 mAh, so we can easily operate the doorbell for several years.

Now, we need to worry about the current draw while the doorbell is idle. If our circuit at idle draws 1 micro amp (uA) or less, then the batteries will last roughly 800,000 hours. With 8,760 hours per year, this is about 90 years. At a sleep-mode draw of 20 nA, we’re talking about 50 times longer!

So, if the specs are correct, then putting our processor and the rest of the device into sleep mode should not appreciably deplete our batteries. The shelf life of alkaline batteries is only 10 years, and most gadgets don’t stick around for even that long — with garage door openers and alarm sensors being the exception. For now, on paper, the numbers look promising, and if the spec sheet writer hasn’t misplaced a decimal point, we can use a low power microprocessor to switch our device on and off.

Design and Testing

A simple pushbutton operated LED is used to test this. I only needed a few parts (which I had on hand), a voltmeter, and David Jones’ µCurrent (see sidebar) to verify I’m drawing very little current when I’ve turned the system off. Figure 1 shows the test circuit. You can see that the processor is permanently connected to the battery. The power switch is between the battery and one of the processor input pins.

Figure 2 shows the prototyped circuit (with a few extras) connected to my current measuring system.

The meter shows that in sleep mode this circuit draws 16. 8 nA. The code for the power switch is written using the CCS PIC® C compiler, and the entire file can be obtained from the article link. Portions are duplicated here to explain how the switch works.