Op-Amps are really quite easy, once you get beyond thinking they're
complicated.  Doing an online search in google will give you all the
info you would need.

I would also recommend the IC Op-Amp cookbook.  Awesome book (some 250+
pages) of nothing but op-amp circuits.


Shawn Wilton
Junior in CpE
MicroBiologist

Phone: (503) 881-2707
Email: shawn@black9.net

http://black9.net


Omer YALHI wrote:
> Adam, thank you for the reply.  However, the circuit I have given is a
> "black box", I am only exposed to the pins A and B.  I think I can still put
> your idea to work by adding another resistor (something smaller than 10K
> perhaps?) at point A and measure the voltage at that point.  Does this sound
> right?  Also, I am not familiar with op-amps, so I wouldn't know where to
> start; therefore I am trying to avoid using an op-amp (unless it becomes
> necessary).
>
> Regards,
>
> Omer
>
>
> -----Original Message-----
> From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
> Behalf Of M. Adam Davis
> Sent: Saturday, April 17, 2004 12:12 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PICLIST] [EE:] Reading different resistor values
>
> This circuit uses large value resisters, which makes it difficult to read
> when directly connected to an analog input pin, but by no means impossible.
>
> You have a simple resister divider.  Hook the analog input pin to the
> junction of R1 and R2 (where B1 connects).  As the switches open and close
> the voltage level at that junction varies.
>
> If the voltage is one volt, then these are the voltages at that junction you
> can expect:
> B1, B2 open -->  .745
> B1 closed --> .109
> B2 closed --> .647
> B1, B2 closed --> .093
> B3 close --> 0
>
> If the voltage going in is higher, then just multiply the above values by
> that voltage, so for 5 volts it would be 5 * .745 = 3.72 and so on.
> You can measure this directly with a multimeter at that point.
>
> The PIC may measure slightly different voltages for both open and B2 closed
> since there is an impedance limit in the PIC - it doesn't like to measure
> voltages through resistors more than 10k (It's a little more complex than
> that, but this will get you started).
>
> However, it will still be able to detect the change and correctly identify
> the various states of the switches.  If it doesn't work very well adding an
> op-amp to buffer the voltage will set it straight.
>
> I hope this helps!
>
> -Adam
>
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