Someone suggested watching for a short reversed-voltage pulse when you connect to the other party, that or a short time (1/2 second?) after you finish dialing would be good times to start the timer. Stop the timer when the telephone is hung up, and call it good, is about all I can suggest (Unless you can get the telephone company there to suggest something better, I get the impression that's the best we can figure here.) Part of the reason is that, here in the US, we're not charged for local calls, just LD calls. It doesn't take THAT long to dial, so if you just "charge" your brother for the time including dialing time, it'll motivate him to dial fast? I'm sure he'll hang up quickly after the conversation is finished... (It's a complex situation - you cannot listen for the last dial tone in a X second window, as it's possible to spend an hour on the phone playing "touch tone tunes" - if LD it'd all be charged time. And you can mix "Tone" and "Pulse" dialing and some (not necessarily all) central switch offices will accept that mix (One friend does something like this, his box stops someone from dialing a pulse {for 1}, followed by area code & number if you have LD dialing disabled. Some people get sneaky...) Mark Javier wrote: > > 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.