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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