Abstract: An alarm device responsive to entering or trespassing comprising:(a) a sensor circuit, including an infrared ray sensor, producing an output having positive and negative peaks based on outputs of said sensor being produced when a target to be sensed passes within the region of vision monitored by said sensor,(b) a level detecting circuit comprising a first detector producing an output when the positive peak of the output fed from said sensor circuit exceeds a predetermined level, and a second detector producing an output when the negative peak of the output fed from said sensor circuit exceeds a predetermined level,(c) a timer circuit comprising a first timer producing an output which continues for a predetermined time interval or above from a time at which the output of said first detector is produced, and a second timer producing an output which continues for a predetermined time or above from a time at which the output of said second detector is produced,(d) an AND circuit comprising a first circuit produc