SIGNAL DETECTION CIRCUIT

To accurately demodulate a modulation signal.SOLUTION: A signal detection circuit 10 includes: a comparator 13 that outputs a comparison signal based on difference between voltage based on voltage of a communication line Lc related to communication of fire notification equipment and reference voltage; and a reference voltage generation unit 14 that inputs first reference voltage or second reference voltage in a level lower than the first reference voltage as reference voltage to the comparator 13 after detecting a frequency modulated communication signal flowing through the communication line Lc on the basis of the comparison signal.SELECTED DRAWING: Figure 2 【課題】変調信号を精度良く復調する。【解決手段】信号検出回路１０は、火災報知設備の通信に係る通信線Ｌｃの電圧に基づく電圧と、基準電圧との差に基づく比較信号を出力するコンパレータ１３と、比較信号に基づいて周波数変調された通信信号が通信線Ｌｃに流れていることを検出した後に、第１基準電圧又は第１基準電圧よりも低レベルの第２基準電圧を基準電圧としてコンパレータ１３に入力する基準電圧発生部１４と、を有する。【選択図】図２