Complete newbie need startup advice

Hi, Sorry if this info i require is wrote somewhere. I want to get into using arduino and have no previous experience. I am quite experienced with alexa and node-red for smart home automation.
I want to make my dumb c02 detector smart with the aid of arduino. This is what i need
1.i have tapped in to the c02 detectors speakers and when the alarm activates upto 3v is supplied. I want the arduino to detect when voltage is sensed.
2. I have a 433 mhz remote if needed and a tasmotised 433 bridge that detects the signal. I want the arduino to power the 433mhz transmitter when it senses voltage. This may or not be required as wifi may render this not needed.
The c02 detector is in a cupboard with the boiler and power supply outlets are next to the unit.
What arduino do i need, which shield and anything else will i need for this project.
I will use node red to complete my notifications etc.
Thanks in advance

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There are several ways to approach this.
First, if I were doing this I would use a Wemos D1 Mini because it has WiFi built in. (And I buy them by the dozen).

For detecting if the Co2 is alarming, I would use a microphone module on the Analog input (Amazon.com : Arduino Microphone). This would be noninvasive and if you ever replace the Co2 detector, you don’t have to hack the new one.

I’ll assume (since you didn’t say) the 433 bridge is the Sonoff RF bridge, so you are using MQTT? Then, why the 433 transmitter when you can publish an MQTT message from the Wemos.

Or, why not simply put a Co2 detector module on the Wemos? Or Arduino Nano 33 IoT if you want to stay with Arduino. (I have never worked with a Nano).

If you really want to hack the Co2 detector, you will need to find a power source in the detector that can supply 80-100mA.