Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Mains intercom

Status
Not open for further replies.

zachtheterrible

Active Member
How do they do that? I would think that the 120v would totally kill the electronics but there must be a way.
 
They generally use FM, injected onto the mains, via isolation caps and filters. The higher FM frequency passes through the filter caps, and the 60Hz AC is blocked.
 
ahh, that makes quite a bit of sense.

This would nesecitate capacitors rated for mains voltage i would imagine. About what value of capacitors?

Does anybody know of any schematics on the net? I searched google and couldnt find anything.
 
I haven't seen any designs for years, they used to be popular in the magazines - as did computer data transmission via the house mains wiring (rather like X10).

They usually use a low RF carrier, something like 80-100KHz, as you already know, you need a specifically rated isolating capacitor for safety!. It's usual to transmit between earth and neural, so there isn't a large voltage across it, but you still need the high rated capacitor for when it's connected to a wrongly wired socket.

Both the transmitter and receiver usually used a tuned RF transformer, and details for winding those used to be given in the articles.

As you're in the states, I suggest you try popping down to Radio Shack?, I bought a wireless mains intercom years ago, at Tandy, the long defunct UK branch of Radio Shack.
 
I've reverse engineered one many years ago for tel/fax bugging.
 

Attachments

  • commax.gif
    commax.gif
    18.7 KB · Views: 4,982
Although it isn't an intercom, its sorta kinda related. I'm talking about BPL...or Broadband over Power Line transmission. As of now, it is still being developed. Check it out. It can replace Dial-Up and ISDN and it may someday replace DSL and even T1. Everybody already has power, so no new wiring will be needed. The down side of it is the noise. Electrically speaking, of course. And who knows what might happen to your computer from a lightning strike on the grid. :shock:
 
Thanks for that schematic sebi. I can't believe one actually exists on the internet :lol:

captainkirk, I read that entire thread on BPL. Very interesting stuff :lol:

Nigel, do you have any idea what magazines they may have been in?

Maybe the electrical is different in the UK, but over here, earth and neutral are connected, i measured a very small resistane between them w/ an ohmeter.
 
zachtheterrible said:
Nigel, do you have any idea what magazines they may have been in?

I've seen them in various magazines over the years, certainly in 'Practical Electronics' and 'ETI', neither of which exist any more, both are now part of EPE.

Maybe the electrical is different in the UK, but over here, earth and neutral are connected, i measured a very small resistane between them w/ an ohmeter.

Same here, the neutral is connected to ground at the substation, but it's a high enough impedance not to short out the RF carrier - another reason for using RF :lol:

You shouldn't try and measure the resistance between earth and neutral, there's usually a voltage across them - only a few volts, but it's not nice to put your ohms range across a voltage.
 
Make two transceivers, and use a power adapter to run them instead of batteries. Its as simple as that.
 
zachtheterrible said:
Thanks for that schematic sebi. I can't believe one actually exists on the internet :lol:

captainkirk, I read that entire thread on BPL. Very interesting stuff :lol:

Nigel, do you have any idea what magazines they may have been in?

Maybe the electrical is different in the UK, but over here, earth and neutral are connected, i measured a very small resistane between them w/ an ohmeter.
As I understand it, earth and neutral are connected together in the UK as well. I'm sure I can remember my father checking the resistance between earth and neutral.
 
JohnBrown said:
As I understand it, earth and neutral are connected together in the UK as well. I'm sure I can remember my father checking the resistance between earth and neutral.

It is, I explained it a few posts above!.

But again, you shouldn't check the resistance between them, as there's a voltage difference!.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top