Telephone Monitoring

Hi,

I want to monitor telephone calls made by users so i could track who made what calls...I created a circuit whereby each user must enter a pin before using the phone. This works good but i am using the dial tone from the CO for them to enter their pin so what i am really doing is sending those four digits that consists of their pin to the central office each time. Once their pin matches what is stored in memory, i display "pin ok" and a relay breaks the telphone line and then reconnects giving the user the option to dial a number since they hear a dial tone once again from the CO. If the pin is invalid the user must re-enter a valid pin when i trip the line.

How do i mimic a dial tone so the user can enter his/her pin and i wouldn't have to keep sending this information to the exchange each time because someone told me that i am dropping the resstance at the exchange each time i do that and i could cause damage to their equipment. Is this true?...Would a mimic tone solve this problem? I need to create a mimic tone for the users to enter their pin in any case. Any help on how to do this please.

I can't imagine how dropping the resistance (which is a normal telephone function) and sending the 4 digits to the exchange each time could damage the exchange.

I would suggest just keep doing it as you are, if it seems to work properly.

Well that is what i said to myself as well because it works fine to my knowledge. However, my lecturer who is my supervisor for this project wants me to mimic a dial tone to enter their user pins instead of using the tone from the central office each time...How to do this?...I'm searching for a chip that does that but is unsuccessfull thus far.

I Googled "dial tone generator chip" and came up with this https://www.google.com/url?sa=t&sou...27jxAQ&usg=AFQjCNHKDCTTghTbN8oN0B4TKY4N9IjpbA from Maxim.

Tone generation

thanks i will take a look at it and try to understand how to implement it to generate the tones. If you understand it already please post it.

Regards.

I haven't used it but if, after reading the data sheet, you have specific questions about its operation, I will try to answer.

Edit: Here's a circuit that uses op amps to generate the tone:**broken link removed**.

Ok thanks.