On ebay there are these audio meter kits for like a buck delivered. I plan on using one of these as an S-Meter in receiver I working on. I know its hard to see in the image, but the 5th LED from the left is lit, the 10 turn pot makes adjusting the signal range a rather trivial matter. Just trim for the max audio voltage from the AF stage and it will be in the ball park. Its certainly no calibrated S-Meter, but it should give a fair indication of the strength. I am just feeding it with a few volts 600hz sinewave from my signal gen.