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Tapped Coil Crystal Radio Kit
In the early days of radio, about 100 years ago, experimenters used what they had to listen to
the strange sounds that seemed to come out of thin air. Radio was very new, and was used
mainly for important messages from ships, businesses and governments. But slowly, radio was
used to talk to regular folks — if they could afford to buy a radio receiver.
Radio receivers were expensive and most people could not afford to buy one. A simple radio
could be built using inexpensive parts and a few things that might be found around the house.
A few feet of wire and a paper tube to make a coil, some tin foil and cardboard for the tuning
capacitor, a razor blade and a pencil lead to make the detector — could be made into a
functioning Crystal Radio. The only special part needed was a pair of headphones.
This kit is a modern version of those early Crystal Radios. It has many of the same parts — a
large coil of wire wound on a paper mailing tube, a modern variable capacitor and a 1N34
diode for the detector — and will produce the sound of radio stations into the earphone.
Everything needed to make a working Crystal Radio is included in this kit. Building it will take a
few hours with simple hand tools. Once completed, a 100-foot roll of wire is attached to the
antenna terminal, and an earth ground is attached to the ground terminal. Turning the variable
capacitor will tune in local radio stations in the daytime, and radio stations from all over the
country at night. Sound from the air with no batteries needed!
Here are the parts for the kit
​
1.
Phenolic chassis
2.
Crystal earphone
3.
Tapped tuning coil
4.
Wires with lugs
5.
Variable capacitor with mounting screws
6.
Tuning knob
7.
B
ag
of Hardware
The
Bag
of Hardware has several different parts:
​
•
Short 8-32 brass screws with washer and nuts (2 sets)
•
Knurled nuts (4)
•
8-32 brass nuts (9)
•
Long 8-32 brass screws (9)
•
Push terminals for antenna, ground, headphones, and diode (6)
•
Jumper (flat brass tabs connected with solder)
•
Diode (tiny black-banded glass body with wire leads)
•
10K resistor (brown-black-orange bands with wire leads)
Assembly
​
1. Attach the Tapped Tuning Coil to
the
Phenolic Chassis
With all of the parts in hand, begin assembly.
Locate the Phenolic chassis. It has a number of
holes with black rubber feet attached to the
bottom:
​
Now, locate the Tapped tuning coil:
​
Using the long 8-32 brass screws and 8-32
brass nuts, attach the nine wires from the coil
to the chassis. The wires should be attached in
order, starting with the thin, red magnet wire on
the left side and ending with the other thin red
magnet wire on the right side. The other seven
red insulated wires go in order between the
magnet wires. The coil will still be loose.
​
Now, use the short 8-32 screws with washers and nuts to attach the black coil tubing to the
phenolic chassis. The screws will come up from the bottom and through the black tubing.
Finish by adding a flat washer and nut on top of the screw inside the tubing. Here are top and
bottom views of the chassis with the coil attached.
​​
2. Adding the Diode Connections
You will need the following parts for this:
•
black push connector (with a flat washer)
•
red push connector (without flat washer
•
the brass jumper
•
black wire with red lugs
•
red wire with red lugs
•
knurled nuts (two)
​
​
a) Separate all the parts of the black push
connector. Insert the connector itself through one
of the lugs on the black wire, then add the black
plastic spacer. Put this assembly through the left
horizontal hole as shown in the picture.
​
On the bottom side of the chassis, add the flat
washer, lock washer and nut. Tighten to just snug.
You’ll tighten all the hardware once more later on.
​
b) Now, take a red push connector (with no
flat washer) and separate its parts. Put the
connector itself with the red plastic spacer
through the right horizontal hole. On the
bottom side, add the Jumper, a lock washer
and a nut. Position the Jumper as shown in
the picture at the right.
Attach the other end of the black wire in 2a) to
the coil connection with a knurled nut as
shown above
c) Finally, attach one end of the red jumper wire to the coil terminal with another knurled nut as
shown below.
