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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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13th Aug 2015, 5:25 am | #1 |
Pentode
Join Date: Jul 2014
Location: Bangalore, India
Posts: 132
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Current-operated 'S' Meter for Crystal Radios
Crystal radios with high-impedance voltage-operated phones require, as the 'S' Meter, 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, crystal radios with relatively low-impedance current-operated phones require, as the 'S' Meter, a low-resistance microammeter in series with the phones. That is, a sensitive ammeter that does not drop headphone current signnificantly. For my crystal radios, using sensitive balanced-armature phones (having an impedance of the order of a few hundred ohms), a voltmeter was picked up from the junkbox. On removing its series and shunt resistors, the resistance of the 1mA FSD movement turned out to be only 60 Ω hence quite suitable for the purpose. The voltage markings on the meter face were removed and the graduations marked 0-10 instead, using a lead pencil. The meter was already mounted on a square bakelite flange. Mounting holes on the flange served as terminal- mounting holes, obviating the need for an enclosure. The result - an interesting weekend project and a handy crystal radio test aid. |