The circuit shown on this page is a reasonably Low Cost and moderately Hi-Tech ammeter for use with DCC systems. The circuit meter output is always has the same polarity regardless of the input polarity. This allows a low cost DC voltmeter to be used to monitor a DCC current.
http://home.cogeco.ca/~rpaisley4/DCCAmmeter.html
This circuit is designed to provide an inexpensive way to to create a High Impedance Voltmeter while making use of an inexpensive analog or digital multimeter. The circuit is specifically designed for testing phototransistors when they are used in the circuits shown at this site. It has a very high input impedance that will not "Load Down" the sensor that is being checked.
http://home.cogeco.ca/~rpaisley4/xHiZmeter.html
The voltmeter is an expanded scale type that indicates small voltage steps over the 10 to 16 volt range for 12 volt batteries and over the 22 to 32 volt range for 24 volt batteries.
http://www.solorb.com/elect/solarcirc/vom/index.html
It measure the voltage on a 12V nominal lead acid rechargeable battery. Lead acid batteries normally spend their working lifetime in the voltage range of 11-15 Volts. This meter circuit was designed to show the voltage range of 10-15V on an analog meter movement, it can be used to show the battery charge state from empty to full.
http://www.solorb.com/elect/solarcirc/xsvmeter/index.html
http://www.tpub.com/neets/book16/68d.htm
This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.
http://www.discovercircuits.com/DJ-Circuits/acampmon.htm