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| Parallel - tuned Compact Loudspeaker Crystal Radio - Schematic 1 |
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| Parallel - tuned Compact Loudspeaker Crystal Radio - Schematic 2 |
Related post: A Chance Crystal Radio Project
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Parallel-tuned Compact Loudspeaker Crystal Radio
Here are the schematics of two parallel-tuned versions of my 'Compact Loudspeaker Crystal Radio'.
The local 200 kW, 612 kHz AM broadcast station, just 20 km away, is heard quite well with either version on my 'Homebrew Horn Speaker'.
Related post: A Chance Crystal Radio Project
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Related post: A Chance Crystal Radio Project
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Tarnished Silver as a Crystal Radio Detector
A fully tarnished, shiny black silver artefact was tried out as a detector in place of the germanium transistor in my series-tuned, shunt-fed crystal radio.
Fortunately the presence of an untarnished spot facilitated the metal to metal connection.
A length of pencil 'lead' was used to probe the tarnished portion to locate the 'sweet spot'.
The local 612 kHz, 200 kW AM broadcast station, just 20 km away, was received quite well using a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, was 300 μA as compared to 750 μA with the germanium transistor and 50 μA with a razor blade as the detector.
It was a confirmation of the shiny black tarnish being silver sulphide (a semiconductor).
A silver coin, stored in a plastic pouch and tarnished a dull grey, in its failure to detect, indicated that its tarnish was silver oxide (a good conductor of electricity).
Related post: Foxhole Radio Detector Variants
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| Series-tuned Shunt-fed Crystal Radio - Schematic |
A length of pencil 'lead' was used to probe the tarnished portion to locate the 'sweet spot'.
The local 612 kHz, 200 kW AM broadcast station, just 20 km away, was received quite well using a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, was 300 μA as compared to 750 μA with the germanium transistor and 50 μA with a razor blade as the detector.
It was a confirmation of the shiny black tarnish being silver sulphide (a semiconductor).
A silver coin, stored in a plastic pouch and tarnished a dull grey, in its failure to detect, indicated that its tarnish was silver oxide (a good conductor of electricity).
Related post: Foxhole Radio Detector Variants
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Carbon Microphone as a Crystal Radio Detector
A vintage telephone carbon microphone, which possesses multiple contacts between carbon granules and metal, was tried out as a detector, replacing the germanium transistor in my
series-tuned, shunt fed crystal radio.
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| Vintage telephone carbon microphone |
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| Series-tuned Shunt-fed Crystal Radio - Schematic |
Its performance improved considerably with a rundown button cell in series.
The carbon microphone behaviour was successfully simulated using two metal plates of size 8" x 8" x 1/16" with carbon granules salvaged from a discarded water filter.
With one plate placed horizontally on the table, a thimbleful of carbon granules was distributed at the four corners and then sandwiched by the other plate. The top plate had to be moved to and fro to find the right spot.
During tests the signal strength was found to be fair with the specimen directly connected and very good with a rundown button cell connected in series with it.
Related post: Tarnished Silver as a Crystal Radio Detector
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A Chance Crystal Radio Project
A vintage unmarked inductor/transformer had been lying neglected in my junkbox for decades and I had to resurrect it.
Resistance checks showed that 2 windings, with a tap each, are terminated in its 6 pins. One measured 70 Ω with a 5 Ω tap and the other less than 1 Ω. A threaded core, with a movement of 5 mm is situated at the top of the unit.
The 70 Ω winding appeared to be suitable for a crystal radio. It was wired up as per the schematic shown below.
As luck would have it, the local 612 kHz, 200 kW AM broadcast station, situated 20 km away
Resistance checks showed that 2 windings, with a tap each, are terminated in its 6 pins. One measured 70 Ω with a 5 Ω tap and the other less than 1 Ω. A threaded core, with a movement of 5 mm is situated at the top of the unit.
The 70 Ω winding appeared to be suitable for a crystal radio. It was wired up as per the schematic shown below.
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| Chance Crystal Radio - Schematic |
(as the crow flies), comes in real loud using a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 250 μA with the core fully in and 300 μA with the core retracted.
The capacitive wire antenna series-tunes the link winding to resonance at 612 kHz, while the self-capacitance of the detector winding parallel-tunes it to resonance at the same frequency, as though the inductor/transformer has been tailor-made for this project.
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| Chance Crystal Radio |
A cosmetic plastic jar makes a good base/enclosure for this chance crystal radio.
Related post: Another Parallel-tuned Air-core Coil Crystal Radio
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Pseudo FM Crystal Radio
The mosfet used was a 3N187 from the junk box.
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| FM Crystal Radio - Schematic |
Now at a dead end, powering it up with a rundown 1.5 V button cell was the only way out. And it sprang to life!
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| Pseudo FM Crystal Radio - Schematic |
A number of local stations were received.
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| Pseudo FM Crystal Radio |
Notwithstanding the problems of hand effects and oscillations, it was music to my ears.
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| Pseudo FM Crystal Radio - Final schematic |
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| Pseudo FM Crystal Radio - Inside view |
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A Tuned Passive Radiator
The problem with my 'Portable AM Crystal Radio' is that it works well only when close to an overhead CATV cable working as a passive radiator.
Hence it was decided to create a tuned passive radiator to further improve its performance.
