Telephone interface questions

[BobK]

Hi Skarr,

Look over the attachment and see if you understand the phone line connections as I have them shown. I put some additional circuitry on the drawing I got from MeLabs'LAB-XT schematic. Let me know if I can help anymore. Also let me know if you are interested in buying a used medical alert system. The kind with the large button on the front!

BobK

[Skarr11]

Hey BobK, thanks a lot for the updated schematic. I understand the sequence you've described and the schematic now.

I constructed your updated circuit (minus the "dial" relay) and tested a little circuit by switching two relays to light two LEDs to make sure I understood the connections and wrote the proper code (per your sequence), and that worked. However, when I connected the phone line to the circuit and executed the programs, I couldn't dial out and the relays just switched back and forth.

I suspect there a grounding issue going on here and that the PIC is resetting itself somehow, but I have no idea how to troubleshoot this. And this only happens when the phone line is physically connected to the circuit; disconnect the line from the circuit jack and the relays work. Any advice?

[Skarr11]

After doing some research I learned that I need bypass capacitors in my circuit: one across the PIC's Vss and Vdd pins, and one across the coil of each relay. I added 0.1 uF caps in these locations as well as across the + and - busses of my breadboard. Adding these caps reduced the erratic behavior of the relays but not completely, and I still can't dial out. The circuit works fine if the phone line is not connected to the circuit. Any thoughts?

[Ioannis]

What kind of power supply are you using for the PIC®?

Does this P.S. have an earth connection?

Is it Switching Power Supply?

Is it the current supplied enough when relays are ON?

Do all relays have Diodes for the back EMF?

Try to have all ground points to one specific point to avoid ground loops.

Are you sure that your Isolation transformer does indeed isolate the Line from the circuit?

Post your latest code. May be something is wrong with the code. Ooh, and what about the MCLR of the specific chip? Is it connected to +Vdd? Check its level.

And by the way, tere is no need to have 2 relays. One can serve as Dial and Hook at the same time.

Ioannis

[Skarr11]

Thanks, Ioannis.

I have a bench-top DC power supply (EZ Digital Co), grounded, not switching. Current is sufficient to switch relays as far as I can tell. All relays have protection diodes across the coils. MCLR is connected to Vdd. The circuit is essentially that shown in BobK's latest post.

Not sure how to determine if the isolation transformer is isolating the line from the circuit. I did have this circuit working and dialing prior to adding the relays, if that is of any help.

I think you may have hit on the grounding problem. My circuit is wired up all over the place on the breadboard. I'm going to rebuild it in a more sensible way and see if that helps. I'll post back when I'm done.

[Skarr11]

I finally have some time to get back to this...

I rebuilt the circuit in a cleaner way so that all the grounds are tied to one rail on the prototyping board, and that seems to have stopped the erratic behavior I was seeing before. I still have bypass caps on both rails of the board and across the relay coil. I have attached a schematic of the circuit I have built so far.

I can *kinda* get this working with my latest program. When the phone line is plugged in to a phone jack on my board, I switch the relay to connect the red line and dial the number. After a few seconds, I switch the relay back. I was expecting to have the phone line disconnected, but instead, the call continues. I have to disconnect the phone and then remove the phone line from the board in order to regain a dial tone. Not exactly what I was looking for...

I'm struggling with how to connect the relays to seize and release the phone line. I was looking at the LABXT board schematic and its line seize setup, also attached. It looks like there is a single DPDT relay used to switch the line, with the red line connected to a common pin on the relay. Couldn't that be an SPDT relay or a reed switch instead? What, if any, connections are made to the remaining contacts?

What I'm also unclear about is how that circuit actually releases the line. I see that when the coil is energized the connection is made, presumably giving you a dial tone, but what happens when the coil is not energized? It seems to me that the phone line would not be released.

Thanks for helping and suffering a fool.

[Dave]

Skarr11, Connect the relay common to the red line (other phone wire).

Dave Purola,
N8NTA