volts reverse bias a diode d5 which is connected between terminal k2 and the battery b now when the telephone is hung up the external voltage of 12 volt is turned off and the diode d5 does not block any longer so that the battery b can provide a current energizing the strobe light s the optical sensor q3 detects the flashes of light from the strobe light s when a flash is detected a 12 volt signal in case of an external energy supply or a battery voltage signal which can be 9 volt in case the unit is operating on the battery is transmitted via pin 2 to the alarm system where it can be used to indicate that the strobe light s is operated for example by driving a light emitting diode since the flash from the strobe light s is extremely brief a capacitor c1 is used to lengthen the operating time of the light emitting diode a pin 3 is provided for resetting the control unit if the strobe light signals are no longer needed a suitable signal is sent via pin 3 that turns on a transistor q1 which then turns transistor q2 off the terminals k2 and k3 of the relay k are then open so that the strobe light s is switched off when the control unit is operating on the battery it will continue to operate even if the connecting cable to the alarm system is severed and even if all of the conductors were to be shorted together it is only the information of the optical sensor q3 which will be lost and it will no longer be.