This could be a solution kicking a bunch of electronics out of the
device. But it requires some modifications. See below...
I'd prefer the sawtooth generated by the NE567, because it has a lower
harmonic content. But, if I find a source of these, I'll consider
them.
Note that this is a hobbyist's project, so I need to buy in small
quantities.
I think you misunderstood the design. The tone is intended to unmute
the speaker, so that the real call can be done by voice. That's why
the call tone needs to be ultrasonic.
Here is the schematic (my webserver is down at the moment, so I had to
use imageshack)
http://img253.imageshack.us/my.php?image=intercom1tb0.png
Some explanations:
The intercom is going to be used to link several flats spread over two
houses. Because people (especially some old women) live there, the
intercom needs to be as easy-to-use as possible, i.e. press button,
speak, release button, await answer.
You too can't expect these people wait for the line to become
inactive, so an interlock is included to avoid collisions on the
line.
That's why the line is raised to 2V while speaking, allowing simple
detection whether the line is in use or not.
Sometimes there will be need to call everyone, so every station will
react on a DC level on the line above 5V and unmute the speaker.
There is also support for an entry-phone. Because one can't expect
strangers to understand what a PTT is for, the PTT is moved into the
other stations.
The TAA4761 is a quad operational amplifier with high-current open-
collector output.
BTW: I'm from Germany so please be gentle with my English...
Ferdinand Meyer-Hermann