Re: one bit of memory?

From: Robert Monsen (rcsurname_at_comcast.net)
Date: 02/05/05 

Date: Fri, 04 Feb 2005 22:58:54 -0800

Thomas A. Horsley wrote:
> In my ongoing quest to waste as much time as possible on my silly
> phone gadget (http://home.att.net/~Tom.Horsley/phonetale/markII.html)
> I added a circuit to "remember" if the phone ever rang so I can
> turn on a flashing LED as a "missed call" reminder.
>
> For memory I used the LM339 as OR-gate sample circuit from the
> NS pdf spec ***, and fed back the output as one of the inputs
> with the output from the 555 timer being the other input.
>
> This worked great till I decided to switch to the much lower power
> LM339 near-compatible TLC3704 chip from Texas Instruments
> (http://focus.ti.com/docs/prod/folders/print/tlc3704.html).
>
> Apparently the push-pull outputs work differently enough that the
> initial power-up state of the chip is "on" rather than "off", so
> my one bit memory circuit doesn't work with the new chip very
> well (but I don't want to go back to the 339 because overall
> the new circuit uses a lot less power than the old one and I don't
> need pull-up resistors all over the place).
>
> Anyone have any suggestions for a nice low power way to provide
> the one bit of memory I need? Or maybe a way to hack around the
> initial power-up state so I ignore that particular bit of feedback,
> but notice it later when I really want it?
>
> (Of course if I really wanted to save power, I wouldn't add all this
> extra junk anyway - the important bits seem to be working fine
> and only draw about 0.5mA - about 10 times less than the first version).

According to your circuit schematic, your comparator has negative
feedback. That means it isn't acting as a comparators, it's acting as an
(uncompensated) opamp. This may well result in oscillations.

Here is a circuit that may work. It's a starting point for some
experimentation anyway. It's a flipflop, with the comparator used to
increase the impedance of the output. If your comparator is open
collector, you may need a pullup resistor on the output.

                              VCC=9V
                    .------------------o-------o-----------.
                    | | | |
                    | | | |
                   .-. .-. .-. |
                   | |100k 100k | | | |47k |
                   | | | | | | |
                   '-' '-' '-' |
                    | | | |\|
                    | o-------)---------|-\ Output
                    | ___ ___ | | | >------.
  From 555 o-|___|-. .-|___|-o---. o-----o---|+/ |
         ___ | 1MEG | | 1MEG | | | | |/| |
   ->|--|___|--o----)-------o | |/ | | | ___ |
         100k | \| '--)-----| | | '---|___|-----'
               | |--------' |> | | | 1MEG
              --- <| | | .-. |
         1uF --- | | | | |47k |
               | | | | | | | Rang=Low
               | | | | '-' |
               | | | | | |
               '----o------------------o---)---o-------o---'
                             GND | |
                                           | |
                                           | |
                                           | T |
                                           | --- |
                                           '----o o----'
                                               Reset

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de) 

-- 
Regards,
   Robert Monsen
"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.
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