How should i connect Weight sensor's,thermal detector's and metal detector's output signal to microcontroller? Should it be connected directly or there should be any sort of circuit in between?
You have to connect with any of the port pins based on your application... If there is direct connection is established via port pins, you can connect directly or use any other protocols in between...
Very ofen sensors output need some adapter ...
What is the level output of each sensor do you want to connect ?
Analog sensor output ?
0-10V
4-20mA ? passive mode or active mode ?
1-5V
mV ?
thermal detector is Dry contact or analog ?
need galvanic insulation or not ?
Metal detector is proximity sensor ? 24V power supply ?
as you see, you must provide many datas if you want to recieve usavle information back.