Telephone line hack

Hi all, I’m just wondering if anybody knew how feasible it would be to hack a telephone landline so that the spark knew when there was an incoming call and could flash lights around the house to indicate that it was ringing for the hard of hearing ( have seen hacks for lightwave rf wifi box that would allow this via php ) … I’ve done some basic research and can see that the line is 50 v, and 100 when there is an incoming call… So I guess the core could measure the difference somehow to identify a call? - would also be cool if it could use the line as a powersource too. - obviously wouldn’t want to impact broadband signal or line quality though. - what do you think? Feasible?


They do have those add on buzzer or light indicator for telephones.

I don’t know but it has been years since we had that after all the changes to digital line here in Singapore.

But i remember there were stuff like that? So it’s gonna be feasible.

Do you can find which pair gets the 100Vac when it rings and use a bridge rectifier and turn it into DC before stepping down for the core as an input/.

-48VDC on hook
~9VDC off hook
~90VAC ringing

I think there are only two wires coming in (in the UK) any way. shows how the wiring works… Although that site suggests -48 v and 0 v which is different to what I’d read earlier.

Have fun! but be careful really XD

Hah… it’s not too bad. We used to strip live tel wires with our teeth when the wire strippers wore down. Sometimes you get a little buzz, but the worst time was when I had my forehead pressed against a tin bulkhead. A call came in as I had the wires in my mouth - that one made my ears ring. It’s a strange sensation, you can almost feel the ring signal in your teeth.

You might be interested in The datasheet has some specification for lines in a few countries

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You could build this and tap in to analog pin (proper voltage/current of course) of Spark Core. This circuit does not tap the phone line, it only goes around the wire with a picup wire.

Phone Ring Pickup Wire Circuit


This would work quite well I am sure. You could delete Q3 and K1 and R3 since you don’t need the buzzer. I would replace the D1 LED with an opto-isolator and connect the other side to a Spark core with a pull resistor to sense the wrings.

Nice idea!

@bko, I found this while working on my phone disconnect device a while back.

Using, say, a 4N25 opto-transistor where would be the best place to pull-down the 4N25 ?

If we kept Q3 the 4N25 could be pulled down by it ?

Would a Resistor be needed between the Core and the Transistor side of the 4N25 ?

Notes about the Contactless Ringer Circuit:


For L1 make 50 close turns of 28 SWG enameled copper wire on any of the two telephone wires.Connect one end of the coil to base of Q1 and leave other end free.

If the circuit not works by try by changing the end of the L1 connected to base of Q1.

A 9V transistor radio battery can be used as the power source.

Assemble the circuit on a good quality PCB or common board.

Use of this type or any other devices other than the allowed telephones and devices with the telephone line may be a law violation in some nations.Check it properly before trying.

I would have put the LED of the opto-isolator where the LED is and then the transistor side gets hooked up emitter to ground and collector to a pull-up resistor and the Spark input pin. You could put it where you have it but I was assuming that would not want the LED and the extra transistor. Your schematic needs a pull-up from A0 to 3v3 (or turn on pull-up on the core input), assumming your wire to ground is the emitter and A0 gets the collector.

ahh this looks a bit easier than I thought it would be - although that schematic looks like it’s using a 9v bat and not power via the line. I’m just wondering how broadband usage will impact on the non invasive circuit though? - I.E it uses the phone line all the time I’m not sure what that will do to the current … and I don’t know for sure but I’m assuming that the change in current is what makes this schematic work ?


I have been wanting to convert this circuit so it uses only the volage from the Core, 3.3v . I have a couple days to build the circuit in eagle but, won’t be back home for a few more days before I can test it on my proto-trainer board.

As far as the trigger. As the 90vac ring volatage passes through the pickup coil “L1” causing Q1 to trip the circuit.

@bko, will 3.3v from the core with 1K resistors be enough for 2N3904 transistors to be in saturation ? or will I need, say, 300 Ohm resistors ? (If I were to use 3.3v insead of 9v supply).

Also, why do we need Opto Isolation if no direct contact is made. If lighthing hits the phone line an Opto is not going to protect the core any ways ?

As you can tell I am not a pro in this, just learn enough to make the LED come on or off :slight_smile:

You don’t really need the optoisolator–that was just an easy way to make connect to Spark. If you want to power it all from one supply that will work fine.

@bko, thank you.

I have an idea how this circuit can be an isoation privacy device that I would like to use with a product I am building.

Would it be okay to PM you to get your feedback on my concept and application to see if it sounds like a marketable device.

I would ultimately make the Eagle Cad files available to everyone with the above circuit with a Spark Core Socket; jumpers to switch from A0 - A7 as input selection; and a socket for a cable to my optional isolation product.


@bko, @kennethlimcp, Let me try this again.

This is a draft - seeking comment - based on above Schematic. Now using 3.3v from Spark Core to power circuit. Using spark core socket, .700" span.

10K pull-up resistor @ R5 but, I also read there is an internal 40K pull-up that can be enabled in IDE code; is this true ?

The taces are 0.016" and should be okay for this shield. No physical contact with Phone Line - using inductive coil around Phone Line. May have to increase or decrease value (sensitivity) of R1 .

What are you guys thoughts ? Any Mistakes ? Values of resistors will be validated next week.

Looks reasonable–I am not sure why you want L1 on the board though. It could be just wound around the phone wire, so it would just be a pad. You have to run one side of the phone line through L1.


Very True about the coiled wire, Thanks !

Could also use a pcb mount 90 degree connector like a 3.5mm mono plug socket so it would not require a user to solder anything and the cable could be disconnected when not in use or if needed to be routed though a hole in an enclouser.

I will set this up on my protoboard next week and test it out; verify values next week; and the max length of cable that can be used, etc.

@kennethlimcp sized his proto-board this way and I like the dimensions a lot.

This board, being that its not using TX or RX could be stacked on other boards that the Core will control as well, like @kennethlimcp soon to be sdCard :slight_smile:

Had time to check circuit & had to adjust R1 to get ring detection. Also added some Proto-Pads. Ordered 3 from for $15.

1 inch x 2.85 inches

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We might be able to squeeze all the components under the :spark: core and half the board size with some SMT :slight_smile:

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@kennethlimcp, Yes ! I do agree with you. These first boards will be proof-of-concept and mod them if needed.

I do have 3 types of boards that I will need to have produced with smt components but, that’s a few months to come; I think ?

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