It was the first circuit i started with, and it can be
build with really junk items.
Apart from the diode (which replacement would need a PbS,
Lead Sulphyde, galena cristal) everything you
can see in the circuit (for the no power version) can be
built with enamel wire, alluminium cooking foil, carton
wax and so on. The real painful part is to find a high
impedance headphone, it is unlikely yo find one at present
day but it is possible. In addition I substituted an high
internal impedance headphone with a opamp and normal
headphone but doing so you need power.
Anyway if you are lucky you can find such a headphone.
The AM signal is captured by the antenna , 10 mt long
horizontal wire, WELL insulated from earth (I mean distant,
to lower the stray capacitance coupling with ground which
will adsorb some signal ). The Inductor and capacitor
forms a resonator, that will tune with the station which
frequency is F= 1/ (2*pi*sqrt(L*C)), so adjust C1 for
tuning. The signal is rectified (demodulated) and smoothed
by C2. The high impedance headphones has fine internal
wiring and lots of windings, so even a small current will
produce an audio output.
Construction of components:
10 mt of electrical wire, WELL insulated from ground (use
plastic bottle caps and hoowup wire to keep it
high). This must be the arrangement |=wall ..=hookup wire
o=plastic cap --=wire antenna :
| 10 mt |
| | |
wind 60 windings of enamel copper wire onto a 2 inch
ferrite core (1 cm diameter or a bit less)
Like for tesla coils, it must be a good ground, otherwire
the signal will be poor. Place a metal nail and connect
to the circuit with alligator clips.
It would be difficult to make it reliable, in addition
needs a capacitance meter, better buing it or finding
it in old broken radios.
But if you want to build it use the parallel plate
capacitance formula and make two carton disks (10 cm
with a alu foil semi disk each separated through paper,
place a nail in center (make sure to not short the
and connect it with small wires, rotating a disk respect to
the other will change shared surface and increase
capacitance, but how i said this is not reliable because as
soon ase you approach it, you will detune it. So better
use a commercial variable cap with insulated lever.
use paper, alu foil or ldpe (but i don't think that you
will be interested in a 10KV capacitor so use thin LDPE)
Buy it. I don't advice you looking for Lead Sulphyde
High impedance headphones: Difficult to find , the only
substitute is an amplifier (see below)
Amplifier + low impedance headphones: Use a general purpose
opamp with feedback resistors and a
low impedance headphone (as these of cassette, cd players),
but you need power... :-(
It is very nice to build, hearing a sound of a radio
station without power is very fun.
I reconstrected it basing on my fathers rememberings and
very old texts, improved a bit with some physics.
Constructing from almost anything is possible , even the
headphone, as many cristal radios have been found
in nazis prison camps build by prisoners from very limited
resources (as everything in a prison camp) and some
in foxholes (called foxhole radios).
Anyway, as i ever say, learn and have fun