One half of my 40 m inverted 'V' dipole antenna was chosen as the passive radiator. The series-tuned circuit was connected to the core of the coax at the shack end and earthed as shown.
Hence it was decided to create a tuned passive radiator to further improve its performance.
One half of my 40 m inverted 'V' dipole antenna was chosen as the passive radiator. The series-tuned circuit was connected to the core of the coax at the shack end and earthed as shown.
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| Series-tuned Passive Radiator - Schematic |
This setup enabled faint reception of the local 612 kHz 200 kW AM station on the portable crystal radio inside the shack.
After tuning the passive radiator to resonance, performance of the portable crystal radio was found to be quite good across the shack roof.
After tuning the passive radiator to resonance, performance of the portable crystal radio was found to be quite good across the shack roof.
Identical performance was obtained when a parallel-tuned circuit was used.
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| Parallel-tuned Passive Radiator - Schematic |
Related post: Portable AM Crystal Radio
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Shortwave Crystal Radio
It was my first attempt at building a shortwave crystal radio.
It's single-tuned and uses a 1N270 to drive vintage sound-powered phones.
The circuit was 'breadboarded' on an old fan regulator baseplate with the coil wound on a white plastic pill bottle.
A 2-section variable capacitor, with slow motion drive, was used for tuning. The 2 sections were connected in series for an effective maximum capacitance of 180 pF.
Using a 60' wire antenna and earth resulted in 'local strong medium wave station breakthrough' and hum.
With earth disconnected, the breakthrough and hum reduced considerably and it was possible to tune-in to a faint shortwave broadcast. It was from AIR Chennai, on 7380 kHz, located 270 km away.
Tuning further up, a weaker Oriental station was heard.
Severe fading was experienced with both the signals.
Similar results were obtained using a 1.5 m long telescopic antenna and earth.
On a later occasion the same stations came in louder, with no breakthrough and hum, using a 20 m inverted 'V' antenna.
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| Shortwave Crystal Radio |
The circuit was 'breadboarded' on an old fan regulator baseplate with the coil wound on a white plastic pill bottle.
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| Shortwave Crystal Radio - Schematic |
Using a 60' wire antenna and earth resulted in 'local strong medium wave station breakthrough' and hum.
With earth disconnected, the breakthrough and hum reduced considerably and it was possible to tune-in to a faint shortwave broadcast. It was from AIR Chennai, on 7380 kHz, located 270 km away.
Tuning further up, a weaker Oriental station was heard.
Severe fading was experienced with both the signals.
Similar results were obtained using a 1.5 m long telescopic antenna and earth.
On a later occasion the same stations came in louder, with no breakthrough and hum, using a 20 m inverted 'V' antenna.
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Portable AM Crystal Radio
Here's the portable version of my 'Series-tuned Crystal Radio'.
The prototype was wired on a piece of perforated board.
At first nothing was heard even on the shack roof. However, on moving around, a favourable spot was found, where the reception is indeed quite good. The spot is close to an overhead CATV cable working as a passive radiator. The cable runs exactly in the direction of the station like a Beverage antenna.
Encouraged by the results, the final version was built using the plastic front panel of a discarded radio. In this version OA79 diodes were used with equally good results.
This radio works quite well, unaided by passive radiators, up to 5 km away from the transmitter.
Related post: A Tuned Passive Radiator
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| Portable AM Crystal Radio |
M0UKD's 'Loaded Quarter Wave Antenna Inductance Calculator' gave a loading coil inductance of the order of 10 mH at 612 kHz (the frequency of the 200 kW AM station located 20 km away). Hence two 4.7 mH moulded inductors were connected in series at the feed point, to resonate the 0.75 m long telescopic antenna at that frequency.
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| Portable AM Crystal Radio - Schematic |
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| Portable AM Crystal Radio - final version |
Related post: A Tuned Passive Radiator
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Minimalist UHF Crystal Radio
Here's the schematic of the minimalist version of my 'UHF Crystal Radio'.
It was wired on a terminal block.
Using this simple setup, digital signals from the nearby cellphone tower were received on the shack roof.
Related post: Some thoughts on UHF Crystal Radios
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| Minimalist UHF Crystal Radio - Schematic |
Using this simple setup, digital signals from the nearby cellphone tower were received on the shack roof.
Related post: Some thoughts on UHF Crystal Radios
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Compact Loudspeaker Crystal Radio
This is the compact version of my 'Loudspeaker Crystal Radio'.
Trials on a number of assorted audio transformers, from the junk box, were not in vain. A push-pull output transformer, intended for a 1 W germanium transistor audio amplifier, gave excellent results.
The centre-tap on the primary is not used.
A discarded blister pack serves as the base.
In spite of its small size, performance is as good as that of its counterpart.
My 'Homebrew Horn Speaker' and 60' wire antenna offer the perfect match for this crystal radio.
Related post: Alternative Detector for a Shunt-fed Crystal Radio
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| Compact Loudspeaker Crystal Radio |
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| Compact Loudspeaker Crystal Radio - Schematic |
A discarded blister pack serves as the base.
In spite of its small size, performance is as good as that of its counterpart.
My 'Homebrew Horn Speaker' and 60' wire antenna offer the perfect match for this crystal radio.
Related post: Alternative Detector for a Shunt-fed Crystal Radio
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