Phone latch

tmack

Member
I have been using DTMF to control stuff for a while. I am now looking for a Phone latching circuit to "answer" the phone and allow me to send my dtmf tones to pan,tilt a camera etc. and then "hang" the phone up when I hang up the other end. I have been able to find surprisingly little about how to make this on the web. I am wondering if anyone has done this before and if the Picaxe can be of any use for this application. The phone I would use would be an analog phone line. I think it goes from about 8-9v DC to about 90V ac when it is ringing. Any help would be appreciated.
 

tmack

Member
I built this http://www.aaroncake.net/circuits/pflash.asp but alls it does is make my relay vibrate the same kinda way the ringer would on the phone (If that makes sense).The relay is turning on and off superquick. I need it to make the relay click on instead of going on and off superquick like it does now. I was wondering if I put a SCR in place of the 3904 transistor would that work? I was thinking about making is so that it disconnected when I put in a DTMF tone.Grounded out or whatever. Thanks for the help.
 

leftyretro

New Member
I built this http://www.aaroncake.net/circuits/pflash.asp but alls it does is make my relay vibrate the same kinda way the ringer would on the phone (If that makes sense).The relay is turning on and off superquick. I need it to make the relay click on instead of going on and off superquick like it does now. I was wondering if I put a SCR in place of the 3904 transistor would that work? I was thinking about making is so that it disconnected when I put in a DTMF tone.Grounded out or whatever. Thanks for the help.
Well from the user feedback from the site that you listed:

"pls connect a 10uf/25 v capacitor accross the base and ground of transistor q1. so that you can avoid relay chattering and arcing of contacts."

Makes sense as the ring voltage is a 20hz one second on and two seconds off signal as I recall. So some way of filtering or timing with say a 555 timer or a picaxe is needed. It might be better if your design 'counted' rings and not respond to short one ring wrong number or noise dialed situations but rather required say 3 rings before answering the call.

Lefty
 

andrew_qld

Senior Member
Just remember that your country's regulators may take a dim view of attaching something to the phone line.

Having got that out of the way, the optocoupler design on that page should work if you just somothe out the AC. As pointed out above, a capacitor from the transistor base to ground, and I'd also put a diode in series with the transistor base.

Depending on the SCR you may need to drive that with a transistor. What sort of laod are you trying to drive with this? If its just a picaxe then you could get by with the optocoupler and a resistor straght into the picaxe.

Andrew
 

Tom2000

Senior Member
One thing you need to watch when coupling to phone line is AC balance to ground. Since the phone line is an antenna that's miles long, even the slightest imbalance will introduce lots of noise into the phone line.

One way to do your signaling while maintaining balance is to do all your signaling at the split center tap of a repeat coil. Unfortunately, the repeat coils used in the telephone industry are usually hard for us hobbyists to obtain, are expensive, and are often much larger, physically, than we'd like them to be.

You can simulate a repeat coil with a pair of cheap miniature audio transformers as I've shown on the attached sketch. The two transformers are wired as a repeat coil, allowing you to do your signaling at the split center tap on the line side, between the A & B leads, just like the big guys do.

If your transformers saturate when you close the hookswitch, add some resistance in series with the A or B lead until both the line stays off hook and the transformers stop saturating. If they don't saturate, and you get clean audio through the repeat coil when you're off hook, don't worry about the resistor.

Good luck!

Tom
 

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andrewpro

New Member
Another thing you could do is use an old modem that has either an internal speaker, or an audio out (like an old "voice modem"). this way you can avoid the pitfalls of connecting homemade equipment directly to the phone line. You would have your audio interface, control, and ring detect all done for you. All you need to do is provide some serial data to the modem to tell it to answer and hangup.

--Andy P
 

premelec

Senior Member
A very simple high impedance ring detector can be made with a neon tube which is in series with a resistor and shines on an LDR - the 80V 20Hz ring pulse will light the neon and lower the resistance of the LDR when the neon + R are connected across the telephone line... I have also seen audio circuits which just listen for the 'bell' - Velleman used to make such a kit... barking dog interference possible :)
 
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