illrrrrrrW

Irving M. Gottlieb W6HDM
M~W
931 Olive Street
Menlo Park CA 94025
APPROACH
~""=~~.........;.---'
I or tbe metalJ oea · tor
-::;~,,; ~ ==~
During the .past several years, a number
of metal locators have been described
in technical literature. An excelle nt example
is the constructional article which
appeared in the July '67 issue of Popu lar
Electron ics. The design philos o phy o f these
locators ha s be en directly inherited fro m
vacuum-tube technology ; it ha s generally
be en found that a decided upgrading in
performance could be reliably attained
from the mere conve rsio n to solid-state
circ uitry . Nonetheless, I have lo ng felt that
a little imagination might be profitably
unleashed toward a somewhat different
approach to the electronic de te ct ion of
metallic objects. After ruling out "change
fo r change's sake," the reasoning evolved as
follows: The transist or AM radio ha s
become such a popular item that actu al
replacement is now the acce pted "servicing"
method. Why not use the very considerable
amplification available from even
the most in expensive of these set s? And, to
make the project just a bit challenging,
why not stipulate that no surgery of any
kind be permitted on the set ?
Shortly after fo rmulation of this idea, it
was discover ed that a superficially similar
approach had been anticipated and imp lev
mented by others. However, in all situations
inve stigated, th ere was either depen ­
dency upon a broadcast statio n, or modif i­
cation of the radio 's circuitry . The se te ch ­
niques were rejected as u ndesirable. On e,
then, was left with the clear obj ecti ve of
dev elo pin g a scheme which would enable
--- ::: ~.---
the deployment of any transistor radio in
an arrangement capable of providing ' a
response to metallic objects: This would be
accomplished without electrical con nection
to the set , and would involve no operation
al dependency on broadcasting stations
. (An added feature of such stipulations.
is that the radio co uld be immediately
used for its intended function if so
desired.) Not only were these objectives
ach ieved in a straightforward manner, but
performance definitely exceeds that gen erally
obtained from the conventional lo cator.
The ensuing dis cussion describes the
theoretical and practical reasons why this is
so :
@I I' REF OSC 1
(F IXED FREQ) I I
,
FROM
'!"",=~"- '
CONVERTOR
~I .
sz
i
~t8,L ,
_ FREQ ~ ~N.CY
I· F CHA NNEL
/et9
/ 12 - LOOP OSC
l KHz
AUDIO S IGNAL
1;2·1 1 APPEARS HE RE
~ /
Fig. 1. Block diagram depicting basic concept of
locator. The two oscillators are "radiationcoupled"
to receiver's i-f channe l. Converter
plays no role in this schem e, and tuning is of n o
conseq ue nce except th at it. is desirable to be
tuned b etween station s to avo id their interference.
terence.
•
10 73 MAGAZINE
,
I
j