Elenco Electronics K-26 Assembly And Instruction Manual
METAL DETECTOR KIT
MODEL K-26
Assembly and Instruction Manual
Elenco
Copyright © 2003, 1989 by ElencoTMElectronics, Inc. All rights reserved. Revised 2003 REV-D 753226
No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.
TM
Electronics, Inc.
PARTS LIST
If you are a student, and any parts are missing or damaged, please see instructor or bookstore.
If you purchased this metal detector kit from a distributor, catalog, etc., please contact Elenco
(address/phone/e-mail is at the back of this manual) for additional assistance, if needed.
RESISTORS
Qty. Symbol Description Color Code Part #
1 R2 4.7kW 1/4W 5% yellow-violet-red-gold 144700
1 R1 15kW 1/4W 5% brown-blue-orange-gold 151500
1 P1 Trim Pot 50kW 191552
CAPACITORS
Qty. Symbol Description Part #
1 C1 680pF Discap (681) 226880
1 C2 .0015mF Discap (152) 231517
SEMICONDUCTORS
Qty. Symbol Description Part #
1 Q1 Transistor MPS5172 325172
MISCELLANEOUS
Qty. Symbol Description Part #
1 PC Board 518026
1 S1 Switch 541102
1 Solder Roll 24” 551124
1 B1 Battery Snap 9V 590098
1 Wire #26 Enamel 45’ 846000
TM
Electronics
PARTS IDENTIFICATION
Resistor Transistor
Capacitor
Discap
IDENTIFYING RESISTOR VALUES
Use the following information as a guide in properly
identifying the value of resistors.
Switch
Battery SnapTrim Pot
IDENTIFYING CAPACITOR VALUES
Capacitors will be identified by their capacitance value in
pF (picofarads), nF (nanofarads), or mF (microfarads).
Most capacitors will have their actual value printed on
them. Some capacitors may have their value printed in the
following manner.
The above value is 10 x 1,000 = 10,000pF or .01mF
The letter K indicates a tolerance of +
The letter J indicates a tolerance of +
Multiplier
For the No. 0 1 2 3 4 5 8 9
Multiply By 1 10 100 1k 10k 100k .01 0.1
Second Digit
First Digit
10%
5%
Multiplier
Tolerance
Note: The letter “R” may be used at times to
signify a decimal point; as in 3R3 = 3.3
-1-
INTRODUCTION
There are many buried treasures waiting to be
found. While everyone dreams of finding a fortune
in gold coins, few do anything to look for them. Now
with the K-26 Metal Detector, you have a chance.
Although you may not find a fortune in gold, you
should find a few silver coins if you work the
beaches or parks. If nothing else, you’re sure to
have fun with this metal detector.
The Metal Detector basically is an oscillator that
transmits in the AM radio band. When a radio is
placed near the oscillator, it will emit an audible
tone. The frequency transmitted by the oscillator will
vary when brought close to a metal object. Thus,
the tone produced by the radio will vary in pitch,
indicating that a metal is present. We shall study the
theory of how the oscillator works and why its
frequency changes when brought near a metal
object.
THE BASIC OSCILLATOR
Figure 1 shows the basic circuit of the oscillator. When the switch SW1 is
closed, a current will flow in the P1-R1 ciricuit. This will turn on transistor Q1
and send a current into the inductor-capacitor circuit (LC circuit). The inductor
is made by winding a 5” diameter coil with 30 turns of copper wire. The LC
circuit is the heart of the oscillator. Every LC circuit has its own resonance
frequency. The resonance frequency of this circuit is in the AM radio band.
If we take part of the energy in the LC circuit and feed it to the emitter of Q1,
the transistor will amplify this signal and cause the circuit to oscillate at the LC
resonance frequency. By varying P1, the current through transistor Q1 will
change, thus forcing the frequency of the oscillator to change slightly.
S1
B1
P1
R1
C1
L1
Q1
C2
R2
THE METAL DETECTOR OPERATION
Obtain a small portable AM radio and place it near
the Metal Detector. Tune the radio around the
midband and away from radio stations, you should
hear only static. If there are AM stations nearby you
will get whistling. This is because the metal detector
oscillator is mixing with the frequency of the AM
station. Now turn control P1 until the static gets
quiet. The metal detector is now tuned to the AM
radio frequency. Now take a piece of iron metal and
bring it close to the detector. Note the change in the
static in the radio or a change in the pitch of the
whistle. This is because you have increased the
inductance of the coil and thus changed the
resonance frequency of the circuit. The radio is now
tuned to the new oscillator frequency.
Magnetic fields move easier in the presence of iron,
nickel and other materials. Thus the inductance of
the coil will increase when these metals are present.
This increase in inductance will cause the LC circuit
to oscillate at a lower frequency. These magnetic
fields are disturbed when in the presence of silver,
aluminum, copper and other highly conductive
Figure 1
metals. Thus, when these metals are brought near
the coil, the frequency transmitted by the oscillator
increases. The radio responds to the oscillator
changes.
Now you have some understanding of how the
Metal Detector works. Go out and look for your
fortune. Wish you lots of luck.
AM Radio
Attach the circuit
board to the radio
Non-metal Support
L1
Figure 2
-2-
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