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.

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.

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 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.

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

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 factors contributing to my failure in building a real FM Crystal Radio could be the distance of the transmitters (6 km away as the crow flies) and their low power 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 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.

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.

Identical performance was obtained when a parallel-tuned circuit was used.


Parallel-tuned Passive Radiator - Schematic

Related post: Portable AM Crystal Radio
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