Dwayne (and all),
Ok, some deeper details (and exposing my ignorance) ... Currently, the
piezo signal passes through a 47K and a pair of 1n4148 diodes configured
as both you and Myke suggest before it feeds a non-inverting opamp then
into a buffer opamp and finally into the PIC RB0/INT. I have 4 available
I/O port A pins that I want to control the sensitivity with. I am now
using these 4 pins (RA0-RA3) by "switching" in 4 resistors to "ground" on
the the (-) input of the opamp to adjust the gain of that stage. With 4
pins I get 16 levels. This works except I can't seem to get acceptable
"even" steps throughout the adjustable range. The extents are fine but
doesn't seem to be in linear increments. By experimentation, I found that
placing a 1K resistor across the piezo to ground knocks the signal down so
that I might be able to use it to help get the overall gain where it works
for me. I would use one of the 4 pins for this resistor and use the
remaining 3 on the opamp as they are. Since my existing protection diodes
are on the other side of the 47K, I'm not sure how effective they are right
at the piezo where I want to connect this new 1K. So Dwayne, in answer to
your impedance question, I will try different resistor values until I find
one that works. Yeah, I'm a bit weak on theory ;-). So, assuming I can
get this to adjust acceptably for me, how likely is the PIC to survive ...
long term even.
Additionally I've also considered using a digital pot IC but they are
relatively expensive and seem to be hard to get. I've also thought of
using a par/ser shift register IC like the '595 but can't find one with
open collector outputs. More levels would be great but this cheap and easy
way should be fine if I can get it dialed in.
Hope this ascii art is sufficient...
Vcc
|
/\ 1n4148
----
47K | |\
Piezo |-----/\/\/\/\/\----------------|- \
| | | \
/ /\ 1n4148 | \--------------
\ ---- | / |
/ 1K | | / |
\ Gnd ---|+ / |
/ | |/ |
\ | |
| |-------/\/\/\/\---|
| /
PIC \
i/o /
RA0 \ 3 of these to adj gain
/
\
|
|
PIC
i/o
RA1-RA3
Thanks guys.
--rob--
{Quote hidden}>Dwayne said:
>Hmm . . .
>
>You need to think this over a bit. If you use a PIC pin to switch in the
>resistor, the protect diodes are going to clamp to about (Gnd-0.6v) and
>(Vcc+0.6v). In other words, the impedance as seen by the piezo is going to
>change as the protect diodes conduct. Is this a problem?
>
>2nd - the current spikes that get routed to the PIC's substrate when those
>protect diodes conduct ** MIGHT ** cause anomolies to the PIC's operation.
>I'd look for problems in the a/d subsection (if any) as well as possible
>program counter type glitches.
>
>I guess it comes down to what you are trying to accomplish. Why do you need
>to change the loading on the piezo?
>
>Note that most any solid state switch is going to have these kinds of
>problems. You may be stuck with a small reed relay.
>
>dwayne
>>Rob wrote:
>> I need to attenuate the signal from a piezo disc by using a port B pin on
>>a 16F84. I want to switch in a resistor (parallel to "ground") by toggling
>>the port pin between a low out and high Z input. Effectively paralleling
>>the resistor with the piezo to ground. The piezo will see some pretty
>>healthy shocks so it will generate a substantial signal. The resistor
>>value looks to be in the neighborhood of 1K or so. My question is, will
>>the port pin survive without any additional external protection?
planet.eon.net>
