Electronic projects

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.

Circuit diagram:

Operating Principle:
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:

Antenna:
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 |
wall |...O...O...O----------------------------------O...O...O...| wall
| | |
to receiver

Inductor:
wind 60 windings of enamel copper wire onto a 2 inch ferrite core (1 cm diameter or a bit less)

RF GROUND:
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.

C1:
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 diameter with a alu foil semi disk each separated through paper, place a nail in center (make sure to not short the capacitor) 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.

C2:
use paper, alu foil or ldpe (but i don't think that you will be interested in a 10KV capacitor so use thin LDPE)

Diode:
Buy it. I don't advice you looking for Lead Sulphyde cristals....

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

author: Jonathan Filippi
e-mail: jonathan.filippi@virgilio.it
web site: http://www.electronics-lab.com