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.

Crystal Radio 'S' Meter
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.

Crystal Radio 'S' Meter - Schematic
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.

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.

The result - an interesting weekend project and a handy crystal radio test aid.