This alarm circuit is very simple, if triggered once, will keep on sounding until either the battery is drained off or the power is cut-off. Pictures circuit below shown an arrangement which shuts off the alarm after about 50 seconds.
The circuit normally sits at a HIGH level. So long as the input from alarm circuit is HIGH, the output of gates 2 and 3 are also HIGH and capacitor C1 is fully charged. A LOW input to gate 1 makes output of gates 2 and 3 also LOW. The HIGH output of gate 4 sounds an alarm when applied to circuit below.
The LOW output of gate 3 discharges C1 through R1 and after about 20-25 seconds, C1 has discharged sufficiently to make inputof gate 2 LOW and restore the original state. The LOW output of gate 4 shuts off the alarm after sounding for 20-25 seconds.