This looks like what I might be looking for... Is there a specific
reason why you used fets instead of bipolar? I'm not very familiar with
fets as they weren't in very common except in military hardware when I
was working as a tech (did I date myself?)


On Sat, 2003-07-26 at 15:27, Spehro Pefhany wrote:
> At 01:14 PM 7/26/2003 -0600, you wrote:
> >First, let me tell you that I have never done any analog design before
> >so I am starting from scratch here :) What I am trying to do is check
> >that this 5 ohm resistor is in the circuit (continuity) using a digital
> >PIC input. Continuity is defined as a TTL logic level HI. I can't use
> >the analog ports as there are not enough (I have 32 identical circuits
> >like below.)
>
> Can you explain what exactly you are trying to do? What range of
> resistance values should yield a "pass"? If it's either 5 ohms or
> close to infinity (like, say, a light bulb filament), then
> you could just do something like this:

I have gone into much greater detail in the message I posted as a reply
to Tom. I envision the resistance to be no more than about 7 or 8 ohms
max, when there is continuity, and infinite like an open connection when
the resistance isn't there. If the resistance is greater than about 7 or
8 ohms, it will cause problems with the current flow and IR losses.

Are thesre multiple sources for these particular fets and are there any
samples available?

>                        +12VDC o
>                               |
>                              .-.
>                          5R0 | |                       Vdd
>                              | |
>                              '-'                       o
>                               |                        |
>                               |                       .-.
>                               |                       | |10K
>                               |                       | |
>     Logic level power MOSFET  |       10K             '-'
>     eg. IRLZ34NS              |       ___              |
>                               +------|___|---+     - --+---o
>                               |              |     |
>             1K0            ||-+              |  ||-+    CMOS in
>              ___           ||<-              |  ||<-
>         o---|___|-----+----||-+              +--||-+ 2N7000
>                       |       |            \ |     |
>     CMOS out         .-.      |              - \   |
>                 100K | |      |       5.1V  / \    |
>                      | |      |             ---    |
>                      '-'      |              |     |
>                       |       |              |     |
>                      ===     ===            ===   ===
>                      GND     GND            GND   GND
>
>
> Best regards,
> Spehro Pefhany --"it's the network..."            "The Journey is the reward"

Thanks for your guidance!

--->Rob

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