Thanks to everybody for answering, but I don4t want to detect or calculate the time a telephone is off hook, I want to know the time I4m conversating (how much is the company billing me !!!) It4s not that easy, is it?? Thanks in advance Javier -----Mensaje original----- De: dave vanhorn Para: PICLIST@MITVMA.MIT.EDU Fecha: Sabado 6 de Febrero de 1999 00:55 a.m. Asunto: Re: Telephone use & time calculation >At 02:22 PM 2/6/99 +1000, Paul B. Webster VK2BZC wrote: >>dave vanhorn wrote: >> >>> You can't reliably detect the conversation itself, but you can measure >>> the current into the phone, and therefore the amount of time it's been >>> offhook. >> >> Your device is in parallel with the line. It detects when the phone >>is picked up by the drop in the line voltage rather than current per se, >>and detects when the phone is hung up by th eline voltage going back to >>(about) 50. > >This is trickier than it seems. I designed this into a line of credit card >verification terminals. >Basically, you have to have at least two overlapping thresholds to cover >all the variables out there. >Detecting the series current into the instrument is much more reliable. >You'll get at least 20mA, and maybe as much as 100mA when the phone is off >hook. Telco equipment is designed around current, not voltage. I made the >voltage detect work, but it took a bit more work than it first looks like. > >For "your phone" you can reliably set a threshold, but I've seen this >number of off-hook voltage vary from 6V or so all the way up to over 24V. >CO voltage (on hook) is typically 48V, but I've seen as high as 56V and as >low as 12V (YES from the telco CO not a PBX!) You have to be careful >checking the voltage. Whatever you measure it with has to conduct very >little current. <5uA is the spec, I did a pulsed measurement that falls >between the cracks of the FCC part 68 spec, sampling the line at about 1mA >but only for a few mS on every sample. You also have to guard against line >voltage plus peak ring voltage, plus any transients. > >For the series current method, a pair of optoisolators will get you there. >Put the LEDs in paralell, opposite polarity, with a 10uF non polar cap in >paralell with both. > >------cap------ >+----A K---+ >+----K A----+ > >Now one or the other opto will trip if the phone is off hook. They will >alternately pulse during ring, which will let you detect ring (incoming >call) or outgoing call. > > >> But you can probably figure it near enough. If the "call" after >>dialling lasts longer than the ringing limit time, it's almost certainly >>been answered for starters. > >Or they're servicing the lines, happens infrequently, without notice, >typically late at night when they think you won't notice. They will drop >your line voltage for a while (usually to zero, but not always) >or when they test your line with an MTU installed (Maintainance Terminating >Unit) They drop the voltage below 12V, and the MTU disconnects all your >in-house wiring from the line, then they can measure their lines fault >currents to ground, and resistance.