Telephone interface questions

[Skarr11]

Woo hoo!

I got it working. I had incorrectly wired one leg of the transformer. I dialed out to a couple of phone numbers successfully. I used the default 4 MHz oscillator.

I'm a bit of a novice at this, so you'll excuse my excessive enthusiasm over seemingly trivial successes...

I noticed that after dialing out and ending the call, I must disconnect the phone cord from my circuit and reconnect it to a real telephone. Once I confirm a dial tone, I then connect the phone cord to my circuit again and confirm that there is a dial tone in the phone. Then I reset the PIC and it dials out. I'm guessing this has something to do with on-hook/off-hook states. Would a dialing IC in the circuit permit changing the on/off hook states using the PIC? Any recommendations?

[BobK]

Hi Skarr,

Just pic'ed up on your post. Have to go to work now but will get back to you tonight on this.

I'm doing the same type of project.

BobK

[BobK]

Hi Skarr,

If you put a relay in the circuit as indicated then you wouldn't need to plug and unplug your circuit to the phone line. Your program would include a line to turn the relay on such as:

PortB.1 = 1 'turn line seizure relay on

Then when you are ready to disconnect from the phone line you would program:

PortB.1 = 0 'turn line seizure relay off

Line seizure refers to a relay that disconnects the house phones and connects the phone line to the dialer. This prevents someone from picking up the phone in an attempt to block the call. Then there is another relay with contacts in series with one side of the phone line that would be closed then the dialing would take place. When the phone call is completed, then both relays would be turned off. The phone line is returned to the house phones until the next call out.

You will need to drive the relay circuit with either a transistor or with a driver package like the ULN2003 or ULN2004.

I have been in the alarm business for a long time and I am quite familiar with dialers and their circuitry and inner workings. I finally decided I wanted to design and build one of my own just for the heck of it. I am in the beginnings of the design phase and will be happy to share ideas with you on this. I've been getting other work related projects out of the way so I can focus more on the dialer project.

HTH,
BobK

[Skarr11]

Thanks for the response, BobK. I appreciate your assistance.

I do have a few solid state relays but I'm not sure if they'll work in this application. I think I have some schematics for hooking this up; I'll stumble through and if I encounter a problem I may ask for some help...

As far as my project goes, I'm helping an aunt with a resuce/service dog. In order to get certified the dog has to perform higher-order functions like dialing a phone in an emergency situation. So essentially it's a dialer circuit with pre-programmed numbers, a large button or two to initiate the call, a small speaker, maybe a small LCD and a microphone, all packaged in an enclosure.

There are special phones that can be purchased, but at $300, I found that rather offensive and agreed to build something homebrew, convinced that this type of device could be made for well under $50 in parts. So I'm progressing slowly through this, learning as I go.

Next up is connecting the relays, and adding the switches and microphone. That should keep me busy for a little while.

What are you building?

[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.