Another Parallel-tuned Air-core Coil Crystal Radio

A 7" long, 2" diameter PVC pipe doubles as coil former and enclosure for this crystal radio. A couple of white plastic cosmetic jar covers serve as the end caps.

Parallel-tuned Air-core Coil 
Crystal Radio
The coil is (280 + 40) turns of 30 SWG enamelled copper wire, close-wound as shown. The PVC variable capacitor is mounted on one of the end caps.

The OA79 diode feeds a pair of sensitive balanced-armature phones.

Parallel-tuned Air-core Coil
Crystal Radio - Schematic
Using a wire antenna 60' long, reception of the 612 kHz - 200 kW local AM broadcast station is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 350 μA.

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Makeshift Crystal Radio Output Transformer

This project was part of my initial attempts at homebrewing an output transformer for my  'Loudspeaker Crystal Radio'.

An air-core crossover network inductor and a ferrite-core one (from the junk box) appeared suitable as the primary and secondary windings of the transformer.

An Air-core & a Ferrite Core Crossover Inductor
The large number of turns of the air-core inductor as primary and the small number of the ferrite one as secondary would give a step-down ratio. With luck, the open ferrite core would enable reasonable coupling.

 Simple coupling of the two inductors
Testing was simple, with the air-core inductor hooked up to the crystal radio and the ferrite-core one (placed within) connected  to the speaker.

The result was encouraging enough for further attempts which culminated in a proper 'Crystal Radio Output Transformer'.
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Crystal Radio Output Transformer

The large size of the power transformer, used as an output transformer for my 'Loudspeaker 
Crystal Radio', was good enough reason to attempt homebrewing a relatively small one.

A search through the junk box yielded a 47mm diameter, Siemens N22 pot core set.

Pot Core used for Crystal Radio Output Transformer
Winding was for an autotransformer, having 500 turns of 30 SWG enamelled copper wire (the maximum number that could be accommodated), with output taps at 50 and 100 turns.


Crystal Radio Output Transformer - winding scheme
The 50-turn tap was to match the existing 3 Ω speaker and the100-turn one a provision for speakers of higher impedance.

A small PCB, also from the junk box, provided termination points for the winding.

Crystal Radio Output Transformer- inside view
A couple of reworked white plastic cosmetic jar bodies lent themselves as enclosure halves.

Crystal Radio Output Transformer
Tests indicated that, inspite of its small size, the output transformer's performance is comparable, though not equal, to that of the power transformer.

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Colourful Parallel-tuned Air-core Coil Crystal Radio

This is the colourful version of my 'Parallel-tuned Air Core Coil Crystal Radio'.

Colourful Crystal Radio
The coil is 75turns of hook-up wire, close-wound on a 2 ⅝" diameter plastic bottle, with a tap at 35 turns.

The 35 turn primary series-tunes the capacitive 60' wire antenna to resonance at 612kHz (frequency of the lone 200kW local AM broadcast station). The secondary is parallel-tuned using a 500+500 pF PVC gang condenser.

Colourful Crystal Radio - Schematic
The plastic bottle serves as the coil former cum enclosure. A screw-type terminal strip is used to connect antenna, earth and headphones.

Reception, using sensitive balanced-armature headphones, is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 400 μA.

Related post: Parallel-tuned Ferrite Toroid Crystal Radio
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Parallel-tuned Ferrite Toroid Crystal Radio

This is the toroid coil variant of my 'Ferrite Loopstick Crystal Radio'.

Parallel-tuned Ferrite Toroid Crystal Radio
A ferrite toroid, measuring OD 16.5mm, ID 10.5mm and H 10.5mm, is used. The coil is 25 turns of hook-up wire with a tap at 5 turns. On the primary side, a 200μH moulded inductor series-tunes the capacitive 60' long wire antenna to resonance at 612kHz (frequency of the lone 200kW local AM broadcast station). The secondary is parallel-tuned using a PVC gang condenser.

Parallel-tuned Ferrite Toroid Crystal Radio - Schematic
A small plastic trinket box serves as the enclosure. A screw-type terminal strip is used to connect antenna, earth and headphones.

Rear of Crystal Radio with Ferrite Toroid Coil
Reception, using sensitive balanced-armature headphones, is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 275 μA.

Related post: Parallel-tuned Air-core Coil Crystal Radio
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Parallel-tuned Air-core Coil Crystal Radio

This is the air-core-coil variant of my 'Ferrite Loopstick Crystal Radio'.

Parallel-tuned Air-core Coil Crystal Radio
The coil is 72 turns of 30 SWG enamelled copper wire, close-wound on a 2½" length of 2" diameter PVC pipe, with a tap at 12 turns.

The primary winding, along with the 200μH moulded inductor and 1000 pf capacitor, series-tunes the capacitive 60' wire antenna to resonance at 612kHz (frequency of the lone 200kW local AM broadcast station). The secondary is parallel-tuned using a PVC gang condenser.

Parallel-tuned Air-cor Coil Crystal Radio Schematic
One half of an acrylic trinket box serves as the base/enclosure. A screw-type terminal strip is used to connect antenna, earth and headphones.

Reception, using sensitive balanced-armature headphones, is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 350 μA.

Related post: Ferrite Loopstick Crystal Radio
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Ferrite Loopstick Crystal Radio

Here's my version of a ferrite loopstick crystal radio.

Parts used are a moulded inductor, a 2200pf capacitor, a MW loopstick antenna, a PVC gang condenser and a germanium diode. The enclosure is a 3" x 3" x 1½" electrical bakelite box.

