This is simple door alarm circuit.You can attach this circuit for your for your main door If somebody opens it alarm will activate.Sw1 is a Read switch.Those switches are operated with magnets.
Parts:
Q1 = BC547
Q2 = BC327
B1 = 3V Battery
R1 = 330K
R2 = 100R
C1 = 10nF-63V
C2 = 100uF-25V
SW1 = Read Switch
Note
# Don't supply more than 4.5V for this circuit
#Attach a magnet for the door frame near by the read switch