[rec.audio] I built an ultrasonic receiver

parnass@ihuxz.ATT.COM (Bob Parnass, AJ9S) (04/15/89)

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                I BUILT AN ULTRASONIC RECEIVER

                     by Bob Parnass, AJ9S

   Both midwestern farmers and  radio  enthusiasts  suffered
   during  last  summer's  record  drought.  Many of us were
   plagued with power line interference, as the wooden power
   poles  shrunk  without  moisture, allowing the power line
   hardware to become loose  and  electricity  to  arc.   We
   could  hear  the  "eggs scrambling" noises throughout the
   shortwave spectrum, even up through 220 MHz in places.

   I spent a few days driving around the  neighborhood  with
   an  Icom R-7000 VHF/UHF receiver on the front seat trying
   to pinpoint the offending power line  fixtures.   I  even
   walked part of the area with a portable scanner, attenua-
   tor, and home made loop antenna.

   It was a challenge.  The fun of the hunt  outweighed  the
   annoyance  of  the  interference.  Although I was able to
   locate a few offending transformers,  the  power  company
   didn't return my call.

   Vowing to build another gadget  to  help  pinpoint  power
   line  leaks  for  the  next  time  this happens, I set my
   sights on building a  device  to  detect  the  ultrasonic
   energy   emitted   by   leaky  lines.   William  Nelson's
   Interference Handbook (Copyright 1981 by  Radio  Publica-
   tions,  Inc.)  shows  an  old  Hewlett Packard ultrasonic
   detector, useful for finding corona discharge in  the  30
   kHz to 50 kHz range (pg. 117).  Autumn rains came, vanqu-
   ishing the noise problem and cooling plans for an  ultra-
   sonic detector.


                   Article Revives Interest

   A magazine article  rekindled  interest  in  ultrasonics.
   February 1989 Popular Electronics featured a construction
   article  on   an  ultrasonic  receiver.    This   was   a
   superheterodyne  circuit,  with a tuneable local oscilla-
   tor.  I decided to build this unit and ordered a  kit  of
   parts from Krystal Kits.

   The kit consists of the printed circuit board, potentiom-
   eters,  ICs, resistors, capacitors, PVC pipe and end caps
   (used as an enclosure).  I bought a  3-3/4"  Radio  Shack
   piezo  tweeter  (40-1382) for use as an ultrasonic micro-
   phone, and supplied my own IC sockets, as none were  fur-
   nished.

   This Krystal Kit is nothing like a Heathkit.   There  are
   no  instructions,  just a reprint of the magazine article
   with a few hand drawn corrections.  Mechanical work, like
   laying  out and drilling holes for the enclosure, remains
   the responsibility of the builder.

   The printed circuit board  eased  construction,  but  was
   only  of  fair  quality.   There  were  extra splashes of
   copper, which required removal with an X-Acto knife.  The
   holes for the mylar capacitors and several resistors were
   spaced too closely, forcing  contorted  bending  of  some
   component leads.

   The oscillator was supposed to tune 15 kHz - 35 kHz,  but
   mine  would only tune 12 kHz - 21 kHz using the specified
   components.  I substituted a 2200  ohm  resistor  for  R5
   (originally  15K)  which  expanded the tuning range to 15
   kHz - 66 kHz.

   Only young listeners can hear pure sounds near  or  above
   20 kHz.  Using my ultrasonic receiver, I have listened to
   sounds I could otherwise not hear from these emitters:

    1.  televisions
    2.  older ultrasonic TV remote control transmitter
    3.  computer terminals
    4.  video cassette recorder
    5.  metal detector searchcoil

   Sound sources radiate almost equally well in  all  direc-
   tions  when  the  wavelength  is  large compared with the
   source.  At higher frequencies, and smaller  wavelengths,
   the radiation becomes more directional.

   Although the receiver  is  sensitive,  there  are  a  few
   shortcomings:   Moderately loud noises in the sonic range
   can leak through the receiver and be  heard.   Since  the
   receiver  is not shielded, it can detect some electromag-
   netic fields as well as acoustic signals.

   In conventional radio  receivers,  the  local  oscillator
   generates  a  fairly clean sinusoidal wave.  By contrast,
   the ultrasonic receiver uses a  square  wave  oscillator,
   which produces strong harmonics.  Thus, you can hear har-
   monics of the same sound at more than one  place  on  the
   tuning dial.

   I am awaiting the arrival of better weather so I can  try
   listening for power line corona and the ultrasonic sounds
   emitted by insects.

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Bob Parnass AJ9S,  AT&T Bell Laboratories  -  att!ihuxz!parnass - (312)979-5414