Need help with a telephone / speakerphone design

[dmiller]

I am looking for design assistance for a circuit based on a current chipset that would include the regular feature set of a telephone / speakerphone combo. I want to be able to dial the phone from another device so I would need some i/o into the system that an external processor could send either dial tones or use the telephone processor via serial or other communication.

I am open to any reasonable relationship on the project.

[email protected]

[Chris Smith]

"I want to be able to dial the phone from another device"

Wire or Wireless?

[dmiller]

and eventually an optional part of another device

[Bob Scott]

Sounds like you want a modem. It communicates serially and has the AT command set allowing you to dial any number- pulse or tone- at any speed up to as fast as the phone company allows. Also it allows you to set the internal speaker on/off and set volume.

Or did you want something else?

Bob

[dyarker]

You didn't say which chipset, but it is likely to have 3 column pins and 4 row pins for connecting a 12 button keypad (1 2 3 ... 9 * 0 #). With 12 transistors, FETs, bilateral switches, or optoisolators (depending on polarity and voltage reference) paralleled with keypad, the chipset will get dialed digits the same as if a person used the keypad. This allows features in the chipset (like last number redial or dialed number display) to still work.

A microprocessor with at least 12 GP I/O pins and a UART, and an RS-232 level converter chip is job done.

Have the microprocessor also control a relay with the N.O. contacts parallel with the hook switch, and your "other device" has on/off hook control too!

Don't forget that the phone line must not get referenced to ground at the user end. Either use a DAA on the line side, or isolate all connections between chipset and your added control circuits.

Have fun with the project,

[Externet]

Hi.
My Motorola telecommunications data book has all the data sheets and application notes for dozen of chips that did that.
They have to be on the web too, somewhere. Now it is Freescale, and should be producing some new generation of those.
Another good search place for telephony chips is the Rohm site.
Good luck,
Miguel

[Robert Reed]

To add to Exters reply, Freescale has turned over much of the thru hole chip manufacturing to Lansdale and they are still manufacturing some of the good old "obsolete" chips. Just Google up Lansdale and you might find just what you want.

[MrAl]

Hi there,

Interesting, i was just thinking of building a speaker extension that
could amplify the phone line to a speaker. This would make waiting
for a 'representative' a bit easier cause you would not have to
hold the phone while waiting.

I guess this is different though...