ed@apricot.sybase.com (08/02/89)
I'm building a project which is essentially a microprocessor based phone answering machine. I have completed all of the basic hardware and firmware and am currently using a device that is manufactured by Cermetek (CH1813) which provides a DAA for my device. It also provides ring-detect, off-hook detect, audio coupling to the phone line and a means for forcing the line off-hook and on-hook using a TTL level signal. However, this thing is expensive and there are number of other things that I would like to do. I don't really have the experience to know how to proceed. Any tips on these things would be appreciated. Also, any tips that indicate the reasons why any of these things could not be done, or would be expensive to do would be very useful. Are there any books available whose subject is exclusively telephone electronics, and includes sample circuits etc.? 1) First off, I would like to duplicate the functions that the Cermetek device provides at a lower cost (< $26.00) 2) Let's say that I have a telephone hooked up to my phone answering device that has a ringer that cannot be turned off. How can I inhibit ringing on this phone while still being able to detect things like the phone going off-hook etc.. I would like to be able to control this with the microprocessor so ideally a good answer would include a TTL level interface. 3) I would like to be able to use the DTMF keypad of a phone that is connected to my device without causing the phone switching equipment to take notice. Is this possible? Maybe there is a sequence of tones that can be dialed to disable further interpretation? 4) How can I detect that a calling party has hung up before the telco finally disables the connection? How about when the telco finally drops the connection? 5) Let's say that I have two phone type devices hooked to my device. I would like to be able to discriminate which of the two devices has gone off hook. Ed Archibald ed@sybase.com
wiz@xroads.UUCP (Mike Carter) (08/03/89)
In article <5337@sybase.sybase.com>, ed@apricot.sybase.com writes: > > > I'm building a project which is essentially a microprocessor based phone > answering machine. I have completed all of the basic hardware and firmware > and am currently using a device that is manufactured by Cermetek (CH1813) > which provides a DAA for my device. It also provides ring-detect, off-hook > detect, audio coupling to the phone line and a means for forcing the line > off-hook and on-hook using a TTL level signal. However, this thing is > expensive and there are number of other things that I would like to do. > I don't really have the experience to know how to proceed. Any tips > on these things would be appreciated. Also, any tips that indicate the > reasons why any of these things could not be done, or would be expensive to > do would be very useful. > First off, let me remind you that there are very FEW companies that make telephone interefaces that are liscensed. Cermetek is one of them. A while ago I was contracted by a company in Atlanta GA to make a similiar device..although its uses slightly different. Registration of your device with the FCC is important unless you wish to get sued if something happens down the block to a lineman with 110VAC. I used the Cermatek CH1810. SLightly cheaper than the 1813. Detecting rings etc is accomplished by using a bridge rectifier tied accross the line. The output fed into an op-amp and into a cap. This will provide a pulse (cap not necessary) each time ringing voltage is applied to the line. WHat you do with the pulse is up to you. YOu can also observe the characteristic drop-off when the calling party disconnects. (CPC). You may not have CPC in your area, most do however. (It's a quick positive going-pulse out of the bridge). The big issue with part 68 registration is liability and ISOLATION from device to phone line. If you think for a moment Aunt Granny and Dr Snobby next door wont complain about the "BUZZING" on the phone line you've got another thing comming....mostly the dreaded PHONE COPS! (These guys are really something). I really advise you to stick with the $26.00 CH1813 . Unless you want to go through the rigamarole and hassles and $$$$$ to get the device FCC registered. Getting flogged by Phone cops brandishing 300 conductor wire isn't fun. -Mike -- ============================================================================= = Mike Carter N7GYX, Phoenix AZ| Q: Why did the Chicken cross the road ? = = hplabs!hp-sdd!crash!xroads!wiz| A: To ESCape the Main Menu . = =============================================================================