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| Stethoscope |
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| Piezo Transducer |
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Makeshift Piezo Earphones
A makeshift piezo earphone arrangement comprises a stethoscope and a piezo transducer.
The sound emanating from the transducer is captured by the bell of the stethoscope, held against its face.
This arrangement was recently used to figure out how to interface a piezo earpiece with a crystal radio. The transducer, a 1¾" telephone ringer, was salvaged from the junk box.
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Series-tuned Moulded Inductor Crystal Radio
'A Series-tuned Air-core Coil Crystal Radio' and 'Another Series-tuned Air-core Coil Crystal Radio' have been made simple by the use of moulded inductors instead of hand-wound coils.
This makes it possible to use a compact plastic cosmetic jar as a housing.
Related post: Another Series-tuned Air-core Coil Crystal Radio
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| Series-tuned Moulded Inductor Crystal Radio |
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| Moulded Inductor |
There is no difference between the two with respect to performance.
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| Series-tuned Moulded Inductor Crystal Radio - Schematic |
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| Another Series-tuned Moulded Inductor Crystal Radio - Schematic |
The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 625 μA.
The performance of a parallel-tuned version is also quite good with a headphone current of 300 μA.
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| Parallel-tuned Moulded Inductor Crystal Radio - Schematic |
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Another Series-tuned Air-core Coil Crystal Radio
This is the air-core coil version of my 'series-tuned crystal radio with alternative detector'.
It is enclosed in a cosmetic jar with the 500 pF PVC gang condenser mounted inside its cap.
There is sufficient clearance to close the jar even though the coil is wound on its body.
The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 750 μA.
Related: 'Gibson Girl' Crystal Radio
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| Series-tuned Air-core Coil Crystal Radio |
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| Series-tuned Crystal Radio - Schematic |
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| Series-tuned Crystal Radio - Coil view |
Related: 'Gibson Girl' Crystal Radio
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'Gibson Girl' Crystal Radio
This series-tuned crystal radio was built using parts salvaged from a vintage / battered SCR - 578 lifeboat radio (popularly referred to as 'Gibson Girl').
It consists of a baseplate, antenna tuning coil, antenna tuning capacitor and terminal strip modified to suit.
The only part required to be fished out from the junk box was a Schottky diode SB 860. It was mounted underneath the terminal strip.
The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 750 μA.
Related post: Series-tuned Ferrite Loopstick Crystal Radio
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| The SCR - 578 |
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| 'Gibson Girl' Crystal Radio |
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| 'Gibson Girl' Crystal Radio - Schematic |
Related post: Series-tuned Ferrite Loopstick Crystal Radio
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Loop Antenna Crystal Radio
A loop antenna crystal radio is of interest on account of its selectivity and portability. But, with respect to signal strength, it is no match for a crystal radio using an external vertical wire antenna and earth.
If signal strength could be considerably improved with increased loop size, then its portability would be compromised.
It may be configured as parallel-tuned or series-tuned.
1. Parallel-tuned version
2. Series-tuned version
If signal strength could be considerably improved with increased loop size, then its portability would be compromised.
It may be configured as parallel-tuned or series-tuned.
1. Parallel-tuned version
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| Parallel-tuned Loop Antenna Crystal Radio |
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| Series-tuned Loop Antenna Crystal Radio |
The loop antenna was rigged up using a 4-legged stool as a makeshift frame. The stool was laid on its side on the table, making it convenient to wind the loop on its legs.
Both the versions were tested with the loop oriented end-on towards the local 612 kHz, 200 kW AM broadcast station.
Signal strength was the same with both, though a far cry from another crystal radio using a vertical wire antenna and earth.
Related post: Another Loop Antenna Crystal Radio
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| 4-legged stool as Loop Antenna frame |
Signal strength was the same with both, though a far cry from another crystal radio using a vertical wire antenna and earth.
Related post: Another Loop Antenna Crystal Radio
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Current-operated 'S' Meter for a Crystal Radio
An 'S' meter, for a crystal radio with high-impedance phones, is generally a high-resistance microammeter connected across the phones. In other words, a sensitive voltmeter that does not bypass headphone current significantly.
On the other hand, for a crystal radio with low-impedance phones, it's a low resistance microammeter connected in series with the phones. That is, a sensitive ammeter that does not drop headphone current significantly.
A voltmeter, discovered in my junk box, was used to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.
On removing its series and shunt resistors, the resistance of the 1 mA FSD movement turned out to be only 60 Ω and hence quite suitable for the purpose. The existing graduations on the meter face were removed and replaced with graduations 0 -10 using a lead pencil.