Ferrite Loopstick Crystal Radio
Ferrite Loopstick Crystal Radio - Schematic
Screw terminals are provided on the rear of the enclosure for connecting the antenna, earthand headphones. Headphones used are of the sensitive, balanced-armature type.

Inside the Ferrite Loopstick Crystal Radio
The loopstick is 125 turns of Litz wire close-wound on a 2" length of ferrite rod with a tap at 25 turns. The number of turns may vary depending on the diameter of the ferrite rod and its relative permeability.

The primary series-tuned circuit consists of the 200μH moulded inductor, the 2200 pf capacitor and the capacitance of the 60' wire antenna. The secondary is parallel-tuned.        

Reception of the local 612 kHz, 200 kW AM broadcast station is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 300 μA.

Reception is as good without an external antenna and earth, when the radio is held close to a CATV cable running over the shack and working as a passive radiator. Signal strength is maximum when the ferrite rod is laterally oriented with respect to it.

Related: Permeability-tuned Crystal Radio
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Permeability-tuned Crystal Radio

This simple, permeability-tuned crystal radio was wired using a variable inductor, a germanium diode/transistor and a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.
 
Permeability-tuned Crystal Radio 
A germanium diode was used as the detector in the series-fed version.     

Permeability-tuned, Series-fed
Crystal Radio - Schematic
 For the shunt-fed version it was a germanium transistor (with its base and emitter interconnected). 

Permeability-tuned, Shunt-fed 
Crystal Radio - Schematic
An empty glue stick and a ferrite toroid were used for the tuning mechanism.

The toroid was fixed to the blue glue stick carrier using rubber adhesive. The coil was 60 turns of 30 SWG enamelled copper wire, close-wound on a homebrewed 1" diameter paper former which fit tightly on the glue stick body.

Tuning mechanism for
Permeability-tuned Crystal Radio
Junk-box parts were used to assemble and wire the radio using a scrap wall wart enclosure as the base.

Reception of  the local 612 kHz, 200 kW AM broadcast station was quite good, with just a 60' wire antenna. Headphone current, measured using a 1mA FSD 60 Ω meter, was 350 μA.

During subsequent trials, it was concluded that a shunt-fed, permeability-tuned crystal radio, with a fixed series-capacitor, could be easily tweaked for best performance.

Permeability-tuned Crystal Radio
with fixed series-capacitor - Schematic
Hence another unit was built, using an empty lip salve stick container as coil former/tuning mechanism. A temporary coil, consisting of 120 turns of 30 SWG enamelled copper wire was close-wound on the 5/8 " diameter lip salve stick body. A 1" length of ferrite rod was glued on to the lip salve stick carrier. A value of 330 pF was chosen for the tubular ceramic capacitor, as a 365 pF variable would be normally set around that value, to receive a station at 612 kHz. A germanium transistor, with its base and emitter interconnected, was used as the detector. After a number of trials, it was found that best reception of the local station was obtained when the number of turns was reduced to 90, keeping the ferrite rod 90% inside the coil.

The temporary coil was then replaced with a proper one, having 90 turns of 30 SWG enamelled copper wire, close-wound and taped.

Assembly/wiring was on a discarded blister pack.

Permeability-tuned Crystal Radio
with fixed series-capacitor
Excellent reception of the local station was obtained with the same sensitive balanced-armature phones and 60' long wire antenna. Headphone current indicated by the 1mA FSD 60 Ω meter was 800 μA.

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Series-tuned Air-core Coil Crystal Radio

A conventional parallel-tuned crystal radio was recently put together using a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.

Conventional Parallel-tuned
Crystal Radio - Schematic
To further improve its performance, a germanium transistor (with its base and emitter interconnected) was used as a substitute for the OA5. This was based on the good results obtained earlier with my first series-tuned crystal radio.

Parallel-tuned Crystal Radio
with alternative detector - Schematic
But the results were disappointing.

However, when the circuit was reconfigured as series-tuned and shunt-fed, there was a dramatic improvement in performance.

Series-tuned, Shunt-fed  
Crystal Radio - Schematic
A 35mm film canister doubled as coil former and enclosure for the PVC variable capacitor and germanium transistor.

Series-tuned, Shunt-fed Air-core Coil
Crystal Radio
A screw-type terminal strip was used for connecting the antenna (yellow), earth (blue) and headphone (red & blue) leads.

The local 612 kHz, 200 kW AM broadcast station came in real loud with just a 60' wire antenna. Headphone current, measured using a 1mA FSD 60 Ω meter, was 700 μA.

Subsequently, an improved, link-coupled, fixed-tuned version was built. The 22 μH inductor, along with the 50 turn link winding, series-tunes the capacitive wire antenna to resonance at 612 kHz.

Link-coupled, fixed-tuned
version - Schematic
A cosmetic jar was used as the enclosure/base and a polythene can body as the coil former.

Link-coupled, fixed-tuned version
Performance was quite good with headphone current measuring 825 μA.  

Related post: Loudspeaker Crystal Radio
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Horn Speaker for a Crystal Radio

This horn speaker is an improvisation for my 'Loudspeaker Crystal Radio'.

Horn Speaker
It requires only 2 additional parts - a dustbin and a food container - with the crystal radio speaker itself serving as the driver.

In order to simplify the mounting, the speaker is made to drive from its rear while its front is sealed by the food container.

Horn Speaker details
Rubber based adhesive is used for assembly.

Audio output is considerably more than with the speaker in its normal enclosure.
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