Current-operated 'S' Meter for Crystal Radios

Discussion in 'Wireless & RF Design' started by vu2nan, Aug 12, 2015.

  1. vu2nan

    Thread Starter Member

    Sep 11, 2014
    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.

    Crystal Radio S-Meter - Schematic
    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.

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