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| Crystal Radio 'S' Meter |
A voltmeter, discovered in my junk box, was used to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.
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| Crystal Radio 'S' Meter - Schematic |
The meter was already mounted on a square bakelite flange. The flange mounting holes served as terminal mounting holes, obviating the need for an enclosure.
The result - an interesting weekend project and a handy crystal radio test aid.
Related post: Series-tuning for a Crystal Radio
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UHF Crystal Radio
A crystal radio was wired up to check for reception of VHF/UHF signals in the vicinity of the shack.
Using a gold bonded Germanium diode 1N65, a 500 mm whip antenna and balanced-armature phones, no signals were audible either in the shack or on the shack roof.
However, using a 250 mm whip, a weak digital signal was detected while moving on the shack roof. Position on the roof and direction in which the whip was pointed were quite critical.
It was presumed that the signals were from a nearby cellphone tower.
A 3 Element Yagi Beam for 33 cm was made using a broken snare drum stick for the boom and 20 SWG bare copper wire for the elements.
The signal strength was not as good. During trials, a chance shorting of the first diode gave a considerable increase in signal strength.
The diode and capacitor were hence discarded. Performance of this single-diode UHF crystal radio is quite good.
A folded dipole version, mounted on a terminal block, followed.
It obviated the need for a coil / additional diode and worked quite well too.
The terminal block arrangement made it convenient to replace the folded dipole with a full wave loop.
Likewise with an AWX antenna.
Results were as good as with the folded dipole.
Related post: Minimalist UHF Crystal Radio
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| VHF / UHF Crystal Radio - Schematic |
However, using a 250 mm whip, a weak digital signal was detected while moving on the shack roof. Position on the roof and direction in which the whip was pointed were quite critical.
It was presumed that the signals were from a nearby cellphone tower.
A 3 Element Yagi Beam for 33 cm was made using a broken snare drum stick for the boom and 20 SWG bare copper wire for the elements.
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| UHF Yagi Beam Crystal Radio - Schematic |
The coil was wound integral with the driven element.
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| UHF Yagi Beam Crystal Radio |
With this setup a higher signal strength was obtained.
A second Yagi Beam was then rigged up, with another diode providing the return path for the audio, instead of the coil.
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| UHF Yagi Beam Crystal Radio with additional diode - Schematic |
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| Single-diode UHF Yagi Beam Crystal Radio - Schematic |
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| Single-diode UHF Yagi Beam Crystal Radio |
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| UHF Folded Dipole Crystal Radio - Schematic |
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| UHF Folded Dipole Crystal Radio |
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| UHF Full Wave Loop Crystal Radio |
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| UHF AWX Antenna Crystal Radio |
Related post: Minimalist UHF Crystal Radio
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Series-tuned Ferrite Loopstick Crystal Radio
This series-tuned crystal radio is housed in a plastic pencil box.
It has 2 windings of 30 SWG enamelled copper wire on a 6" long, 3/8 " diameter ferrite rod. The primary has 10 turns and the secondary 90, both close-wound.
The primary series-tuned circuit, which includes the 200μH moulded inductor, the 3600 pf capacitor and the capacitance of the 60' wire antenna, is resonant at 612 kHz (the frequency of the local 200 kW station, located 20km away). The secondary is also series-tuned using a 500pf PVC variable capacitor. The detector, a germanium transistor with its base and emitter connected, shunt-feeds sensitive balanced-armature phones. Reception of the local station is quite good, with a headphone current of 625 μA (measured using a 1mA FSD 60 Ω meter).
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 (with headphone current measuring around 350 μA).
This radio also works quite well, even up to 5 km away from the transmitter, unaided by external antennas, earth and passive radiators.
Related post: Series-tuned Air-core Coil Crystal Radio
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| Series-tuned Ferrite Loopstick Crystal Radio |
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| Series-tuned Ferrite Loopstick Crystal Radio - schematic |
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| Series-tuned Ferrite Loopstick Crystal Radio - inside view |
This radio also works quite well, even up to 5 km away from the transmitter, unaided by external antennas, earth and passive radiators.
Related post: Series-tuned Air-core Coil Crystal Radio
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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.
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.
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| Parallel-tuned Air-core Coil Crystal Radio |
The OA79 diode feeds a pair of sensitive balanced-armature phones.
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| 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.
Related post: Colourful Parallel-tuned Air-core Coil Crystal Radio
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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.
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.
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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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.
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| An Air-core & a Ferrite Core Crossover Inductor |
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| Simple coupling of the two inductors |
The result was encouraging enough for further attempts which culminated in a proper 'Crystal Radio Output Transformer'.
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