This article introduces the voltage monitoring circuit diagram. The principle of the circuit and easy to understand, is also very practical. The depth of understanding of the circuit components are related to your understanding of the circuit, such as you can see and buy these components: UA741 .
When the input voltage is 0 LED. LED to stop when the voltage rise to light level will depend on the R2. Reverse and - needle reverse operation mode. Set the voltage at the cause, (1) set to 0 v in the input. (2) set the input voltage in the ideal level. (3) the adjusted R2 to point out after correct leadership.
http://pcb.hqew.net/?s=544864