Hello everyone,
My English Language is weak
But I try ask you
I study Process Control, I need help about how i can design sample circuit alarm.
please help me.

 

Do you mean simple circuit alarm rather than sample?

Where will you use the alarm?

 

yes
maybe I use alarm of fire or any thing you can help me

 

A simple alarm for fire to be used inside a building can be built from an LDR, a transistor and a few resistors. If you want you can build it with an op amp like here:

http://www.reuk.co.uk/LM741-Light-Dark-Sensor-Circuit.htm