| 摘要 |
A nullification device of the invention comprises a nullifying signal generator 15 for producing a nullifying signal, and a controller 14 for measuring a predetermined period of time and on/off-controlling a mode change switch 16 provided between the signal generator 15 and a transmitting antenna 18. On the other hand, an activation device comprises a generator 23 for producing an activating signal for the nullification device. The activation device and the nullification device have connection terminals a, b and a', b', respectively, where the devices are in contact with each other, with the nullification device placed in a accommodating recess in the activation device. The controller 14 turns on the switch 16 in response to the activating signal, starts to measure time upon cessation of supply of the activating signal, and turns off the switch 16 upon lapse of the specified time period, whereby the nullification device for forcibly ceasing the generation of an alarm by an alarm unit is prevented from being used wrongfully.
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