Foxhole Radio Detector Variants

In the absence of a foxhole radio, a quick way to test out a safety-razor-blade-detector was to use it in place of the OA5 in my first crystal radio.

My First Crystal Radio - Schematic

Detection was established, on the very first attempt, with a new (not blue!) blade and solid hookup wire for contact.  It was found that only printed areas of the blade were effective.

However, the local broadcast station sounded weaker with the blade than with the OA5. Results were equally bad with a pencil for contact.

A spare carbon brush of a mixer/grinder was then tried out. It worked fine on most parts of the blade's surface and the signal strength went up multifold, though not as strong as with the OA5.

Safety Razor Blade & Carbon Brush

The spring made it possible to vary the contact pressure for optimum signal.

A fixture was then homebrewed using junk-box parts. A piece of copper-clad board was used as the ground contact, enabling the blade to be moved around while adjusting the spring force to locate a 'hot spot'. A lock nut was provided to retain the setting.

Safety Razor Blade Detector

Trials confirmed easy/reliable/repeatable set-up and adjustment.

Other rusty/oxidized/plated steel parts were also tried out in place of the razor blade. The best of them all turned out to be a piece of broken hacksaw blade which even outdid the razor blade!

A 'pencil contact' was then homebrewed as a replacement for the carbon brush. Results were not that good and it was not as easy to locate a 'hot spot' with it as with the carbon brush.

Pencil with spring

A discarded extruded-carbon-block water filter was the inspiration for the next version. It served as the base cum detector contact. After removing the fabric cover, a self-tapping screw was used to secure one of the lugs on the block. The blade was lightly held in place using a rubber band.

Carbon block water filter

Connection to the blade was made by sliding the other lug under it whilst a wrap of electrical insulation tape isolated it from the carbon block. Results with this 'giant', yet simple, detector were unbelievably good.

The performance of these blade detectors was considerably enhanced by connecting a run-down button cell in series (~ 0.1V with negative terminal to carbon brush).

Button Cell & 'Ginger Cell'

Next, the button cell was replaced by a 'Ginger Cell' - a piece of ginger into which a brass screw (+ve terminal) and a steel one (-ve) were screwed in. Its open-circuit voltage of 0.5V dropped to around 0.1V in-circuit and it was as good as the button cell.

Later, a cat's whisker detector was made with a black oxidized-brass screw mounted on an old plastic pulley. 

Cat's Whisker Detector details

The copper-wire cat's whisker was soldered to the plated screw and the rectifying spot located by adjusting the screw.

Cat's Whisker Detector & Screw

It was tricky to adjust, but its performance was quite good.

Related post: Carbon Microphone as a Crystal Radio Detector

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