[VoIP] Asterisk and Payphones

[Martin Harriss]

Mark Rudholm wrote:
> Martin Harriss wrote:
>> Mark Rudholm wrote:
>>> Well, I'm glad to hear there is significant interest in this.
>>>
>>> For the moment, I'm working on coin-relay control.  My first
>>> draft of the schematic is available here:
>>> http://rudholm.com/coin-control.pdf (use "File, Rotate" to
>>> fix the orientation in Acrobat Reader).
>> Mark,
>>
>> Looks like there's problem in your schematic.  If you activate the 
>> return/collect line, you will turn on both the LCC120 SSR's.  This will 
>> short the 130v line.
> 
> As I drew it, the only way to short the 130v supply line would
> be if you energized one of the LCC120 SSR's and not the other,
> but that shouldn't be possible since they're energized by the
> same control line.
> 
> I'm not sure how clear it is in the PDF unless you zoom really
> far in, but the LCC120 chip is two SPST switches in one package.
> One switch is normally-open and one is normally-closed.  I gang
> those two internal switches together (by shorting pins 6 and
> 7) to turn each LCC120 into an SPDT switch.  Then I gang the two
> LCC120's together (by tying their control lines together) to make
> a DPDT relay.  So as long as they both energize and de-energize
> together, there should be no unwanted shorts.

Now I see -- I didn't realize that the LCC120's were NO + NC.  I thought 
both halves were NO.  Duh.

> Granted, there may be a few milliseconds of overlap when the
> relays are changing states, but the data sheets show the transitions
> as being about 5ms worst-case scenario, which is fine, especially
> considering that R1 limits the load to 120mA even if something
> downstream becomes a short.  That 1:1 isolation transformer is
> 130mA unit and the SSR's can all handle 170mA, so even a serious
> mistake shouldn't set any fires :)

Yes, shouldn't be a problem.

>> Also, I think I see what you're trying to do with the strobe line.  Why 
>> not just have the processor (Asterisk, or whatever) turn the line on and 
>> off to do the strobing?  You may not even need that strobe once you get 
>> the other part of the circuit sorted out.
> 
> Well, the reason for doing the timing in hardware is that I don't
> trust the PC or the software to not set the strobe line high and
> leave it there, which would leave my coin-relay energized all day,
> possibly damaging it (I don't have the relevant BSP, but I doubt the
> coin-relay would be happy with a 100% duty-cycle).  So yeah, you're
> absolutely right, I could do the timing in software, but I trust
> C2 more than untested PERL or shell script code :-)

Yes, it's nice to have some protection in hardware; what I often do for 
something like this is to write the software and test the outputs with a 
LED or a meter before connecting the real hardware, or before applying 
the 120 volts.

> As far as one control line goes, I thought about just sending the
> control voltage pulse out whenever the return/collect line changed
> states, but increases my part count, and it means that you couldn't
> do "collect, collect, collect" (which ACTS does when you're on a
> non-local call).  I suppose I could do something with a tri-state
> output that floats the line when it's "neutral" but <shrug>, adding
> that strobe line seemed easier and less uncertain.

Yes, as long as you have enough output ports available this makes the 
most sense.

> Thanks very much for the feedback.

You're welcome.  Let us all know how you make out with this!

Martin