I am redesigning my PCB and I would like to ask the best spot to place my 1uF for the output given by the IR receiver going to the input (+) of LM324. I have two PCBs and the first PCB has the LM324 with the PIC and the 2nd PCB has the 4 IR LEDs and the 4 IR receiver. They are connected by the 6 pins connecting wires.

1.Is it better to place the 4 IR receiver's filter capacitors beside the LM324? beside the IR Receiver? or on the first PCB beside the connecting wires?

2.Do I also need to use filter capacitor for LM324 to be hooked on Vcc to ground? I don't see any application circuit figure on the datasheet.

Thank you in advance

 

The decoupling capacitor should ALWAYS be placed a close as possible to the pin it is meant to decouple. In this case it will be the VCC supply rail, pin 4 of the opamp. Your project is not a production model. So just solder it on bottom side between pin 4 and pin 11 (GND). Lay the cap on its side
As an aside. Then you use a dual supply opamp configuration. Both the supply pins should have a decoupling cap to ground.

 

As the rooster says:

The decoupling capacitor should ALWAYS be placed a close as possible to the pin it is meant to decouple.

I think you are talking about the decoupling capacitors for IR reciever 3 pin packages, if so put them as close as possible to the IR receivers as possible.
You do also need one for the opamp, value should be in the datasheet.