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