At 09:50 PM 7/28/2003 -0600, you wrote:
>Spehro,
>
>Because I am feeling humbled by your knowledge, I have to ask a simple
>question, and please don't chastise me in public for asking: Where would
>I put a simple LED and current dropping resistor in this circuit so that
>it would light when there is continuity, without affecting the existing
>functionality?

Hey, I don't bite. If you don't mind

a)      A bit of current through the load when it is suppose to be
        de-energized

b)      The LED going off when the load is energized (whether or not it's
         open)

Then you can connect the LED+series R across the power MOSFET:


> >                        +12VDC o
> >                               |
> >                              .-.
> >                          5R0 | |                         Vdd
> >                              | |
> >                              '-'                          o
> >                               |                           |
> >                               |                          .-.
> >                               |                          | |10K
> >                               |                          | |
> >     Logic level power MOSFET  |             10K          '-'
> >     eg. IRLZ34NS              |              ___          |
> >                               +----x-----x--|___|---+-----+---o
> >                               |    |2K0  |4K7       |     |
> >             1K0            ||-+   .-.   .-.         |  ||-+    CMOS in
> >              ___           ||<-   | |   | |         |  ||<-
> >         o---|___|-----+----||-+   | |   | |         +--||-+ 2N7000
> >                       |       |   '-'   '-'       \ |     |
> >     CMOS out         .-.      |    |     |          - \   |
> >                 100K | |      |   ---    |      5V1/ \    |
> >                      | |      |   \ /  \ |         ---    |
> >                      '-'      |   --- \  |          |     |
> >                       |       |    |   LED          |     |
> >                      ===     ===  ===   ===        ===   ===
> >                      GND     GND   GND  GND        GND   GND


The 4K7 resistor should have been in the previous schematic and is required
(LED or no) to make sure the 2N7000 turns off (when the 5R0 load goes
above about 50K ohms). A total of about 9mA goes through the load when
the power MOSFET is off.

If you really need the LED to always indicate when the load is "good" then
you'll have to do something a fair bit more complex. One method is to
remove power from the MOSFET briefly every little while during the "on" time
and look at continuity, and latch your error signal (if any) in the micro.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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