Circuit for the principle: The alarm circuit consists of two parts, the transmitter and receiver. Figure a shows the transmitter circuit, GT is a smoke break reed switch, usual
ly shed door when the magnet close to the GT, GT inside contacts open. lC1 is 555 circuits, and R3, R2, C3 constitute self-excited multivibrator, the oscillation frequency of 1 kHz. lC2 is built-in antenna type transmitting circuit T630, contains an internal pulse modulation, amplification shaping, firing several parts circuit, operating frequency is 150 kHz.