​​​
3. Adding Antenna and Ground Connections
You will need the following parts for this:
•
black push connector (without flat washer)
•
red push connector (with flat washer)
•
long black wire with red lugs
•
short black wire with red lugs
​
a) Separate the parts of the red push
connector. Now, put the connector through
the other end of the red wire you added in
step 2c). Add the red plastic spacer and put
​
it in the upper vertical hole of the chassis.
Add a flat washer, lock washer and nut on
the bottom side of the board and tighten to
just snug.
b) Separate the parts of the black push
connector. Put the connector through one
end of the short black wire and add the
black plastic spacer. Put the assembly in the
lower vertical hole.
c) On the bottom side, add one end of the
long black wire with red lugs, a lock washer
and a nut. Tighten to snug. Use the pictures as a guide.
​
​
4. Adding the Headphone Connections
You will need the following parts for this:
•
black push connector (without flat washer)
•
red push connector (without flat washer)
​
Separate the parts of the red push connector.
Add the red plastic spacer to the connector and
put it in the upper vertical hole on the right side
of the chassis. it should go through the other
side of the Jumper you added in step 2b). Add a
lock washer and nut and tighten to snug.
Separate the parts of the black push connector.
Add the black plastic spacer to the connector
and put it in the lower vertical hole on the right
side of the chassis. On the bottom, add the
other side of the long black wire with red lugs
you added in step 3c). Add a lock washer and
nut and tighten to just snug.
​
5. Adding the Variable Capacitor
You will need the following parts for this:
•
Variable Capacitor with mounting screws
•
long black wire with red lugs
•
knurled nuts (two)
​
Connect the other end of the short black wire
​
you added in step 3b) to the far left-hand coil
terminal. Add one end of a long black wire to the
same terminal. Add a knurled nut and tighten to
snug.
​
Thread the long black wire under the other wires
all the way to the right side of the chassis. It will
be trapped between the chassis and the variable
capacitor frame.
Remove the mounting screws from the tuning
capacitor and set aside.
Align the capacitor on the two remaining vertical holes on the right
side of the chassis and put the upper screw in loosely. Now, align the lug of the long black wire
between the capacitor and the chassis so that the remaining screw will go through the lug
when it is screwed into the capacitor. Gently tighten both screws.
​
6. And the rest of it . . .
You will need the following parts for this:
•
Tuning knob
•
Diode
•
Resistor
•
Earphone
•
Antenna
•
Ground
Start by gently tightening all of the hardware to make it secure. It is best to align the holes of
the push terminals as shown in the picture below. Then:
1)
Attach the tuning knob by placing it on the shaft of the variable capacitor and tightening the
set screw with a small slotted screwdriver.
2)
Connect the diode to the push terminals by pushing down on the terminal and inserting
each wire lead through a terminal.
3)
Connect the resistor leads to the headphone terminals.
4)
Connect the earphone to the same terminals
5)
Stretch the antenna wire out and connect one end to the antenna terminal
6)
Connect the ground terminal to a cold water pipe, the neutral (ground) connection on an
electrical outlet or a ground rod.
Start listening!
​
Operation
Crystal radios require a wire antenna (the longer the better) and a good earth ground. The kit
includes a 100-foot roll of #22 hookup wire and should be long enough to hear some stations.
Unroll the wire and string it from any convenient support. It doesn’t have to be too high, so the
lower branches of trees or even along a wooden fence will work.
The ground is essential. There are several ways to get one. Possibly the easiest is to connect a
short wire from the ground terminal on the radio to the central screw that attaches an electrical
outlet cover plate. An exposed copper or iron cold water pipe will also work. Or, you can drive
a piece of copper pipe or ground rod directly into the ground outside. Try each method to see
what works best.
Once you have an antenna and ground, connect the piezo earpiece. Listening to the earpiece,
you should hear one or more radio stations as you turn the big knob through its rotation.
Daytime listening will be limited to local stations, but night time will be much more interesting.
This is because layers of the earth’s ionosphere (the ionized layers in the earth’s atmosphere)
change to higher levels after the sun goes down. The higher ionized layers then reflect radio
signals from much greater distances. At night you may hear radio stations from all over North
America.
The coil settings provided in the instructions should produce good results, but moving the tap
for the detector or the antenna may improve reception. A longer antenna will also improve
performance.
​
