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'[PIC]:voltage drop'
2001\05\25@082411 by sam woolf

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I'm sending a pulse from a p16f84 down a fairly long wire to some ultrasoonic transducer circuitry.
By the time it gets there the voltage has dropped significantly. Whats the best way to overcome this problem? do I need to build some kind of booster circuit? Sorry if this is a overly simplistic question- I'm an electronics newbie..
Sam,.

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2001\05\25@084937 by Dale Botkin

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On Fri, 25 May 2001, sam woolf wrote:

> I'm sending a pulse from a p16f84 down a fairly long wire to some
> ultrasoonic transducer circuitry. By the time it gets there the
> voltage has dropped significantly. Whats the best way to overcome this
> problem? do I need to build some kind of booster circuit? Sorry if
> this is a overly simplistic question- I'm an electronics newbie..
> Sam,.

Bigger wire?  What size wire and how long are you using?

Dale
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2001\05\25@085714 by Chris Eddy

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Sam, with a coax cable, you must present the signal to the cable with a
series resistance equal to the cable impedance, and you must end at the
load in the characteristic impedance.  I assume you are stuck at the
load with the mismatched transducer, probably looking for some sort of
reflection from space.  But, I would still endevor to terminate in 50
ohms, or whatever it is.  That may require an inductance in parallel
with the transducer.  On the pulse generate end, you want to drive the
signal through a 50 ohm resistor into the center conductor.  All of
this, of course, could be 50 ohms, 75, or 92, or whatever you have.

In this case, using a pulse, your signal is made up of frequencies all
the way up into the clouds, and as such any mismatch will reflect, and
probably spread the frequencies out in time, making your pulse look like
a lumpy blob at the far end.

Presumably, if there is no target in the front of the transducer, and
you are all matched up, you will get no reflection at all.

Chris~

sam woolf wrote:
>
> I'm sending a pulse from a p16f84 down a fairly long wire to some ultrasoonic transducer circuitry.
> By the time it gets there the voltage has dropped significantly. Whats the best way to overcome this problem? do I need to build some kind of booster circuit? Sorry if this is a overly simplistic question- I'm an electronics newbie..
> Sam,.
>
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2001\05\25@190517 by David Cary

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Dear Chris Eddy and Sam Woolf,

Chris Eddy <EraseMEceddyspam_OUTspamTakeThisOuTNB.NET> on 2001-05-25 07:57:40 AM talked about impedance
matching, then said
> On the pulse generate end, you want to drive the
> signal through a 50 ohm resistor into the center conductor.

I don't think so. If I have a signal generator, a long wire, and a transducer at
the other end, adding a resistor in series with the wire *decreases* the energy
that makes it to that transducer. Isn't that exactly the opposite of what Sam
Woolf asked for ?

Impedance matching is the solution to some *other* problem.

sam woolf <samwspamspam_OUTCOGS.SUSX.AC.UK> on 2001-05-25 07:19:41 AM wrote:
> I'm sending a pulse from a p16f84 down a fairly long wire to some ultrasoonic
transducer circuitry.
> By the time it gets there the voltage has dropped significantly. Whats the
best way to overcome this problem? do I need to build some kind of booster
circuit? Sorry if this is a overly simplistic question- I'm an electronics
newbie..
> Sam,.

Are you saying that the transducer circuit works fine when directly attached to
the PIC, but isn't loud enough once you insert that long wire between them ?

Lots of different options. The ``best'' one depends heavily on what exactly is
on the far end that you're trying to drive.

(Is this a FAQ yet ?)

- Same voltage, more current: inverters, or line drivers like
 TI TPS2828DBVT
 TI TPS2815
 TC1412
 (less than $1 at DigiKey)(each can drive 1 A into capacitive loads)
, or a simple discrete transistor driver. (simplest)

- Same power, more voltage: impedance matching using inductors; pulse
transformers.

- More voltage and more current: op amps; for maximum power people use discrete
transistor driver + transformer.

Is your ``transducer circuit'' a
 6500 Sonar Module
 http://rdrop.com/~cary/html/ultrasonic.html
?

If not, I'd be interested in learning more.

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David Cary

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2001\05\26@051939 by Peter L. Peres

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The easyest way is to treat the line as a transmission line and adapt its
impedance at both ends. A thin single wire running in a bunch will likely
be near 600 Ohms impedance. To adapt it, you add a 600 Ohm R in series
with the PIC output and a 600 Ohm load at the far end, to GND. The
amplitude of the signal will be halved but the pulse will be clean. You
can probably decrease the series R to 470 or lower (on account of the PIC
output impedance). The coupling to (and from) the other wires in the bunch
can become very significant after ~60 feet. Use a twisted pair or a
shielded twisted pair for better results.

Depending on how long 'fairly long' is you may want to go to differential
transmission, carrier transmission, coax, or something else. Also pay
attention to the resistance of the wire. Fairly long thin wire runs tend
to add up ohms quickly. Also if this is a pulse wire some attention should
be paid to how it is laid out. Sharp corners around conductive objects can
cause nasty effects.

Peter

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2001\05\26@051956 by Peter L. Peres

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David Cary <RemoveMEdcaryTakeThisOuTspamBRUNSWICKOUTDOOR.COM> wrote (about matching the line):
> I don't think so. If I have a signal generator, a long wire, and a
> transducer at the other end, adding a resistor in series with the wire
> *decreases* the energy that makes it to that transducer. Isn't that
> exactly the opposite of what Sam Woolf asked for ?

That is correct but there is no easy way around it. To maximize power
transfer and adapt at the same time means adapting at the operating
frequency, which is easiest done using reactive elements. In particular a
properly chosen coil in series with the wire and transducer at the PIC end
will series-tune the assembly and the transducer (complete with cable).
This assumes a capacitive transducer and that the transducer impedance is
not extremely different from the cable's. Some damping resistors may be
required to avoid ringing in a long line.

Peter

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2001\05\27@195647 by Gennette, Bruce

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Why don't you use active Lo signaling?

Have the transducer supply a voltage to a 10K resistor at its end of the
long wire.  Monitor the other end of the resistor, it should read full
voltage.  Have your PIC also supplying the same voltage to its end of the
long wire through an I/O, and pull it to 0V for the pulse.  You *WILL* get a
full strength, negative going signal with this scheme which is used *A LOT*
in interconnected electronics.  (You may need to invert the signal, or not
depending on the design of the transducer circuitry).

Bye.

{Original Message removed}

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