Variometer Crystal Set
This simple crystal set is described in ‘The Boy’s Book
of Wireless’ by Ernest H. Robinson, published by Cassell and Co. of
I have built this set following closely the instructions
and diagrams as set out by the author and have tried to make it look
by using materials similar to those available to a constructor in 1923.
would have been nice to wind the coils with cotton covered wire but it
difficult to find these days. Brass and nickel plate parts and an
wooden base board give that antique look.
In the circuit diagram a 0.002 capacitor is shown
the headphones. The author suggests that this is optional and he leaves
on the construction diagrams. I have found its inclusion makes no
the performance of the set as most magnetic headphones have ample self
Using a variable inductor as the tuning element was
common in early crystal set designs as it avoided the purchase of an
variable capacitor but it did have some drawbacks. Selectivity is broad
tuning range is restricted, not covering the entire broadcast band.
though that in 1923 even a large city such as London had only one
station so adjacent channel interference was not a problem! The theory
operation is that the inductor acts as a loading coil to bring the
resonance, so the ‘tuned circuit’ is the inductor plus the capacitance
short (less than a quarter wavelength) antenna. Things would probably
too well if the antenna was longer than a quarter wavelength and
that would mean an antenna length greater than 75 meters or 250 feet.
I buy most of my hardware and
electronic bits and pieces
from an Australian outlet called Jaycar Electronics. They do a very
order service and stock nearly everything you could ever need. They
website and catalog in US dollars. Check out http://www.jaycarelectronics.com
online catalog and to look at the parts I mention and http://www.jaycarelectronics.com/catalog
to order a paper catalog .
The base board is made from 3mm thick black acrylic
and measures about 125mm x 175mm. I found some acrylic which was glossy
side but matte on the other. The matte side looks like the traditional
board used in early radio equipment. I get my acrylic from a factory
makes plastic advertising signs. They always have cheap off-cuts. A
12mm x 12mm
timber edging gives room under the base board for the wiring. A small
picture frame makes a ready made edging. Check out K-Mart or similar.
NEVER solder any connections to components while mounted on the acrylic
board – ACRYLIC WILL MELT! Use solder tags then bolt it all
catalog number HP-1350)
I make these from nickel plated brass 10mm long
spacers (Jaycar catalog number HP-0900). These spacers are tapped
a standard 3mm metric thread. Drill a 2mm hole through the side of the
about 5mm from one end such that a wire placed through the hole will be
firmly by a screw screwed in from the end of the spacer. Get some 3mm
fit. (Jaycar catalog number HP-0400)
look, remove the nickel plating with fine
sandpaper and polish up the underlying brass.
Cats Whisker Holder
The diagram explains the construction pretty well.
brass strip and threaded rod came from a hobby shop. The brass strip
6mm wide by 0.6mm thick by 105mm long and was bent around a 25mm
of PVC pipe. The threaded rod is about 40mm long. The brass adjusting
nuts/knobs came from the tops of old dry cell telephone batteries. You
have to hunt through your junk box for something similar. The
fine bare copper wire soldered in place.
The crystal holder is made from a cut down male and
female ‘F’ connector. This type of connector is used on RG-6 and RG-59
coaxial cable. What you need here is an F59 screw on plug (male) and a
chassis or wall plate socket (female).Have a look at the pictures in
catalog. (Jaycar catalog numbers PP-0637 and PS-0645).
Start with the male plug. What we want from
here is the threaded coupling ring. Slip a small hacksaw blade behind
coupling ring and carefully cut the body of the plug in half. This will
the ring to fall free. Now take the female socket and using a pair of
pull out the connector pin from the back of the nylon center of the
Take twist drill that will fit into the front of the socket and drill a
hollow in the nylon center of the socket. Do not drill away any metal.
suitably sized piece of crystal (Galena) into the front of the socket
have just drilled out and secure it by screwing on the coupling ring
from the male F connector plug. Mount the crystal holder through the
and secure with the nut and washer supplied. A solder connection is
made to the
side of the washer before mounting. (otherwise you will melt the
board). By the way, there is a good description on this website about
make Galena crystals using lead, sulphur and your mother’s best tea
but don’t tell her. (See Fun Tips and Projects – page 2)
How to wind spiderweb coils is described elsewhere
this website. The diagram gives the dimensions of the cardboard formers
in this case are not removed when the coils are finished. Be careful
coils are wound and mounted so that the wire turns go in the same
wound 32 turns of plastic insulated wire on each of my coils, as much
could get on the formers. Varnished winding wire will allow more turns. The original notes suggested 40 turns but the
exact number to tune the broadcast band will vary slightly depending on
length of the antenna you are using. This is because of the simplicity
design where the antenna and the variometer coils form the tuned
plenty of lead length at the start and end of each winding to allow for
flexible connections. A coat of varnish
will help hold all the windings in place.
This set is no DX performer but is a simple design
close in domestic listening, selectivity is broad but local stations
nicely on 50 feet of antenna wire and a good water pipe earth. I have a
national broadcast station about 15Km (9 miles) away and when tuned in,
headphone volume is almost too loud. Tuning lower powered and more
is possible but the 50KW is always there in the background.
Written and Photos by
How a Variometer Works
A variometer is a device that uses two coils to
continuously variable inductance. If two coils are connected in series
spaced apart so that their magnetic fields do not interact, then the
value of inductance is simply the sum of the inductance of each coil.
If the coils are placed in a position where their
magnetic fields can interact then interesting things happen. The value
total inductance is found to be either larger or smaller than the sum
individual coil inductances, depending on the positioning of the coils
whether the wire turns are in the same or opposite directions.
anti-phase). The amount of inductance that is gained or lost is due to
phenomenon called Mutual Inductance.
If the coils are placed in-phase (the windings of
coils go in the same direction) then inductance is gained. If the coils
anti-phase (the windings go in opposite directions) then inductance is
Let’s check out the spiderweb coils used in the
variometer crystal set. Using the formers as described and winding on
of wire gives a coil measuring about 110uH on my inductance bridge.
Connecting them in series, with the end of the first
connected to the beginning of the second coil and the coils well
total inductance is as expected around 220uH.
If the coils are
now carefully laid one on the other in-phase (both windings in the same
direction) the total inductance increases to about 400uH. That’s nearly
the value of inductance we would expect from just adding the inductance
of both coils.
On moving the
coils apart the total inductance value will fall back to 220uH. In fact
inductance between 220uH and 400uH and be produced by careful
By reversing the connections to one coil (end of first
coil to end of second coil) or by just flipping the coil over puts it
anti-phase (windings in opposite directions). When one coil is laid on
other a decrease in total inductance to around 50uH is seen. Again,
coils apart will produce any value between 50uH and 220uH.
So here we have a way of smoothly tuning our crystal
by moving coils rather than using an expensive variable capacitor. But
a problem. If you have built the variometer crystal set you will have
that the change in inductance is not enough to tune across the whole of
medium wave broadcast band. The way around this is to tune the bottom
band with coils in-phase and switch to an anti-phase connection to tune
of the band.