My nearby seismic buddy and I have established a local area tripartite 
network for the study of microseisms.  We record for short periods 
simultaneously at three sites, and require accurate timing of the three 
records in order to calculate time differences of arrival.  We use Dataq 
RS-154 and RS-194 digitizers which have a digital input for registering time 
marks on the records.  We use WWVB controlled wall clocks for generating 
minute marks.

  My buddy uses large LCD RadioShack clocks, and photo-optically looks at the 
lower right segment of the tens-of-seconds digit.  This segment is black for 
all digits except for the digit "2," so every minute it disappears at 20 
seconds past the minute for 10 seconds.  His pickup circuit has a logic high 
output whenever the segment is black and low when it is absent.  The Dataq 
recorder will make a mark at either of the transitions.  He uses the 
high-to-low transitions, so gets minute marks 20 seconds past the minute.

  I use an analog wall clock, brand name "Atomix," distributed by Cheney 
Instruments.  I removed the plastic face, glued a . 25-inch circle of 
aluminum foil reflector to the sweep second hand, and replaced the face.  The 
pickoff is held in place by a rubber band and string, and is positioned on 
the clock face to detect the second hand when it is at the even minute.  The 
pickoff circuitry is mounted in a wooden block, painted black.  The light 
source is a red LED and the detector is a phototransistor, coupled to a 
medium-gain NPN transistor.  There are some potential problems with this 
scheme.  For one thing, the clock goes through an adjustment procedure every 
few hours to synchronize with WWVB.  It starts running fast and then slows 
down to fall into alignment with the radio signal.  Except for the few 
minutes when this is going on, the clock is very accurate.  Another potential 
problem is that the second hand does not simply jump from one second to the 
next, but overshoots before settling down.  The pickoff must be carefully 
positioned to avoid sensing the bounce from second 59 and well as the dwell 
at second zero.  I purchased two of these clocks from HeartlandAmerica, and 
one of them had too much bounce to be useful.

  We have tested both marking schemes at the same site on a single digitizer, 
and have determined that there is no detectable differences in our timing 
(except for the fixed 20-second offset.)

  With analog wall clocks, it is also possible to generate one-second marks 
by inductively picking up the clock's stepper motor's current pulse.

  If anyone is interested in trying the above schemes, I will be happy to 
give further details.

Bob McClure
  My nearby seismic buddy and I have established=20=
a local area tripartite network for the study of microseisms.  We recor=
d for short periods simultaneously at three sites, and require accurate timi=
ng of the three records in order to calculate time differences of arrival.&n=
bsp; We use Dataq RS-154 and RS-194 digitizers which have a digital input fo=
r registering time marks on the records.  We use WWVB controlled wall c=
locks for generating minute marks.



  My buddy uses large LCD RadioShack clocks, and photo-optically looks=20=
at the lower right segment of the tens-of-seconds digit.  This segment=20=
is black for all digits except for the digit "2," so every minute it disappe=
ars at 20 seconds past the minute for 10 seconds.  His pickup circuit h=
as a logic high output whenever the segment is black and low when it is abse=
nt.  The Dataq recorder will make a mark at either of the transitions.&=
nbsp; He uses the high-to-low transitions, so gets minute marks 20 seconds p=
ast the minute.



  I use an analog wall clock, brand name "Atomix," distributed by Chene=
y Instruments.  I removed the plastic face, glued a . 25-inch circle of=
 aluminum foil reflector to the sweep second hand, and replaced the face.&nb=
sp; The pickoff is held in place by a rubber band and string, and is positio=
ned on the clock face to detect the second hand when it is at the even minut=
e.  The pickoff circuitry is mounted in a wooden block, painted black.&=
nbsp; The light source is a red LED and the detector is a phototransistor, c=
oupled to a medium-gain NPN transistor.  There are some potential probl=
ems with this scheme.  For one thing, the clock goes through an adjustm=
ent procedure every few hours to synchronize with WWVB.  It starts runn=
ing fast and then slows down to fall into alignment with the radio signal.&n=
bsp; Except for the few minutes when this is going on, the clock is very acc=
urate.  Another potential problem is that the second hand does not simp=
ly jump from one second to the next, but overshoots before settling down.&nb=
sp; The pickoff must be carefully positioned to avoid sensing the bounce fro=
m second 59 and well as the dwell at second zero.  I purchased two of t=
hese clocks from HeartlandAmerica, and one of them had too much bounce to be=
 useful.



  We have tested both marking schemes at the same site on a single digi=
tizer, and have determined that there is no detectable differences in our ti=
ming (except for the fixed 20-second offset.)



  With analog wall clocks, it is also possible to generate one-second m=
arks by inductively picking up the clock's stepper motor's current pulse.


  If anyone is interested in trying the above schemes, I will be happy=20=
to give further details.



Bob McClure