The circuit described here is an intrusion alarm particularily suited for an automobile but could be used for home, safe storage, restricted area etc.
The alarm is triggered by opening the car door.
Approximately 10 seconds later the siren will sound; unless the ignition key has been inserted and turned which will then disable the circuit.
Additionally, after withdrawing the ignition key (driver leaving vehicle) the circuit will self-arm in about 2 minutes.
Gives the car occupants time to gather up their things and exit the car.
The door switches (all paralelled) are used for circuit trigger, the dome/courtesy lamp now enabled on by a transitor in the electronics.
Most commercial car alarms require the user to arm them with a keyring transmitter (needs a battery sometimes) and all of the attendent turn signal lamp flashings and horn beeps etc.
An experience I had with a commercially build auto-alarm, was that:
1) The electronics was mounted inside the siren-horn enclosure and got an ultrasonic/sonic blast every time it was set/reset, resulting in fractures in some solder joints - necessitating repair.

2) The presence of moisture condensation inside the electronics (cold night etc)
i.e. electrical leakage on the PCB, would cause the alarm to reset!
Hence it was not much use as an ALARM !!!
and in fact the late night indigenous Australians took note of this when no visible flashing LED could be seen - and took my car for a joyride.
Fortunately I got it back with minimal damage. Usually stolen cars are set on fire once the fuel runs out !
Circuit assembly is mounted under the vehicles dashboard.
This circuit is self arming (one less thing for you to remember).
A 555 1Hz oscillator flashes a Led inside the car.

Circuit Operation.

When S1 is opened C1 begins to discharge through R1 and R2.
When the voltage on C1 falls below the threshold voltage of gate 2 then the flip-flop consisting of gates 1 and 2 is set.
This holds the input (pin 13) of gate 4a high, which means that when one of the door switches (represented by S2) closes, the flip-flop consisting of gates 3a and 4a is set.
The alarm is triggered, and even closing the door (opening S2) will not reset the flip-flop.
C2 was previously charged through R6 and D3 from the output of gate 4a.
When C2 has discharged the output of gate 2b goes high.
This takes pin 6 of gate 1b high. Since the other input is held high by R13 the output goes low,turning on T1 and T2 and sounding the siren.
C8 charges through R14 and R15 in about 3 seconds, taking pin 13 of gate 3b high.
Meanwhile C6 slowly charges through R11 and it is this time constant that determines the duration of the initial blast of the siren (about 30 seconds).
When the voltage on C6 exceeds the threshold voltage of gate 3b, the output of this gate goes low, grounding pin 5 of gate 1b through C7 and R12.
The output of 1b thus goes high and the siren switches off.
C8 now discharges through D7, R14 and the output of gate 1b.
C7 slowly charges through R13 and R12 and this time constant determines the 'off' period of the siren (about 10 seconds).
When C7 has charged to the threshold of gate 1b the output goes low and the siren again sounds. C8 charges through R14 and R15, and this time constant determines the subseguent 'on' periods of the siren (about 3 seconds).
After this period the output of 3b goes low, grounding pin 5 of gate 1b through R12 and C7 and the whole cycle repeats.
Gates 1b and 3b thus form an asymmetric multivibrator which causes the siren to produce short blasts at 10 second intervals.
In adddition each time the siren is switched off a differentiating network consisting of R8, R9, C4 and C5 feeds a reset pulse to pin 9 of gate 3a so that if the doors are closed during the siren 'off' period the siren will not sound again and the alarm will be reset.
I have constructed several alarms to this design and find they work really well !

REF : Elektor book 75 page 98 "Thief suppression in Cars"