At 12:54 AM 12/13/01 -0600, Tim McDonough wrote:
>Has anyone been involved with circuits to take a phone line off
>hook and then dial a number on command from the micro? In
>general, the parts I'm still researching are:

Been there, done that, in spades.

>- Detect if the phone line is already in use without disturbing
>an in-progress call

Can be done by detecting the line voltage, using a sampling technique.
Not 100% reliable, short CO loops and high resistance equipment cause false
"free line" indications unless you add multiple threshold selection

>- Go off-hook and dial a number (suggestions for the tone
>generation part?)

Easy, Holtek chips, or PWM it.

>The device/phone line connection should be a pre-approved DAA. We
>don't want to jump through all those hoops ourselves unless
>there's a big cost savings at the end.

There is, if you're doing more than a few.
The DAA guys promote the "scaaaary part 68" bit, but it's not that bad.

>Low cost is very important. I'm trying to avoid having to
>purchase any of the available OEM modem modules or the stand
>alone dialers that I've found so far.

See above

>A modem built up from the Silicon Labs Si2400 chipset runs around
>$21-$27. The Cermetek modem modules are around $35. We don't
>really need a true modem for the information we need to send. It
>seems like it should be possible to do a much simpler dialer for
>half to two-thirds of those costs.

You can also send DTMF faster than you think, but I do this sort of stuff
for a living.

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