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.

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).

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.

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.

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 well, unaided by passive radiators, up to 5 km away from the transmitter.

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.

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.

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.

Piezo Transducer
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.

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.

Series-tuned Moulded Inductor Crystal Radio
This makes it possible to use a compact plastic cosmetic jar as a housing.

Moulded Inductor
There is no difference between the two with respect to performance. 

Series-tuned Moulded Inductor 
Crystal Radio - Schematic
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 sound-powered phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 625 μA.

Another Series-tuned Air-core Coil Crystal Radio

This is the air-core coil version of my 'series-tuned crystal radio with alternative detector'.

Series-tuned Air-core Coil Crystal Radio
It is enclosed in a cosmetic jar with the (500+500) pF PVC gang condenser mounted inside its cap.

Series-tuned Air-core Coil Crystal Radio - Schematic
There is sufficient clearance to close the jar even though the coil is wound on its body.

Series-tuned Crystal Radio - Coil view
The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and sound-powered phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 750 μA.