I'm currently doing a home automation project which involves my intercom system being controlled by a NodeMCU. The intercom system runs off 12V AC. As for the actual automation/wiring itself, I think I have that fairly well covered. However, I would love to be able to power the NodeMCU directly from the 12V AC rather than having to bring in an extremely long extension cord just to supply 5V DC.
Whilst I am aware that there are plenty of options available for getting 5V DC from 12V DC, I have been almost completely unsuccessful in finding a simple convertor to do exactly what I intend at a low price.
I would really appreciate it if someone could give me some guidance as to how they may think to overcome this problem.
Thank you in advance for any help.
Best Answer
Generating 5V @500mA is easy, all you need is a bridge rectifier, largish capacitor and a 5V switching regulator.
The switching regulator is a replacement for the older linear regulators, you can use something like the R-78E-0.5 from Digikey or there are dozens of cheaper units on Ebay.
Since you are only adding about 2.5-3.5W maximum to your 12V AC input supply, I'd imagine you would have no problems.
simulate this circuit – Schematic created using CircuitLab
BUT THERE IS A PROBLEM ...in all probability the 12V AC is converted into at least one supply within your intercom. This means you CANNOT connect DIO pins or ground on the ESP8266 to points within the intercom as it may damage one or both.
You can overcome this problem in several ways:
Update:
Since #3 seems to be a better fit from the comments, this would be the schematic:
simulate this circuit