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How do I separate AC and DC signals from phone line to go to 2 circuits

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Bobby Cooper

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Hi, New to the forum and glad to be here.

My project: I built a very known Contactless Telephone Line Ringer Detector circuit and it works as designed but, I want to benefit two fold by spliting the DC and AC signals out and have my Micro-controller control a relay based on the state of AC and DC at any given time. If I place an uninsulated alligator clip on the insulated vinyl jacket phone line this device detects the -48volts vdc for ON HOOK.

How would I split the AC to one ciruit and DC to another circuit ? (Separate the AC and DC telephone Signals for use in other circuits)

Thanks, Bobby

Here is what I have so far:
core_phone_isolator_V1r08-05.jpg
 
Use something like this:

The R and C values determine the cut off frequencies for the DC and AC.
 

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Use something like this:

The R and C values determine the cut off frequencies for the DC and AC.

Thank You Very Much. The AC is running at 60 Hz and DC is only voltage. I know you have saved me a great deal of time and I appreciate it. Bobby
 
Thank You Very Much. The AC is running at 60 Hz and DC is only voltage. I know you have saved me a great deal of time and I appreciate it. Bobby

Looking at the Schematic you provided....

Would it be simple enough as to just use a Capacitor to block the DC and allow the AC to flow to the 2nd circuit.

  1. I pickup (inductively) prior to the Capacitor ?
  2. Then after the Capacitor I pickup (inductively) the AC part at 90vac (Phone Ring Voltage USA) ?

I don't need to worry about Frequency in my project and I am a very new to electronics.

Thanks, Bobby
 
I hope I am using the proper term "Inductive pickup". I am using the below Contactless Ring Detector Circuit to detect votage present on a phone line. My board is designed after the below circuit but, does not use Q3, S1, or K1 Piezo Buzzer. The trick I found was to wrap an "insulated wire" (5 wraps) around the RED (RING) wire of the phone line and the circuit will detect when a 90VAC (DC Drops to -20VDC) when call happens.

I also found out that by using an uninsulated alligator clip over both Telephone Lines I could pick up the "ON HOOK" -48VDC as well.

So my quest is to use (when present) the -48VDC "ON HOOK" to keep a relay OPEN and to use the 90VAC to cause the relay to Close.

By your schematic it appeared I just needed a capacitor to stop the DC and inductively pickup the 90VAC down stream from the capacitor.

L1 of Q1 is the inductive wire wrap (If I am saying that propertly)

Again, thanks for your help.

(SEE Below image for Notes provided with circuit below)

contactless-telephone-ringer_circuit.JPG


The contact less telephone ringer circuit can produce a ring as well as a visual indication when a call comes.The main advantage is that since there is no direct contact between the phone line and the circuit there is no chance for a loading or disturbance in the telephone line.

When the telephone rings 60 Hz AC signal is generated which produces a proportional magnetic field around the telephone lines.These magnetic field will be picked up by the coil L1 due to electromagnetic induction.A proportional voltage is developed across L1 and it will bias transistor Q1 to ON.This results in the conduction of transistors Q2,Q3 andQ4.The buzzer will ring and the LED will glow.The switch S1 acts as an ON/OFF switch.
 
Ah, a picture is worth a thousand words! You did use the word inductively properly. If you power the relay directly from the phone 48v it is possible, if its resistance is too low, that it will pull down the line voltage and make your phone look like, to the exchange, that it is picked up already. Will think about your circuit a little more.
 
Do you want the relay to close only when the ring is present(90vac)? Or do you want the relay to close for the duration of the phone being "picked up/answered"?
 
Do you want the relay to close only when the ring is present(90vac)? Or do you want the relay to close for the duration of the phone being "picked up/answered"?

Thanks for your comments: I do need the Relay to stay closed when the -48VDC drops to -20VDC which happens when the phone is answered. When the Phone call is ended the return to -48VDC should close the relay (complete the cycle) and be waiting for the ring 90VAC to close the Relay again and remain closed until the phone is hung up.

The Software will control timing and delay and on/off of relay. I just need 2 signals by induction both the 90VAC and -48VDC.

