Electronic – arduino – LVD solution with low power indication

First, let me comment on the timer circuit. This will work, as long as your batteries are all of relatively the same age and kept in the same conditions. In 6 months when you are still using this and your batteries are all 6 months older you will need to update the timer. Functional solution, but not the best one.

You can divide down the voltage for your input with a resistor network that has a high enough voltage not to affect your lifetime(you can use a network that does load, you just replace your batteries more often). There is one catch, you need to Load a battery to see a true value of it's life left. You will find the more loaded a battery is the more the discharge curve looks like a line. It will never be a line, there will still be clear phases, but you can dependably correlate a loaded batteries voltage with your life left.

If your PIC is on during the measurement you will probably get a decent measurement. Have the pic spend time measuring your battery and look at the resulting voltage curve until your device dies. If the curve stays relatively flat, and then suddenly drops and your batteries die then you will want to use a transistor and load resistor to increase current draw during battery measurements. There is a large amount of information on batteries on battery university. Often microcontrollers fail to pull enough current to get a curve that is sloped the entire way(I have seen this problem with ultra low power uC like MSP430). You will probably be fine with just your PIC running.

Research into AA battery chemistry has fielded some results. It does look like they show pretty flat discharge curves with low currents(<500 mA). This will mean that you will likely want a resistor discharge circuit coupled with a transistor to allow the voltage measurements to be more valuable.

Please forgive me if this was not clear enough. If you comment and questions or suggestions I will update it.

Your options basically fall into two broad catagories: Some sort of semiconductor device, like an opamp or analog CMOS switch, or some sort of mechanical switch (a.k.a. relay).

The problem with using a semiconductor device is that you really want your power rail to be higher than the audio. For a headphone that would put your power rail somewhere in the +/- 5 to 12v range.

A latching relay would be nice, and they are available. I doubt that you can find one that works at 1.2v, but it would only consume power when actually switching. A DC/DC converter might be required, but it only has to be on when switching the relay. Plus, it is very hard to beat the audio quality of what's effectively a piece of wire.

Of course, let me ask the obvious question: What's the problem with a simple switch? Zero power consumption. No power rail issues. Super reliable. Great sound quality. If a simple switch will work then this is what you want to use.