"Nico Coesel" wrote in message
...

I'm pretty sure that it won't work that simple. There are other
factors at play that make controlling things with a telephone a lot
harder to achieve than you think... For everyone's safety I'm not
going into the details.

You could, only there are no details.

Using a ringer's voltage levels is indeed easy, there is really nothing to
hide because every EE student can figure out the details for himself. Let's
use an example. Since I'm not a terrorist, I assume that the circuit is used
to light a lamp as an aid for a hearing-impaired person who could than turn
on his hearing aid in case it was off, and do not encourage anyone to put
anything else in place of that lamp. To make it even less usable for certain
people, the following example relies on the ringer NOT being removed from
the circuit so it would ring (this way an additional time delay cannot be
used). The amplifying of the ringer signal can be done with only one
transistor, the voltage offsets be provided with a normal and a shottky
diode. Let's also assume that the ringer is a dynamic and not a
piezoelectric one, since with a piezo the circuit would differ slightly. A
reed relay could do the switching. Using an NPN, the circuit would look like
this: Negative ground, connected with ringer's "-", to battery "-" and
through a forward-biased shottky to the emitter. The transistor's base
connected through a 2K2 resistor to battery "+". Base also connected to
diode "+", while diode "-" is connected to ringer "+". Collector through
relay coil to battery "+", a capacitor across the relay coil. That's it. The
relay contacts can be used to switch on a lamp, connected to the same
battery and placed so that the hearing-impaired person can easily see it.
Note to hearing-impaired preople: this circuit may not always work, it
depends on the type of ringer and on the volume setting. I did not test it
with any ringers either, but I think many old-style ones should do.