I have been told that I can not detect the -48VDC by EMF but, my circuit does indeed excites Q1 Transistor (Saturates it) 3VDC is pulled down on Q4 to light the LED. In my design I do not use Q3, Piezo and SPST Switch. It is powered by 3.3v (battery)

I can do a video tonight that demonstrates how I can detect the ON HOOK -48VDC with my board that people told me is not possible if that would help.

Let me know if I need to clearify anything at all.

Thanks, Bobby
 
Would this help;
https://www.electronics-circuits.com/tech/telephone-status-indicator-with-led/
The ring LED is powered thru a cap and the others are DC driven thru a bridge rectifyer.

4pyros - Hi and Thanks for your comment.

I can't use a contact to the phone line type circuit. The circuit of my board is non-contact and seems to be the beginings of where I want to end up. I am trying to understand how when -48VDC is applied to the phone line, how my board can picked that "inductively" via a non-insulated aligator clip ?

I was able to put a .1 uf Dielectric Capcitor in the circuit on my breadboard and detected very, very low current of the VAC which I need to prove will be reliable.

This non-contact circuit will not have to be FCC Certified. My lawyer states, "If its connect to the home owners line in side their home and does not make an actcual connection to the phone copper wire. A home owner has a right to do that without any type of license or certification". Its the same is if I laid a screw driver actually against my phone line the laws of nature allow the screwdriver to affect the EMF generated by the phone company; not the screwdriver.

So, non-contact is what I am trying to advance from the above "AC ONLY DECTEDED" to both AC & DC where people say I can't do it but, I have. Detected with no phone ring and line measures NO VAC only -48VDC.

Any ideas how to proceed or addition circuits would be helpful. Thanks,

Bobby
 
I think getting an inductively picked up AC signal is easy. Just like a transformer.
Trying to get a reliible inductively picked up DC signal will be almost impossible, transformers don't work with DC.

The readings you are seeing on your breadboard are just stray stuff you are seeing because of your wires.
These stray readings can not be counted on for readings, you may never see them again once you go to a PCB.
 
I agree with 4pyros. You can pickup the AC inductively but not the DC. He is right, any DC readings you are getting are from poor insulation. Equipment attached to telephone lines has to be rated for isolation(breakdown voltages) to be certified. This is because they are subject to high voltage events, like lightning strikes! You will find within a phone an isolation transformer and an isolation area on the PCB rated for these voltages. It isolates the user from the potentially fatal high voltage events, at least in theory.
 
Not real sure what your trying to accomplish here, but do some searches on Telephone DAA (Data Access Arrangeement). e.g. **broken link removed**

So, for the most part -48 is on the line when it is ON HOCK and about 5V is on the line when OFF HOOK. Ring is about 90 VAC at 20 Hz or so. Not 100% sure of the actual specs. Loop current is about 20 mA when off hook. Meaning, the best way to get two phones to talk to one another unaided by the phone company and no signaling use a currnt source. You have to assume that the ring signal can be present when on and off hook, I believe.

Another useful piece of information is the call progress tones. https://en.wikipedia.org/wiki/Call-progress_tone

And caller ID is passed between the 1st and second ring.
 
Hi KeepItSimpleStupid, I agree with moffy & 4pyros about the DC not being detected by EMF induction methods.

What I am trying to do is create a non-contact EMF inductive pickup "Trip Circuit" if you will that I can determine "OFF HOOK" and "RING".

Your specs are very close as you state for the phone line specs only thing being is a normal 35mA current draw by a telephone but, could be dropped by Central Office to 20mA but, never more than 70mA.

It appears what I want to do is very much impossible.

thanks for your comment.
 
You could use a saturable core transformer. It will saturate (if you design it) if say 20ma current flows through it. It is non contact, and can maintain isolation. You detect whether the core has saturated and that tells you if a current is flowing. You already have the ring detector worked out.
 
Capacitive and inductive coupling infer changing electric fields. DC just doesn't change. A Neon bulb may be able to pick up a ring signal.
 
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