For the power supply detection circuit, I used a circuit built 
around a Harris Semiconductor CMOS Micropower Over/Under Voltage 
Detector (ICL7665S).  The lead-acid battery was determined to fail at 
approximately 10.5 volts, so I made a resistor network which would supply
the correct voltage to the ICL7665S at that voltage value:

                         12V battery (+)
           ___________________|________________
           |     ________|_________    |      |               R1 = 6.8K ohms
           |     |       V+       |    > R1   |
           |     |                |    <      |               R2 = 1M ohm
           |     |           Hyst2-----|      |
           |     |                |    > R2   |               R3 = 140K ohms
       C1 ---    |    ICL7665S    |    <      >  R4
          ---    |            Set2-----|      <               R4 = 1M ohm
           |     |                |    > R3   |
           |     |                |    <      |               C1 = 0.1uF disk
           |     | Ground    Out2 |    |      |               
           |     ----|--------|---|    |      |               O  = 100K Pot
           ----------|        |        O      |               
                     |        |        |      |               |  = Ground
                     |        |        |      |               =         
                     |        |        =      |
                     |        |               |
                     =        ------------------------ To MAX232CPE
                12V battery
                    (-)


	I connected the 12V battery leads to the wearable's DC-DC
converter (Datel UWR-5/3000-D5) and to the circuit as shown above.
The TTL signal generated by the ICL7665S was passed to a Maxim 232CPE
chip which drove appropriate RS-232 logic.  From there, it went to pin 1 (DCD)
on a standard 9-pin serial cable, so as not to interfere with the operation
of the Twiddler keyboard on that serial port.

                             -------------
                             |           |                       R5 = 20K ohms
                             |           |
                      R5     |           |
   From ICL7665S ----^v^v-----T1in       |
                             |      T1out------------- To Serial Cable
                             |           |               Pin 1 (DCD)
                      --------C1+     C2+--------
                    + |      |           |    + |                R5 = 20K ohms
                  C2 ---     |           |  C3 ---               
                     ---     | MAX232CPE |     ---               C2 = C3 = C4 = C5 =
                      |      |           |      |                C6 = 1.0 uF tantalum
                      --------C1-     C2---------
                             |           |
                      --------V-      Vcc-------------------- +5V from converter
                      |      |           |      |     + |
                  C4 ---     |           |  C5 ---  C6 ---
                     ---     |           |     ---     ---
                      | +    |           |      | +     |
                      |      |         V+--------       |
                      =      |    GND    |              =
                             ------|------
                                   |
                                   |
                                   =

Notes:  1.  You must tie the 12-volt and 5-volt grounds together on the DC-DC
              converter in order to use them for voltage supplies in this circuit.

        2.  The 100K pot is used to tune the failing voltage set by the resistor
              network.


	For software, I used Alessandro Rubini's Powerd v. 2.0, with
these changes in the default powerd.conf file:

serialline      /dev/cua0
 monitor        DCD
 failwhen       low


Kevin Pipe
morgoth@media.mit.edu