Hi John,
For starters you might contac Smith-Kettlewell in San Francisco.

The have a couple of blind engineers who work for them.

In the past they have published a magaZine called The Smith Kettlewell Techn 
ical file that showed blind persons how to build various types of equipment.

This magazine is also up on the web but unfortunately I don't remember the 
url.

Hope this helps.

There is also one commercial manufacgtorer that makes a talking voltmeter I 
believe.

Cordially,

Eric Clegg