m-e Bell-556 TX Betriebs- Und Montageanleitung Seite 21

CONNECTION (SIGNAL VOLTAGE)
The radio converter can be activated with a signal voltage which must not be
below 3V or above 12V. It can be activated with direct and alternating
voltage.
The signal voltage is connected to the two red connection lines (1 R), no
polarity is required. This way it is possible to connect the radio converter in
parallel to a wired gong, as shown in this example:
Trafo
8V AC
Bell push button
Note: Please insulate the black wire so that it does not cause a short circuit.
Note: The radio converter cannot be connected to a purely battery-operated
gong (without transformer). In this case, the existing chime must be removed
and the radio converter connected as described in the section "Connection
(potential-free)".
To Converter
Gong
21
Bell-556 TX
(1 R+R)
Red
Red
GB
GB