CIRCL E 185 ON FREE INFORMATION CARD
BATTERY TESTERS
Thank yo u fo r the
"Battery
Test-
ers" sect ion in Hard ware Hac ker
(Radio
-Electro
nics
,April 19
91
). It
was short, yet provided a lot of usef ul
information . However, I wo uld li ke t o
point out that the t e
ster
include d with
Du racell 's
AA-t
yp e ce lls me asur es ,
500 mA o nly when 1.5 volts are being
tested. Since power (and a resist or's
heat) is meas ured in watts , not am-
peres, th e " milliamme ter " is actually
a wa
tt
mete r. It sh ould s how 0.75
watts full-scale (1.5 volts ti mes 0.5
amps)
DARREN HOM
Walnut
Cr
eek, CA
circuit t o calibrate t he Gauss mete r.
At the sa me tim e, I underst and t hat
hand-wound coils have been used for
several decades by co untless elec-
tronics e nthusiasts w ith satisfactory
results . Mayb e I'm loo king for preci-
sion in every aspec t of my work, even
when it's not required.
DAVID KATZN ELSO N
Willingb
or
o,
NJ
Iapprec iate and ag ree with yo ur co n-
cern over the
diff
iculties
of
winding
the l
ar
ge calibration coi l. Ioffer the
foll
OW
ing sugges tions: Wind the coil
conc entrically first on a fixed form ,
and then stretch it out to the one
met
er
length: or use an alternative
non- so len
oid
de
si
gn, s uch as
tw
o
separate co ils in a Helmhotz-type ar-
rangement . Please no te that, in rec-
og ni
tio
n of
exact
ly
yo
ur
kind
of
con cerns, th e met er 's p ickup co il is
inte
gr
ated
in the PC bo
ar
d.
REI
NHA RD METZ
GOTTCHA!
Regard ing the " Poor M an's Laser
Pri
nter"
(
Radi
o-Ele
c
tr
onics
,April
1991): First, mo st of tod ay's co piers
wo uld dete ct the exposure lamp (lig ht
bulb) not illuminating and would ge n-
erate a fault code. Lamp illumination
is detec ted by a CDS cell pointed at
the lamp or on the lamp regulator
board . Sec ond, it would b e simpler t o
pull a lead off the lamp rather than to
remove t he lamp, if you have a cop ier
that does not d et ect the lamp il-
lumination. Lamp removal is not usu-
ally a DIY project. Finally, the focal
plane of co piers is at the surface of
the original glass, so resoluti on might
be a problem due to the monitor 's
curvat ure.
MIKE
ZYC
H
Elk Grove, IL R-E
CIRCL E 58 ON FREE INFORMATIO N CARD
~IE
4251 CYPRESS DRIVE
~
E
JACKSON
,MISSISSIPPI
39212
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FOILED BY COILS
The proj ect ent itled " Build This
Mag
net ic Fiel d M
et
er " (R
adio
-
Electronics
,April 1991) brought to
mind a difficulty I have always had
w ith th e arc haic "art " of coil Winding.
Factory-made co ils are co nst ructed
using highly specialized jigs and fix-
tures, so that their specified values
are usu ally reliable . Since the average
hobbyi st is equipped wit h a maximum
of two hands, it is not illogical to pre-
sume that a fairly co mp lex home-bui lt
device is required to lay down mag-
netic wire on a co il form with acce pt-
ab le r esu lt s . (A cc
ept
ab le , to me ,
me an s ri
gid
ly a nc ho red , eve nly
space d w ind ings th at co n
for
m to
95% of a co mparable factory stan-
dard. In ot her word s, why st uff a cir-
cuit boa rd with high-quality silico n if
the int erfacing discret e co mponents
are not approx imately e qual in quality
reliability?)
The one-meter-Iong co il shown in
Fig. 6of the article raises my coil-
winding hack les to a fever pitchIThe
ext reme ly w ide va riat ions in to ler-
ance that would surely result from
f
abr
ic
ating
th at e lep ha nt
ine
coil
makes me wonder about using that
COOK'S
INSTITUTE
OF ELECTRONICS ENGINEERING
WARNING:
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and
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1
-'-
16 _ _No
Florida
Sales
!
CIRCLE
186
ON FREE INFORMAT ION CARD
(f)
o
z
~
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Our
Record
I-
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1rl
ju ry that all products purchased,now and in the f uture, will
-J
o nlybe us ed on Cab le
1V
systems with prope r a uthorization
W f rom local officials o r ca ble
co
m
p
a
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officials in accordance
o
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Date: _