Possible to Power RPi 3 & Touchscreen with Old Intercom Wires?

Question

So this question could be a dumb one, as I have little to no experience with electricity. Anyways, here it is:

My current project is using the RPi Official Touchscreen (7 inches), along with a RPi3 and putting it in my wall where an old Nutone intercom (Nutone IS408) used to be. The intercom has various wires, which I'm assuming some of them carry power for the speaker. These wires come from the main unit downstairs, which is a Nutone IMA-4006. Would it be possible to connect these wires to somehow power the RPi3 and RPi Touchscreen by connecting those wires to the Pi somehow? Whether it's through the GPIO Pins, or splicing into a Micro-USB cable?

I want to do this because there is no easily-reachable power outlet near this wall, and this being able to use those existing wires would probably be ideal if it's possible.

Answer 1

I prefer not to use them directly, But as you said you didn't have any other easy way.I suggest you take either end of the wires out

1. on the raspberry mounting end connect those wires to 7805IC and output of ic to splice MicroUSB

2. On the other end connect those wires to 12v supply

You should be running them both without any problem

Answer 2

It is infeasible to power a Pi3 remotely over a distance more than a couple of meters, even if you used ludicrously large wires. 1.5m 23/.010 speaker cable (which I use) with a 5V supply is working at its limit.

If you do want to remotely power the Pi (or any other 5V device) you need to use a higher voltage (at least 12V) supply and a step down regulator.

Attempting to use wires used for some other purpose is just asking for trouble.

Answer 3

There is a way do do what you want. You will need to know what the maximum load current is and the size and length of the existing wire * 2). Multiply the current by 125% (efficiency adjustment) so you have some cushion. With these numbers go to this calculator: https://www.wirebarn.com/Wire-Calculator-_ep_41.html with that you can calculate the maximum your current your wire will support.

You can raise the load current by putting a buck converter at the load set at 5V or whatever you want and then raising the input voltage to the converter enough to get the current and voltage your wiring will support.

You need to connect your power supply at the other end that will supply this voltage and at a minimum the calculated current. Using the buck converter without considering efficiency you can divide the load current by the voltage ratio of your input voltage to calculate current capacity of your wires. Example if you have a 3A load at the output, supplying it with 10V will reduce the current to 1.5A.

Note, the buck Converter will probably be in the 90%+ efficiency or better, if not increase the efficiency adjustment percentage. It would be a good idea to place some bulk capacitance and high frequency bypass by adding two capacitors. This should get you going. You can test this out on the bench with the parts and some tungsten light bulbs to validate everything. Hosehould lamps would be OK.