hi, i need to build a voltage indicator.
when the voltage will be 12V the led to turn green,
if i am lower than that i need the led to be red.
what parts can i use for this purpose?
It looks like you need a couple of comparators. You will have to set them up to control one or the other LED and or turn on a power supply for the LEDs. Let me know if you have specific questions about the parts.
The ADMP356 may be useful
http://www.analog.com/en/special-linear-functions/comparators/adcmp356/products/product.html
Depending on how much current you need a linear regulator or a switching regulator could also be used/ Something like the ADP7104 or ADP2301
http://www.analog.com/en/power-management/linear-regulators/adp7104/products/product.html
http://www.analog.com/en/power-management/switching-regulators-integrated-fet-switches/adp2301/products/product.html