Series-fed Compact Loudspeaker Crystal Radio

Here's the schematic of the series-fed version of my 'Compact Loudspeaker Crystal Radio'.

Parallel-tuned Compact
Loudspeaker Crystal Radio - Schematic
The local 200 kW, 612 kHz AM broadcast station, just 20 km away, is heard quite well through my 'Homebrew Horn Speaker'.
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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.

Series-tuned Shunt-fed Crystal Radio - Schematic
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 sound-powered 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).
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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.

Vintage telephone carbon microphone

Series-tuned Shunt-fed Crystal Radio - Schematic
Its performance, after a bit of tapping, was as good as that of a razor blade detector (akin to moving the pencil or carbon contact on the razor blade to find the sweet spot).

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.
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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.
Chance Crystal Radio - Schematic
As luck would have it, the local 612 kHz, 200 kW AM broadcast station, situated 20 km away
(as the crow flies), comes in real loud using a 60' wire antenna and sound-powered 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.

Chance Crystal Radio
A cosmetic plastic jar makes a good base/enclosure for this chance crystal radio.
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Pseudo FM Crystal Radio

After several failed attempts to build an FM Crystal Radio with a diode detector, a dual-gate mosfet version was tried out but also in vain.

The mosfet used was a 3N187 from the junk box.

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!
Pseudo FM Crystal Radio - Schematic
A number of local stations were received.

Pseudo FM Crystal Radio
Notwithstanding the problems of hand effects and oscillations, it was music to my ears.

Pseudo FM Crystal Radio - Final schematic
The 330 μH moulded inductor was a later addition that solved the oscillation problem.

Pseudo FM Crystal Radio - Inside view
The limiting factors to my success in building a real FM Crystal Radio could be the location of the transmitters (6 km away as the crow flies) and their rating (1 to 10 kW).
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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 overhead
CATV cables, which serve as passive radiators.

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.

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.
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Shortwave Crystal Radio

It was my first attempt at building a shortwave crystal radio.

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.
Shortwave Crystal Radio - Schematic
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 wire antenna (35' long) and earth resulted in 'local strong station breakthrough' and hum.

With earth disconnected, the breakthrough and hum disappeared and it was possible to tune-in to a faint shortwave broadcast. It was from AIR Chennai, on 7380 kHz, 300 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.
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Portable AM Crystal Radio

Here's the portable version of my 'Series-tuned Crystal Radio'.

Portable AM Crystal Radio
The prototype was wired on a piece of perforated board.

Two 4.7 mH moulded inductors, connected in series at the feed point, resonate the 0.75 m long telescopic antenna at 612 kHz (frequency of the 200 kW AM station located 20 km away).

Portable AM Crystal Radio - Schematic
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 overhead CATV cables, which serve as passive radiators.

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.

Portable AM Crystal Radio - final version
This radio works quite well, unaided by passive radiators, up to 5 km away from the transmitter.
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Minimalist UHF Crystal Radio

Here's the schematic of the minimalist version of my 'UHF Crystal Radio'.

Minimalist UHF Crystal Radio - Schematic
It was wired on a terminal block.

Using this simple setup, digital signals from the nearby cellphone tower were received on the shack roof.
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Compact Loudspeaker Crystal Radio

This is the compact version of my 'Loudspeaker Crystal Radio'.

Compact 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.

Compact Loudspeaker Crystal Radio - Schematic
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.
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