Sometimes you have to do
it the hard way, even if doing it the easy way is an option. That is
the case here. The intention is to add a second doorbell in parallel
with the existing bell. This does not, in principle, require any
electronic components. You would simply connect the second bell to the
first one. But if the existing bell transformer is not rated for the
additional load then this is not a good idea! An option is to buy a new
and larger transformer. But bigger also means more expensive! Moreover,
replacing the existing transformer can be an awkward job, for example
when it is built into the meter box. So we follow different approach.
This circuit is connected in parallel with the existing bell. This
is possible because the current consumption is very small compared to
the load of the bell. The bridge rectifier rectifies the bell voltage
when the push-button is pressed. This will then close therelay contacts.
These contacts are the ‘electronic’ button for the second bell,which is
powered from its own cheap bell transformer.
Author: René Bosch – Copyright: Elektor July-August 2004