IJAZ AHMED wrote:
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3) Some sort of authentication so that your micro knows that it is a call for remote control application, and not a normal telephone call.

IF OFF-SITE ACCESS to the control system in the home is the mechanism that Mr Gupta desires then the requirements set out by IJAZ are right on track. It is important to note however that the authentication system can get complex if Mr Gupta's gadget seizes the line before the authentication takes place. This is becasue the normal usage of the phone gets thwarted to large degree if an addon device takes the line when it was actually intended for a voice call. There are products in the market that try to solve this problem in the case of arbitration for FAX versus VOICE call but in my opinion these products fail miserably. An alternate authentication system should thus be considered. One scheme that may work if the remote access can come from a known universe of phone numbers is to put a Caller ID decoder in the gadget box. Another scheme is to use the coded ring detection in areas where a phone company feature called "distinctive ringing" is available where it is possible to have the line incoming to the house to have two or more different ringing cadences. Yet another scheme is the ringing counting scheme where by the remote control site places a call and lets the line ring a set number of times and then hangs up and the calls back a second time and lets the line ring a different set of times wherein the gadget microcontroller detectes these predetermined ring counts and seizes the line after they have occurred.

Yet another scheme is to not have the gadget seize the line immediately in the normal sense. Instead use a standard answering machine to grab the line. Then during the OGM phase (out going message) the remote phone can present DTMF which the gadget microcontroller detects and upon validation will then latch onto the line while waiting for the answer machine to disconnect. In this scheme it is highly advantageous for the gadget to be attached to the TEL side of the answering machine as opposed to the LINE side so that the answer machine will give up the line and let the gadget then maintain the line connection.

NOW ON THE OTHERHAND if Mr Gupta actually wanted to use the keypads of the phones in the house that are on the same line (and his post does not really clarify this point) then the set of "reguirements" of how to interface will be VERY different than the remote access via the telephone central office.

Michael Karas