Computing stuff tied to the physical world

Flashback – Batteries came later

In AVR, Hardware, Software on Sep 30, 2013 at 00:01

During all this early experimentation in 2008 and 2009, I quickly zoomed in on the little ATmega + RFM12B combo as a way to collect data around the house. But I completely ignored the power issue…

The necessity to run on battery power was something I had completely missed in the beginning. Everyone was running Arduino’s off either a 5V USB adapter or – occasionally – off a battery pack, and never much more than a few days. Being “untethered” in many projects at that time, meant being able to do something for a few hours or a day, and swapping or recharging batteries at night was easy, right?

It took me a full year to realise that a wireless “node” tied to a wire to run for an extended period of time made no sense. Untethered operation also implies being self-powered:


Evidently, having lots of nodes around the house would not work if batteries had to be swapped every few weeks. So far, I just worked off the premise that these nodes needed to be plugged into a power adapter – but there are plenty of cases where that is extremely cumbersome. Not only do you need a power outlet nearby, you need fat power adapters, and you have to claim all those power outlets for permanent use. It really didn’t add up, in terms of cost, and especially since the data was already being exchanged wirelessly!

Thus started the long and fascinating journey of trying to run a JeeNode on as little power as possible – something most people probably know this weblog best for. Over the years, it led to some new (for me) insights, such as: transmission draws a “huge” 25 mA, but it’s still negligible because the duration is only a few milliseconds. By far the most important parameter to optimise for is sleep-mode power consumption of the entire circuit.

In September 2010, i.e. one year after starting on this low-power journey, the Sleepy class was added to JeeLib, as a way to make it easy to enter low-power mode:

class Sleepy {
    /// start the watchdog timer (or disable it if mode < 0)
    /// @param mode Enable watchdog trigger after "16 << mode" milliseconds 
    ///             (mode 0..9), or disable it (mode < 0).
    static void watchdogInterrupts (char mode);
    /// enter low-power mode, wake up with watchdog, INT0/1, or pin-change
    static void powerDown ();
    /// Spend some time in low-power mode, the timing is only approximate.
    /// @param msecs Number of milliseconds to sleep, in range 0..65535.
    /// @returns 1 if all went normally, or 0 if some other interrupt occurred
    static byte loseSomeTime (word msecs);

    /// This must be called from your watchdog interrupt code.
    static void watchdogEvent();

The main call was named loseSomeTime() to reflect the fact that the watchdog timer is not very accurate. Calling Sleepy::loseSomeTime(60000) gives you approximately one minute of ultra low-power sleep time, but it could be a few seconds more or less. To wait longer, you can call this code a few times, since 65,535 ms is the maximum value supported by the Sleepy class.

As a result of this little class, you can do things like put the RFM12B into sleep mode (and any other power-hungry peripherals you might have connected), go to sleep for a bit, and restore all the peripherals to their normal state. The effects can be quite dramatic, with a few orders of magnitude less power consumption. This extends a node’s battery lifetime from a few days to a few years – although you have to get all the details right to get there.

One important design decision in the JeeNode was to use a voltage regulator with a very low idle current (the MCP1700 draws 2 µA idle). As a result, when a JeeNode goes to sleep, it can be made to draw well under 10 µA.

Most nodes here at JeeLabs now keep on running for well over a year on a single battery charge. Everything has become more-or-less install and forget – very convenient!

  1. That’s great! How do you know when a battery needs to be changed? Is there an email or SMS that goes out to you from housemon?

    • Heh – well, one could easily add that functionality to HouseMon, but I don’t live in such a push-world. I just check the status summary once in a while…

  2. That’s OK for house monitoring I guess, but when you start using battery powered sensors for triggering events (e.g. switching a bathroom ventilator based on humidity), things become a bit more ‘complicated’. OTOH, to be honest, I have the habit of ignoring low-battery notifications for sensors of which I know that they’re not being used as trigger.. ;-)

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