An easy way is using a low votage transformer, lets say 110 x 6 Vac.=20
Tricks as zener diode, transistors, you choose how you would check if
the voltage is below certain point.  The transformer allows you to play
without so much risk.

                                          +5V     +5V
                                           |       |
                                           |       |
                                           |       R  10k
                                           |       R
                   1N4001   Zener 5V1      R 10k   b----> - Low AC
   ---.     .---------|<----->Z------.     R       |
       S | S                         |     |       |
       S | S                         R     |    | /
110    S | S  9Vac         Variable  R<----o----|/  2N2222
       S | S            Pot 2 kOhms  R     |    |\e  NPN
       S | S                         |    ---      |
   ---'     |                        |    ---      |
            |                        |     |       |
            '------------------------o-----o-------o------GND

Without 110, the 2k Ohms adjustment makes the 2N2222 conduct, via its
base polarization of the 10k resistor, so outputs goes to ground.
When AC goes in, its negative side of the wave goes through the zener
(that can be eliminated if you have a 6 or 4Vac transformer) discharge
the capacitor (could be a 1=B5F tantalum), when the senoid goes
positive,
this discharge stops, the capacitor starts to charge via 10k Resistor,
the next negative side of the wave discharge cap again and the
transistor never conducts.  If the input AC voltage goes lower than a
certain point, the 10k resistor can load faster the cap than the
negative wave can discharge it, so the transistor will conduct in few
senoids.  It is only a matter of adsjustment.

You can also use optocoupler with a series of 30V zeners... 3 zeners is
enough, resistor and feed the led of the optocoupler.  The output
transistor is connected to +5V and feeds a resistor to ground. When
voltage drops below 90V, the LED does not lit anymore, transistor cuts,
resistor to ground gives you the negative signal.  The capacitor, as
usual, is to avoid output goes to zero during the other side of the
senoid that is not lighting the led.

     3 x 30V Zener   Diode 1N4004    4k7 - 10k (experiment)
o---->Z-->Z-->Z----->|--------------RRRRRR----------.=20
                                                    |=20
110 Vac                                             |
o------------------------|<-------------------------'
                     OptoCoupler 4N35  =20
                       -----
  +5Vdc o---------------/ \e-----o-----o----> -LOW AC
                                 |     |=20
                                 |     |
                          1=B5F   ---    R
                                ---    R  10k
                                 |     R
                                 |     |
                                 |     |
                                GND   GND

Wagner



Bill Bennett wrote:
>
> I am need of someway detecting when AC line voltages drops. What I
> would like to do is be able to receive a low when voltage drops to 90
> volts and then as line voltage rises receive a high when it reaches
> 105 volts. Usually around here line voltage runs 125-126 volts would
> like something able to withstand 140 volts.
>
> Thanks,
>
> Bill