A user on Electro-tech-online.com posted that it might be possible to use a saturable core transformer to detect the DC current by non-contact means as the current flows by the core.
Know of this ? Is it possible ?
They also mentioned that the use of a Neon Bulp could be used to accomplish DC detection as well ?
?
I did some research on the Saturable Core Transformer can could not find one locally or even on line that was smaller than the size of a car battery. And, I also checked Google Patents and someone has the patend on this techology. So with frustration comes options to open new doors.
I think I can use the -48vdc (contact) to delatch the relay.Ā i can use a āSelf-Delatchingā circuit confined to a separte relay board but, not connected to my Ring Detection Board. Here is the process I think will
work:
A. Phone is ON-HOOK with -48VDC, relay is open isolating phone line from my home.
B. A phone call comes in:
On a Separate Relay Board that is connected to the phonline wires:
90VAC by induction excites the sense Transistor attacted to the inductor L1The relay is tripped close and phones begin to ring.
Phone is answered - line voltage drops from -48VDC to -20VDC (OFF-HOOK Voltage)The relay is latched only by a transistor that will desaturate (opto-isolated) when the -48VDC returns to the phone line.
Phone is hung up - line voltage increases back to -48VDC te relay is de-energized and phone line is isolated from GRID and waiting for another call.
Do you guys think this would work ?
Using a DTDP relay rated at 120VACĀ has to be connected to the phone line to provide isolation and must have at least 800 Ohms of resistance per FCC Regs.
So I need a self-contained automatic Latching / De-latching relay circuit connected to the phoneline wires to handle isolation / pass-through / reset of relay.
The Spark Core Micro-controller Isolation / Detection board will only use the 90VAC Only by induction for automation / monitoring / logging call info
This will work for both Landline and VOIP phone systems.
Should I go this way ?
A small separate Relay Board that is isolated form the Spark Core that will automatically close the relay on a Ring of 90VAC from phoneline. then Delatch automatically when the Line voltage returns to -48VDC ?
All comments welcome ! This is dwiving me nuts !
My underground lab has turned in to a Rabbit Hole 
If you pass the cable thru a ferrite core a few times then you should be able to pick up the dc current with a hall effect sensorā¦ All you need is a very slight change to detect. Try cutting a slot in the core to tightly fit a hall sensor then use that hole to wrap the core with 20 loops of one of the phone wires.
You shouldnāt need a pre made commercial highly accurate current measuring deviceā¦ just a current detecting thing a me jig that says yep or nar.
And Reed relays are great to use for things like thatā¦ In dip8 size too so easy to use on proto board
Good for a few hundred volts at low current so perfect for isolating your line
Hi Hootie81,
I will try both of your suggestions. In my quest to keep it simple and stay out of FCC Certification issues Iām not sure if I can use a Reed Relay.
What about using a reed switch as a reed relay like this article ?
I have worked with Hall Effect sensors some so I can test your suggestion with no problems.
The main problem I have is trying to sense the DC -48VDC (Non-Contact).
Also, anything that is physically in contact with a FCC Regulated phone line (the copper wire) must have at least 800 Ohms resistance.
For the dc current measurement I would leave the insulation onā¦ itās kind of like making a electro magnet in primary school. The more wraps the stronger the magnet. Hall effect seniors come in 2 types from memory. . One measures just one polarity and the other measures both you would need both if the current changes direction
And yep itās the reed relays not the switches Iām thinking of. i cant see the difference between using the 2ā¦ essentially you are making a reed relay using a reed switch?
Once your line is isolated you will need some kind of resistance connectedā¦ otherwise no current will flow?
@Hootie81 @peekay123 @kennethlimcp,
Thanks for previous comments they are helping me go to the right places but, since this device / cicuit I want to obtain does not exist I am finding possbile components (Maybe) to use.
How about this Hall Effect Sensor Chip by Allegro - there new and experimental (so I read): ??
Could I use this to detect 20ma - 70ma DC current draw when phone is answered ?
To me that looks like overkill and makes things a bit more complicated.
I would probably use something like the UGN3503UA. its alot more common and cheap as chips and should do the job quite well. trick will be cutting a ferrite core to fit it perfectly and lining it up perfectly with the actual sensor in the package.
if the supply is 5v the output will sit at 2.5v and any current will either reduce that or make it higher. if you arent getting a big enough change to see on an analogue input on the spark then wrap the line a few more times, if its peaking out at 5v or 0v then take a few outā¦ its going to be a trial and error thing and will depend on so many different factors.
Thanks for that info overkill - yep, I know - needs to be a better circuit.
Will be ordering this chip tomorrow.
I know this has turned in to a long project but, I struggle with the design part.
Help would be appreciated and I will be testing this whole circuit this coming weekend.
Do I need to Place A Bridge Rectifier at āAā and a Opto-Coupler at āBā on the below schematic as 90VAC will be present when the Phone Company sends a Call and phones ring ?
If I wanted to tell the Spark Core there is either 48VDC or 6VDC on the telephone line would I use a Comparator Circuit that produes Analog Signal 0-3v : OR
OR, Could I use a Zener Diode & Resistors to accomplish same 0-3v to the Spark Core A1 Pin ?
What I have added so far:
Here I present a more advance version of my Spark Core Phone Isolation Shield.
Needs Isolation at āBā / ? Bridge Rectifier at āAā ?
http://spydrop.com/files/SparkCore_Phone_IsolatorV1_1.08.jpg
Thats a whole lot more complicated than what i would doā¦ i like KISS
Cant tell whats happening around the RJ11 connectorā¦ in Oz our phones only use 2 wires on pins 3 and 4. if we have dual lines i.e. separate phone and fax then we can use 3 and 4 for phone and 2 and 5 for faxā¦
I would do something like this:
1 socket for input, and another socket for output. SPDT reed relay between the two socketsā¦ one way is output line isolated and a 10k resistor across the line (for current to flow) the other position connects to the output socket. (might need to do 2 SPST reed relays if you cant find a 3.3v SPDT)
All pick up of all of the voltages is done by measuring current with a hall sensor. (thats on the line before the isolating reed relay.)
edit: cant find a 3.3v SPDT reed relay online so will have to use 2ā¦ but thats OK. See pic below for what i was thinking.
RJ11 on my schematic (from top to bottom)
Will make a special RJ11 (six wire) Cable for connections of:
pin 6 = External 5V supply for 555 / SSR control.
pin 5 = Inductive Wrap pickup wire inside special RJ11 Cable
pin 4 = Telco line Green + to SSR Contract
pin 3 = Return Telco line Green + from SSR to Phones in House.
???-----------------
On your drawing, what is the first Reed Relay for ?
Wouldnāt that put a load āOff Hookā signal to the phone company if this relay closes itās contact ?
It possibly would yesā¦ but there must be a threshold on there?