I bought this Universal Avo Meter Model 7 (believe it is 1940s or early 1950s vintage) off eBay several years ago. I didn't expect to receive a fully working meter, however, it was beyond repair, looks to have experienced a surge, with much of the wiring missing or burnt away.
It sat as more of a display piece for a couple of years before I thought to take another look inside, turns out the meter movement is still working fine, and I came up with the idea of turning it into a VU meter, the result is this project.
Utilising the existing battery compartment as a way get the 12V power supply into the unit and trying to minimise any change to the outside of the case. In the end, the only addition has been a 6mm TRS jack.
The circuit utilises a TL072 op-amp followed by a diode rectifier and a capacitor to provide peak hold. The resistance of the coil in the meter movement bleeds the voltage from the capacitor.
No working multimeters where harmed in this conversion.
One final tweak, behind each of the AC and DC switches are some mechanical switches. These are operated by small cams on the back of the rotary mechanism. This of course is a convenient way to turn off the unit without needing to add any more switches.
With that along with a nice cable I'm calling this project done...
This is a "slightly" tweaked version from circuitlib.com the main change being R3 to work better with my particular meter movement.
The rest of the build was largely around laying out and building the circuit onto a PCB and making the brackets to mount it. The brackets where made from metal salvaged from a rice cooker that already had M4 threads tapped.
Power is supplied using the existing battery contacts, an old school braided power cable would really finish the look.