I'm designing a circuit involving 2 digital sensors and 2 analog sensors on an Arduino Mega 2560. Do I need to connect a transistor to the sensor before interfacing them to the Arduino pins.
Secondly, since the Arduino 3250 doesn't have a chip like the ATMEGA 328 on Uno boards, how do I transfer this circuit of mine to a printed circuit board.
The Mega2560 is a 5V processor and cannot be connected via I/O pins to 3.3V sensors without logic level shifters.
These days, almost all sensors are 3.3V. Some hobby module suppliers add the required level shifters, but since that adds expense, many of the cheap modules on Amazon, Alibaba and the like don't have them.
There is a risk of destroying both boards if the connection is made without those shifters.
since the Arduino 3250 doesn't have a chip like the ATMEGA 328
What is an "Arduino 3250"?
It's a typo, I actually meant "Arduino Mega 2560"
Might help if we knew which 4 of the tens of thousands of different sensors you have. 
The chip on the Mega 2560 is the big brother of the ATmega328, with more memory, more options, more pins, but with the same instruction set.
A PCB has to be designed for the exact layouts of the chips you plan to use. If that is news, then perhaps it is a bit early to be thinking about a PCB.
Hi, @david-silva
Welcome to the forum.
Can you tell us what your project is and what sensors you will be using?
Thanks.. Tom.... 
I'm currently working on a bioreactor project that involves controlling various actuators(DC pump, DC fan, DC heater, DC solenoid valves) and reading data from multiple sensors using an Arduino Mega 2560.
The sensors I'm using are :
I'd need more insights
Interfaces directly with a single digital GPIO on your Arduino.
Interfaces with the I2C bus (two pins) on your Arduino.
Purchase one that's part of an actual module; this will already have the signal conditioning circuitry on board. It'll draw something like 150mA from a 5V supply for the heating element. No problem even running an Arduino from USB, but your project will require a decent power supply for your motors etc. anyway. These modules use an analog pin for signal readout and a digital pin for control, so you need two pins.
The Gravity DFRobot pH sensor is also a sensor + signal conditioning board. It outputs an analog signal.
So for your sensors, you need on your Arduino:
Thanks very much.
Do you suggest I use an 8 channel relay module for controlling the actuators?
I'd use MOSFETs, personally. Smaller, cheaper, more quiet and more flexible (pwm).
I'm actually going with the MOSFETs to control the actuators.
So my circuit requirements are as follows
Sensors:
Drivers:
I'm also using a 20× 4 LCD to display values of the parameters and push buttons to set values of my various parameters
How do I choose an appropriate power supply for this project. I'm looking at a 7805 voltage regulator though with 100uf capacitors at the 5v output and 12v input.
Secondly, can this be done using an Arduino Uno with 2 ATMEGA 328 chips to split the pin requirements
Or should I just fit everything on an Arduino Mega 2560 as my end goal is a PCB.
Hi, @david-silva
Can you please tell us your electronics, programming, arduino, hardware experience?
How are you approaching the PCB design, although get the prototype running first, use a Mega and make a PCB to plug onto the controller PCB like a shield?
OR
Make a complete PCB with the 2560 controller IC embedded in the PCB design?
Have you got any of the hardware yet?
Tom..
I'm about developing the prototype.
I've not actually gotten all the hardware but I got my Arduino uno and Mega boards along with couple of components already
Power to your project is 12V? Way too much heat to dissipate in a 7805 linear. Buy a "buck converter", one with 3 or greater ampere output, adjust it to 5V and you're much better off.
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