The comparator input should have a series resistor (1 meg or so) to protect
it from transients picked up by the foil or wires.



-----Original Message-----
From: Thomas McGahee [mailto:tom_mcgahee@SIGMAIS.COM]
Sent: 07 January 1999 13:26
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: Ideas for Pic-based Motion detector?


Foster,
Be aware that unless the two wires are both enclosed in some sort of
Faraday cage (such as a simple grounded metal enclosure) you will
have a nice 30 foot antenna that is going to pick up all kinds of
local RF. If you want to go with a variable capacitance type circuit
I would change from using wires to using two thin foil strips
instead. That will allow a MUCH greater capacitance value as your
starting value, and will make the circuit less prone to errors due
to the proximity of other objects.

Since you are not interested in absolute position information, but
only want to detect rapid CHANGES, you might want to investigate
using a circuit similar to the following:

   O +Vcc
   |
   |
   \         V Vref (adjustable)
   /         |
   \         |    |\
   /         -----| \ comparator
   |              |  >---->  Output pulse
   *--------*-----| /
   |        |     |/
   \       ---
   /       --- 30 foot foil capacitor
   \        |
   /        |
   |        |
   |        |
  ---      ---
   -        -
The comparator must be one with a very high
input impedance and capable of operating with
very small input currents... on the order of
picoamps. Most FET type op amps will do.

The two resistors should be high values, such as
22 Megohms. They set a midpoint voltage of Vcc/2.
Normally the capacitor is just charged to that
level. But if the plates of the capacitor are
moved rapidly, then the voltage at the junction
will change. If the comparator has been properly
adjusted then a pulse will appear at it's output
indicating the change was detected.

If the capacitance is too small to begin with, then
the two resistors will have to be made even larger.
Instead of a comparator you could connect a
buffer op amp to the capacitor and look at the
output on a scope (use AC input selection to
remove the DC bias). Turn the gain up and you
should see a "noise" that represents the system
vibration or movement.

Hope this helps.
Fr. Tom McGahee
Electronics Department
Don Bosco Technical High School


----------
> From: adastra <foster@ADASTRAN.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Ideas for Pic-based Motion detector?
> Date: Wednesday, January 06, 1999 6:52 PM
>
> Greetings, PICMASTERS:
>
> I need to devise a system which will detect a small movement between two
> adjacent wires.  The wires would be about 30 feet long and spaced
> (non-uniformly) about 1 inch apart. They are normally stationary (fixed to
> moveable objects), and I need to detect if they are suddenly moved either
> closer together or farther apart, approximately plus-or-minus 1/4 inch, as
a
> result of an impact on one of the objects.  (For various reasons, simple
> contact switching will not work in this application.)
>
> I am thinking of a system where an RF signal(say,100 khz)is applied to one
> wire and picked up by the other. The gain of the return signal could be
> automatically set to establish an accurate baseline level, and an output
> signal generated if the level changed suddenly.
>
> Perhaps the PIC could generate the RF signal, modulate it, and
synchronously
> detect the return.  (Maybe the synchronous detection is overkill?) I also
> need the PIC to generate some specific digital output data when the
movement
> is detected, but that is less of a mystery to me than the transmit-receive
> scheme I am contemplating, as outlined above.
>
> Any comments or links pertaining to this (or any alternative)approach
would
> be much appreciated.
>
> (I guess this is essentially an analog problem, and I apologize if it is
too
> far off topic.)
>
> Thanks, Foster