hi everyone, good evening. I am designing a circuit that allows the user to call in from anywhere and control appliances remotely using the phone. There are several questions which i hope you will be able to enlighten me. Most telephone/modem circuit uses a 1:1 dry transformer for impedance matching purposes. These are also call 600:600 ohm transformer. Does that means that as long as this transformer is plug-in, then it will provide a 600 ohms impedance to the telephone line? If this is not true then that means external components is needed and basically the transformer can then be 700:700, 800:800, etc ? I am using ETAL P3191 and the datasheet provides the matching recommendations _____________ -----600 ohm-----o------- 6.8 uF -----| |-------|----- | | | | | | Transformer | | Line Source | | 430 ohm | | | | | | | | ----------------o---------------------|_____________|-------|----- Must the cap value be follow completely? 6.8uF at 250V is very expensive. I saw some design and found that every one is using different values. Are there any calculations involve? if i plan to use a 0.68uF,is it ok? In addition, to generate the off hook condition, i have to put a resistor across the phone line ( before the cap ) of around 500 ohm. I have tested and found that any value in between 0 ohm and 1.5 Kohm can do the job. Does this method of generating off hook condition affect the impedance? I connected a DTMF decoder encoder ( MC145740 ) at the transformer secondary and presently, i am not able to detect any valid DTMF from the IC.In your opinion what would be the error over here? ( I uses a relay to do the switching to on hook and off hook circuitry ) Thanks a lot. _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads