Does anyone know of a circuit to reduce the telephone ringing voltage down to something I can input to one of the Arduino pins
a circuit to reduce the telephone ringing voltage
You have to rewind the coil on the telephone.
down to something I can input to one of the Arduino pins
Do you mean output from an Arduino pin?
If so no. The amount of power is not available from an arduino pin.
However if you actually mean how can you drive a telephone with the output of an arduino then you need a circuit known as an inverter, composing of a transformer and transistor.
I think that he wants to detect ring voltage (inbound call), Mike.
I don't encourage people playing with the POTS. [If there's a mishap, it probably won't involve only your line.]
Personally I wouldn't try to change the voltage at all in the phone's system. I would try to use a logic output optocoupler, with the ringer driving the diode on the opto. This type of opto lets you convert signals to a logic level of your choice with isolation, modifying your ringer circuit as little as possible.
Edit: Oh, and the Arduino probably won't like the frequency of the ringing (Might not detect anything), so instead of outputting the opto straight into the Arduino I'd go into a one shot multivibrator and then into the Arduino.
I have don't this by taking a tiny lil POTS phone from a yard sale. And taking the signal from the point where the ringer pizo or whatever it has for a ringer hooks up. Run that signal through a 4n25 opti-coupler and a series resistor (size dependent on your phones ringer type). Then you have a ringer signal you can use for what ever and you didn't have to mess with the POTS at all.
Due to the laws involved, esp if you mess up...
An alternative would be to slap on a piezo near to the phone, amplify the signal use software to determine if it's the phone or microwave going ding, it may not be as easy, but you aint risking a law suit either..
I'd say so, but I suspect Mike knows that too. 
A optocoupler is the way to go - one of these would be pretty nifty as they detect both halves of the AC cycle. You use a capacitor (0.22µF 400V, not electrolytic) and resistor (10k) in series with the input side to deal with the nominal 70V at 16 Hz or so ringing voltage.
Well, we could give him the benefit of the doubt and assume some competence ...
Unit_One:
Oh, and the Arduino probably won't like the frequency of the ringing (Might not detect anything), so instead of outputting the opto straight into the Arduino I'd go into a one shot multivibrator and then into the Arduino.
If you write the code properly, the Arduino will detect it just fine. No interrupts required. 
Paul__B:
[quote author=Runaway Pancake link=topic=254729.msg1802479#msg1802479 date=1405181055]
I don't encourage people playing with the POTS. [If there's a mishap, it probably won't involve only your line.]
Well, we could give him the benefit of the doubt and assume some competence ...
[/quote]
No offence to anyone, but if that were likely then the question wouldn't have been put in the first place.
Everyone's house wiring has a breaker box, so if you mess up on that it's just you, but the PSTN /POTS isn't that way.
ok we're getting a little carried away with law suits and bringing down the rest of my street if i do somthing wrong i'm a retired phone technician what i do with my phone line will not affect anyone else so i take it the best course would be an optical isolator does anyone have a link to one as i'm not familiar with one of these not sure which one would be appropriate
with this
Even with an opti you will probibly have to drop the voltage but if you search AC opti-coupler on jameco.com you will find some that work I would NOT think that you need zero cross detection. And I don't know of any that are good to 90v
Trickyrick:
ok we're getting a little carried away with law suits and bringing down the rest of my street if i do somthing wrong i'm a retired phone technician what i do with my phone line will not affect anyone else so i take it the best course would be an optical isolator does anyone have a link to one as i'm not familiar with one of these not sure which one would be appropriate with this
Yours was the first mention of "law suits".
http://transition.fcc.gov/wcb/iatd/part_68.html
http://brohogan.blogspot.com/2009/12/telephone-interface.html
So best to use an old phone and use its ringer signal it is already a usable voltage and safe to use given if your circuit fried it would never hurt the anything on the outside
The ringing voltage of a ringer in a phone is the same as the ringing voltage coming into the house around 80 volts at 25 cycle.
