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 understand crystal radio / piezo earphone interfacing. 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.   

Moulded Inductor
This makes it possible to use a compact plastic cosmetic jar as a housing.

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

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

Series-tuned Crystal Radio
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 500 μA.

'Gibson Girl' Crystal Radio

This series-tuned, shunt-fed crystal radio was built using parts salvaged from a vintage / battered SCR - 578 lifeboat radio (popularly referred to as 'Gibson Girl').

'Gibson Girl' Crystal Radio - Schematic
It consists of a baseplate, antenna tuning coil, antenna tuning capacitor and a terminal strip modified to suit.

'Gibson Girl' Crystal Radio
The only part required to be fished out from the junk box was a Schottky diode SB 860. It was mounted underneath the terminal strip.

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 500 μA.

Loop Antenna Crystal Radio

A loop antenna crystal radio is of interest on account of its selectivity and portability. But, with respect to signal strength, it is no match for a crystal radio using an external vertical wire antenna and earth.

If signal strength could be considerably improved with increased loop size, then its portability would be compromised.

As usual, 2 configurations are possible - parallel-tuned-series-fed and series-tuned-shunt-fed.

1. Parallel-tuned-series-fed version 

Parallel-tuned-series-fed Loop Antenna Crystal Radio
2. Series-tuned-shunt-fed version

Series-tuned-shunt-fed Loop Antenna Crystal Radio
The loop antenna was rigged up using a 4-legged stool as a makeshift frame. The stool was laid on its side on the table, making it convenient to wind the loop on its legs.

The 4-legged stool used as a makeshift Loop Antenna frame
Both the versions were tested with the plane of the loop oriented towards the local 612 kHz, 200 kW AM broadcast station.

Signal strength was the same with both, though a far cry from another crystal radio using a vertical wire antenna and earth.

Current-operated 'S' Meter for a Crystal Radio

An 'S' meter, for a crystal radio with high-impedance phones, is generally a high-resistance microammeter connected across the phones. In other words, a sensitive voltmeter that does not bypass headphone current significantly.

On the other hand, for a crystal radio with low-impedance phones, it's a low resistance microammeter connected in series with the phones. That is, a sensitive ammeter that does not drop headphone current significantly.

A voltmeter, discovered in my junk box, was used to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.

On removing its series and shunt resistors, the resistance of the 1 mA FSD movement turned out to be only 60 Ω and hence quite suitable for the purpose. The existing graduations on the meter face were removed and replaced with graduations 0 -10 using a lead pencil.

Crystal Radio S-Meter - Schematic
The meter was already mounted on a square bakelite flange. The flange mounting holes served as terminal mounting holes, obviating the need for an enclosure.

Crystal Radio S-Meter
The result - an interesting weekend project and a handy crystal radio test aid.

UHF Crystal Radio

A crystal radio was wired up to check for reception of VHF/UHF signals in the vicinity of the shack.

VHF / UHF Crystal Radio - Schematic
Using a gold bonded Germanium diode 1N65, a 500 mm whip antenna and sound-powered phones, no signals were audible either in the shack or on the shack roof.

However, using a 250 mm whip, a weak digital signal was detected while moving on the shack roof. Position on the roof and direction in which the whip was pointed were quite critical.

It was presumed that the signals were from a nearby cellphone tower.

A 3 Element Yagi Beam for 33 cm was made using a broken snare drum stick for the boom and 20 SWG bare copper wire for the elements.

UHF Yagi Beam Crystal Radio  - Schematic
The coil was wound integral with the driven element. 

UHF Yagi Beam Crystal Radio
With this setup a higher signal strength was obtained.

A second Yagi Beam was then rigged up, with another diode providing the return path for the audio, instead of the coil.

UHF Yagi Beam Crystal Radio
with additional diode - Schematic
The signal strength was not as good. During trials, a chance shorting of  the first diode gave a considerable increase in signal strength.

Single-diode UHF Yagi Beam
Crystal Radio - Schematic
The diode and capacitor were hence discarded. Performance of this single-diode UHF crystal radio is quite good.
Single-diode UHF Yagi Beam Crystal Radio
A folded dipole version, mounted on a terminal block, followed.

UHF Folded Dipole Crystal Radio - Schematic
It obviated the need for a coil / additional diode and worked quite well too.

UHF Folded Dipole Crystal Radio
The terminal block arrangement made it convenient to replace the folded dipole with a full wave loop.
UHF Full Wave Loop Crystal Radio
Likewise with an AWX antenna.

UHF AWX Antenna Crystal Radio
Results were as good as with the folded dipole.