How many sensors can be supported by a single Arduino unit?

I think you want to know how many sensors 1 Arduino can support. But just in case, I am answering both interpretations of the title of your question:

1. How many different types of sensors can an Arduino support?:

With the correct software and hardware, the Aruidno can interface with almost any kind of sensor. The exceptions include but are not limited to sensors which produce more data than an Arduino can process (such as a video camera) or sensors that require fast responses (such a time of flight laser distance sensors (LIDAR)).

2. How many sensors can an Arduino support at the same time?:

This depends on what processor resources are needed. If the sensor needs the ADC there 6 channels on most Arduinos. If you need more, there are hardware solutions. But many here have suggested to simply add a 2nd Arduino.

To specifically address your laundry list:

room temperature

Thermistor connected to ADC. But most are not linear, so you will have to create a look up table.

humidity

I think there is a SPI module on the market to do this.

carbon monoxide

These sensors usually require an amplifier. And change with age & temperature. So a lot of software here as well as hardware.

carbon dioxin

Not sure about this one.

buzzer

Self osculating? Then you only need a logic output.

1 LCD display

Alpha & numeric? Most have 4 or 8 pin Hitachi interfaces. But you can look for a SPI or serial port LCD so as to conserve on Arduino pins.

ESP8266

This is just a serial device. You will need the Ardino's serial port.

I'll leave it up to you to make a detailed assessment of the necessary processor resources. Because you need to first pick out the specific hardware you will be using.

That's half the battle. The other half is verifying the libraries for all these sensors and communication devices will not interfere with one another. That I will leave up to you as it is not an easy task to wade through all the code for all the different libraries you will be using. And, again, until you pick out exactly what sensors and devices you will be using it is impossible to even start analyzing as we wouldn't know which libraries to look at.

The question itself is a bit broad, but I'll try and provide some insight here.

The number of sensors that can be supported, and the number of pins they require, will depend on what sort of interface they use.

• If your sensor has an analog interface, you will need an analog input pin for each sensor.
• If your sensor has an I2C interface, you can have multiple devices sharing the same pins (a bus) provided that they have different I2C addresses (often configurable with jumpers on the device).
• If your sensor has an SPI interface, you can have multiple devices using the same pins (a bus), except that each will need a chip select PIN connected to a digital I/O pin on your Arduino so that it can 'select' each device.
• If your sensor has a serial interface, you can generally only have one device.

To add complexity to the problem, you can get expansion modules for the Arduino that will give you extra analog pins or other communication interfaces.

With the long list of sensors you have above, you may also be hard-pressed for program space, as most sensors will need code and/or libraries to interface with them. The Arduinos with more pins (such as the MEGA) also have more program space.

I would encourage you to do some further research on the sensors you want to use, and finding out what interface they use, and what Arduino libraries are available for them. This way, you can fully understand the requirements and limitations of any parts you want to use.

Further reading:

• https://www.arduino.cc/en/Reference/Wire (I2C interface)
• https://www.arduino.cc/en/Reference/SPI (SPI interface)
• https://www.arduino.cc/en/Reference/Serial (Serial interface)
• https://www.arduino.cc/en/Guide/Libraries (Using libraries)
• https://www.arduino.cc/en/Tutorial/HomePage (Arduino tutorial)