Hoe about a current transformer? When voltage is detected on the line , arduino fires off...
But, since that was your job ..
@cjdelphi
Nice
@trickyrick
I am using the old phone (cordless) way right now in a loud room to flash a 100w Dj strobe on a triac and it works great. I only had an audio signal at my ringer I guess it may be because it had changeable ring tones but yea no high voltage at all. It stands to reason that you should always check stuff like that before hooking it up.
Trickyrick:
I'm a retired phone technician what i do with my phone line will not affect anyone else so I take it the best course would be an optical isolator does anyone have a link to one as I'm not familiar with one of these not sure which one would be appropriate
So in fact you do know what you are doing, perhaps more so than many here.
Ahem. I gave you a link. And the phone line side of the circuit with which you will be well familiar. The output side of the optocoupler you connect between an Arduino input and ground with the internal pull-up enabled.
I think that you want a circuit that signals the Arduino when the phone rings.
A schematic of a gizmo called a "ring detector" from Velleman is attached. Here's a link to the product page, where you'll find a link to the user manual, the source of this schematic: [u]https://www.vellemanusa.com/support/downloads/?code=K8086[/u]. The manual is long on assembly instruction, and short on theory. Note that Velleman's blurb for this device doesn't mention any regulatory approvals, so I presume it has none. Proceed at your own risk.
The device blinks an onboard LED, and optionally operates an onboard relay. Power for the relay coil comes from an offboard power supply, presumably because it takes more current that can be surreptitiously drawn from the phone line. That supply doesn't appear to come standard with the device. I don't think you want a relay output, anyway, so you can do without it, and without the circuitry that drives it. You can replace everything to the right of the red line with a pullup resistor, an input pin's internal pullup resistor, or, as has been suggested in this thread, a retriggerable one-shot multivibrator. You can do without the LED, if you don't need a visual indication.
The purpose of 1M resistor R1 escapes me, but it limits current to microamps, so it almost might as well not be there. A purist might omit it. As a phone technician, you'll know a lot more about the possible effects of that resistor on the system than I will. Maybe you can illuminate?
I have previously believed that ringing voltage was supplied by reversing the polarity of TIP and RING. Today, I'm not so sure. You'll know, though, and you can use that information, along with knowledge of transients that Might appear on phone circuits, to select the right voltage rating for capacitor C1.
Otherwise, the circuit looks OK to me. Capacitor C1 blocks DC, so there's no current when the line is on-hook. The back-to-back zener diodes block current when the AC peak voltage is below 20V, so there's also no current when the phone is off-hook. he full-wave rectifier ensures that it's balanced, and will operate when TIP and RING are reversed. The only time the circuit conducts is when the phone rings. The phone system used to ring electromechanical bells, with coils and switches, so I'd expect that it can deal with considerable chaos during ringing, and I wouldn't expect it to be annoyed by anything this circuit could do.
Alternatively, you could just buy one of these devices. Amazon has them for about 23 USD, and I've seen a price of about 11 pounds, exclusive of VAT, on a UK site.
I have, and have used, this
http://www.broadcastboxes.com/pdf/mpc2book.pdf
http://www.broadcastboxes.com/pdf/mpc2prmr.pdf
Presented in the interest/s of education only.
tmd3:
I think that you want a circuit that signals the Arduino when the phone rings.
That appears to be the requirement.
tmd3:
A schematic of a gizmo called a "ring detector" from Velleman is attached.
Yep, that's a more sophisticated version of the circuit I described, using the Zeners. Using a purpose-designed optocoupler such as the EL814 does away with the bridge rectifier.
And again, the same circuit, sans Zeners (whose main purpose is to suppress response to the impulses generated by decadic dialling), since the "hook" switch disables the ring detector in this case.
Y couldn't I just use something like this. the input voltage is 80 to 240 VAC
All I want to do is trigger an event on the arduino board so I could use this as a switch when the phone rings