This Auto Anti-Hijack Alarm Circuit Diagram was designed primarily for the situation where a
hijacker forces the driver from the vehicle. If a door is opened while
the ignition is switched on - the circuit will trip. After a few minutes
delay - when the thief is at a safe distance - the Siren will sound.
Where it differs from the first two alarms - is in what happens next.
I'm obliged to Victor Montanez from the USA who suggested that the
engine cut-out should not operate - until the vehicle comes to a stop.
That way - the engine will not fail suddenly or unexpectedly. And the
hijacker will retain control.
I haven't been able to implement Victor's excellent suggestion
completely - because I couldn't think of a simple, reliable and
universally applicable way of sensing when the vehicle has come to a
stop.
Instead - I have postponed engine failure until the ignition is
switched off. Once the thief turns off the ignition - the engine will
not re-start. Clearly - there is no certainty as to when this will
occur. But I think it will occur sooner rather than later. Because
there's a strong possibility that the hijacker will turn off the
ignition - in an attempt to silence the siren.
Auto Anti-Hijack Alarm Circuit Diagram
As well as acting as a Hijack Alarm - this circuit offers some added
protection. Like the Enhanced Hijack Alarm - it incorporates Jeff Chia's
suggestion. That is - every time the ignition is switched on - the
alarm will trip. So it will protect the vehicle whenever you leave it
unattended with the ignition switched off - even overnight in your
driveway.
Importance
Before fitting this or any other engine cut-out to your vehicle -
carefully consider both the safety implications of its possible failure -
and the legal consequences of installing a device that could cause an
accident. If you decide to proceed - you will need to use the highest
standards of materials and workmanship.
Notes
You're going to trip this alarm unintentionally. When you do - the
LED will light and the Buzzer will give a short beep. The length of the
beep is determined by C4. Its purpose is to alert you to the need to
push the reset button. When you push the button - the LED will
switch-off. Its purpose is to reassure you that the alarm has in fact
reset.
If the reset button is not pressed then - about 3 minutes later -
both the Siren and the Buzzer will sound continuously. The length of the
delay is set by R8 & C5. For extra effect - fit a second siren
inside the vehicle. With enough noise going on - you may feel that it's
unnecessary to fit the engine cut-out. In which case - you can leave out
C7, D8, R12, R13, Ty1 & Ry2.
When the ignition is switched on - C3 & R4 are responsible for
tripping the alarm. By taking pin 1 low momentarily - they simulate the
opening of a door. If you don't want the alarm to trip every time you
turn on the ignition - simply leave out C3 & R4.
Because the voltage on C3 may be reversed - the capacitor needs to be
non-polarized. But connecting two regular 22uF capacitors back to back
as shown - will work just as well. Because non-polarized capacitors are
not widely available - the prototype was built using two polarized
capacitors.
To reset the circuit you must - EITHER turn off the ignition - OR
close all of the doors - before you press the reset button. While BOTH
the ignition is on - AND a door remains open - the circuit will NOT
reset.
The reset button carries virtually no current - so any small
normally-open switch will do. Eric Vandel from Canada suggests using a
reed-switch hidden behind (say) the dash - and operated by a magnet. I
think this is an excellent idea. As Eric said in his email: - "... that
should keep any thief guessing for a while."
Veroboard Layout
How you prevent the engine from starting is up to you. It should
happen when Ry2 de-energizes. The contacts of Ry2 are too small to do
the job themselves. So use them to switch the coil of a larger relay.
Remember that the relay must be suitable for the current it's required
to carry. Choose one specifically designed for automobiles - it will be
protected against the elements - and will give the best long-term
reliability. You don't want it to let you down on a cold wet night - or
worse still - in fast moving traffic!!! Remember also that you must fit a
1N4001 diode across YOUR relay's coil - to prevent damage to the Cmos
IC
YOUR relay should drop-out when Ry2 de-energizes. Wire YOUR relay so
that when it drops-out the engine will not start. Because turning-off
the ignition will cause both Ry2 and YOUR relay to de-energize - the
standby current will be low - and the engine will be disabled while the
vehicle is parked.
The circuit board must be protected from the elements. Dampness or
condensation will cause malfunction. Fit a 1-amp in-line fuse AS CLOSE
AS POSSIBLE to your power source. This is VERY IMPORTANT. The fuse is
there to protect the wiring - not the components on the circuit board.
Please note that I am UNABLE to help any further with either the choice
of a suitable relay - or with advice on installation.
Both the Siren and the Buzzer will go on sounding until the alarm is
reset. The circuit is designed to use an electronic Siren drawing up to
about 500mA. It's not usually a good idea to use the vehicle's own Horn
because it can be easily located and disconnected. However, if you
choose to use the Horn, remember that Ry1 is too small to carry the
necessary current. Connect the coil of a suitably rated relay to the
"Siren" output. This can then be used to sound the Horn.
