Automatic phone dialler - how to detect pickup?

[Mr RB]

Hi, I am contemplating making a auto phone dialler for standard landline telephones. I can do the DTMF generation in software using a PIC to dial the outgoing number, and can do the off hook control easy enough with a transistor to pull the line low.

Does anyone have experience building one of these, specifically regarding detecting that the person receiving the call has picked up the phone?

All I can think of is to detect the ringback tone as the phone is ringing, then wait for an absense of ringback tone for X seconds. It's not a pretty solution as the PIC will have to detect and interpret the sound, with the ring pauses, and when the phone stops ringing it will most likely be replaced by other sounds like the person talking.

Is there something I'm missing that would be an easier way to detect the person has answered? Any changes in line voltage or other easy way to detect?

I appreciate any help.

 

[audioguru]

A phone line in most countries gives a pulse when the distant end answers the call.

 

[Rexlan]

What you're missing is that it is illegal and you obviously don't know anything about interfacing to the network.

You will have a lot of difficulty generating proper DTMF with the twist and you certainly do not want to "pull the line low with your transistor".

Your POT's line will be off PDQ.

 

[KeepItSimpleStupid]

Search for "call progress monitor" or "call progress indicator" or "call progress tones"

 

[Mr RB]

Thank you Audioguru and Keepitsimplestupid for the suggestions to research. I will check into that.

To Rexlan, I think you may have mistaken me for a noob hobbyist. I have already developed PIC code for generating extremely accurate xtal locked DTMF sine frequencies and can easily be twist compensated, see Zero-error 1 second timing algorithm. Secondly, most cheap answering machines use a transistor to pull the line low and take it off hook, it's a very common system used in commercial products. Thirdly, there is nothing illegal about researching and developing a potential new phone product.

Please speak up if you have anything of value to add, your reply seems to indicate that you have some experience in this field and I would much appreciate any input you can provide.

Thanks to all.

(edit) OK, I've read up on the "call progress tones" it was mainly stuff I had already seen. This is still looking like a worst case scenario where the PIC will need to recognise the ringback tone and detect when it ends when the person picks up.

Audioguru, can you provide more info on the pulse you said was provided when the person picks up? We get a "click" sometimes here in Australia when they pick up but I thought it was more a switching artifact than a deliberate and reliable system to detect pick up?

 

[audioguru]

The phone system in North America was designed by Bell and Northern Telecom. I worked in the PBX business years ago so I can't remember the details but the line has a pulse caused by momentary interruption of the 50V battery or by momentary voltage reversal when the distant line is answered. The voltage is not detected, the line current change is detected.
But it cannot be guaranteed for a long distance call to a different kind of telephone system that might not give the pulse.

It was a challenge for a PBX to determine the exact cost of a long distance call since the moment the call was answered was important to begin the timer. I think an answering machine also detects the pulse when the line calling line is disconnected.

 

[Rexlan]

Thank you Audioguru and Keepitsimplestupid for the suggestions to research. I will check into that.

To Rexlan, I think you may have mistaken me for a noob hobbyist. I have already developed PIC code for generating extremely accurate xtal locked DTMF sine frequencies and can easily be twist compensated, see Zero-error 1 second timing algorithm. Secondly, most cheap answering machines use a transistor to pull the line low and take it off hook, it's a very common system used in commercial products. Thirdly, there is nothing illegal about researching and developing a potential new phone product.

Please speak up if you have anything of value to add, your reply seems to indicate that you have some experience in this field and I would much appreciate any input you can provide.

Thanks to all.

(edit) OK, I've read up on the "call progress tones" it was mainly stuff I had already seen. This is still looking like a worst case scenario where the PIC will need to recognise the ringback tone and detect when it ends when the person picks up.

Audioguru, can you provide more info on the pulse you said was provided when the person picks up? We get a "click" sometimes here in Australia when they pick up but I thought it was more a switching artifact than a deliberate and reliable system to detect pick up?

You did not indicate you were "researching and developing" ... as unlikely as it may be.

The phone line is NEVER just pulled line low to go off-hook. There are strict requirements to interface to the PSTN.

There are several call progress chips on the market and we used them for years. A good DTMF transceiver chip will do the entire job in one tidy package ... but you will still need to take the phone off-hook properly and decouple the DC to your audio.

Detecting a missed ring is also quite easy and an excellent indication that the called party has gone off-hook. Teloc standards are universal in the US and signal timing is very reliable.

 

[Mr RB]

Audioguru- Thanks! That sounds like a system that can't be relied on as more and more exchanges are going to digital and to 3rd party service suppliers. Come to think of it, when people answer the phone now I haven't heard that old "click" in years...

Rexlan-

You did not indicate you were "researching and developing" ... as unlikely as it may be.

My first sentence said "I am contemplating making an auto phone dialler". That's pretty plain. As for calling it unlikely, that sounds like you are calling me a liar. I'll put that down to you knowing nothing about me and the fact that I generally don't discuss my commercial ventures much on this hobby forum.

The phone line is NEVER just pulled line low to go off-hook. There are strict requirements to interface to the PSTN.

I should have clarified that I would use a transistor to pull the line low and a resistor to keep it in spec, as done in most cheap phone devices. Please let's not get too pedantic, the point I was attempting to communicate was that this is a minimum parts low cost design being contemplated.

I'm aware of the DTMF transceiver chips, but the device must be small and cheap and a DTMF chip will not be used as the PIC microcontroller can already do the majority of the tasks including generating the outgoing DTMF. I'm a PIC guy so I never use a specialty chip that's hard to source, high MOQs, limited supply options etc when I can do all the tasks with the main PIC.

Thanks for the info on detecting the cessation of the ringback tone, that was always going to be my solution to fall back to if nothing clever could be found. Which is starting to look like the case!