This was designed, keeping in mind, that sometimes we have few components available and you could not carry your DMM somewhere, but you have to make some instrument for a quick need with what is available.
IC3 LM555 is to generate -5V from 9V. That reminds me of a story, I came across a Book called CMOS Hot Ideas of Intersil. I had a chance to observe, build and learn some things here. The ICL7107 circuit was the most fascinating, I also read the books of National Semiconductor here.
In those days small firms used to make DPM’s digital panel meters with 7107. In 1987 when i got a oppurtunity to design a DPM for a firm, I put the 555 clock in place CD4009 clock shown in intersil, to derive the -5.
Potentiometer Dial GIF for above circuit to be used to make a hand-held avo-meter is here.