Telephone interface questions

[BobK]

Hi Skarr,

I don't think the solid state relay would work in this type of situation. However some new devices called MOS relays have been seen in alarm communicator designs for a few years now.

I took the time to draw out the circuit you posted along with the relays you would need in your interface. It is attached to this post.

I am currently looking at actually building a receiver first using regular alarm dialers. If I'm still up to it then I will progress to my own dialer. The dialers that I have been using the past 8 years have only cost me between $50 to $75 and I can program them and remotely access with my laptop but the receiver seems more interesting now. I own a few professional receivers that cost over $10K and they can receive multiple types of alarm communications but I will focus on one type that uses DTMF tones.

Will be out here if needed.

BobK

[Skarr11]

Thanks for the schematic, BobK! I'll play around with it later, I won't be able to work on this for a few days.

[Skarr11]

I had to figure out how to interface a small SPDT relay (Radio Shack, PN 275-240) to my PIC, and was able to do a little test and turn on an LED, so I have that working. Now it's just a simple matter of hooking the relays to the the tip and ring lines.

I'm unsure of how to connect all this up to the house phone such that I can dial out with the circuit when I want to, but upon disconnecting from the circuit I regain the house line. It's not clear to me from your schematic, BobK, how I implement that with phone jacks. Any tips?

Thanks.

[BobK]

Hi Skarr,

Take an unused phone cord and cut one end off and strip the red & green wires. Connect them to your project on the incoming telephone line. When you are ready to test, plug the other end of the phone cord into a phone jack. I wouldn't worry about seizing the phone line at this point in your development.

In actuallity, to use line seizure properly you need to bring the outside incoming phone line to an RJ31X jack and then from there you bring the house phone side to the jack. The RJ31X jack has internal connections that maintain the phone line to house phones until you install the proper RJ45 plug into it. Then your equipment will have a DPDT relay that will maintain the connection until your project goes to call out.

Digest this and I'll get back to you tonight when I get back from work.

HTH,

BobK

[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?