I've built a triple variable power supply with two 0...+21V outputs and 
one 0...-21V outputs. I want to finish it with a dedicated triple
voltmeter, 
using an 16f877 and a 2x40 character LCD. 

I'll use a voltage divider to get each supply into the + or -5V range 
and then multiply in software to get the actual value to display. 
Just one problem - how to change the minus voltage to positive voltage, 
so that I can measure all three sources using a single 16f877? 
(Each 16f877 ADC input must be 0...+5V.) 

I have a 7660 which converts positive voltage to negative nicely. 
The minus output follows the positive input to within .01V. 

But I couldn't find any part to do the reverse job (minus to positive). 
If I were to reverse the inputs (connect the power supply Ground to the
7660 V+ 
and the power supply V- to the 7660 Ground), then the minus voltage
coming out 
would be relative to the -V input, not the Ground. 
(For example -4V in, results in -8V out with respect to the system
ground 
which is also the 16f877 Ground. 

I wish I could run it "backwards", but applying minus voltage to the
7660 V- 
output did not produce positive voltage at the 7660 V+ input ;) 

I think there must be a simple solution, but I'm just a beginner. 
Any ideas would be appreciated. 

   0...21V ---                         
              | 
              \ 
              /                          
              \ (0...5V) 
              |------------------------  
              /                        | 
              \                       AN0 
              |                       -------- 
   Gnd ------------------------------| 16f877 |-- +5V 
              |                       -------- 
              \                       AN2 
              /                        | 
              |--------------------- ????? (magical - to + voltage
inverter) 
              / (0...-5V)                  (using 7660 or ?) 
              / 
              \ 
              | 
   0...-21V -- 



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