Re: proposed anti-theft device

[Jason Brunberg <stoopid@idirect.com>]

Hey all, check this one out. I found this a couple of months ago and
have yet to attempt installation in my car as yet...hopefully as she
hibernates in my garage this winter....

jason
the mad bastard



Here is a description of the circuit that I have installed in my Celica
to
disable the ignition ("ignition kill switch").  I'm not too good at
ASCII
art so I will try to describe the circuit instead.

The operation is as follows:

1. Turn the key to the ignition position.
2. Place a small magnet on a certain location of the dash board to
enable a relay.
3. Once enabled, the magnet can be removed.
4. Turn the key to start the engine.


The magnet activates a hidden reed switch that is located behind the
dash
board.  It could also be located in the base of the drivers seat.

Disabling the starter is the least effective method.  Disabling the
ignition 
or EFI computer will allow the engine to turn over but not start. 
Disabling 
the fuel pump will allow carbureted engines to start and run for a
minute or 
so until the fuel in the carb runs out.  On my Celica with dual Weber
DCOEs, 
I can actually drive quite a while before this happens.  On EFI engines,
the 
engine *may* start and run for a few seconds, until the fuel in the fuel
rail loses pressure.

Parts Required (total cost approx. $10 to $15):



Automotive relay with a 12V coil and normally open contacts.  The
current rating of the relay contacts should be high enough to handle the
current to the ignition coil, fuel pump, EFI computer, starter, or
whatever
item in the car you decide to disable.  I like a relay with a rating of
10
amps minimum; 20 amps or 30 amps is even better.  The relay coil
resistance
should be between 20 ohms and 200 ohms.  These can be purchased at auto
parts stores, or Radio Shack.

It will be necessary to calculate the current required by the relay
coil.
 This is equal to the battery voltage divided by the resistance of the
relay 
coil.  For example, with a relay coil resistance of 120 ohms, the
required
current will be:


Battery     Relay Coil     Required
Voltage     Resistance     Current

14 volts    120 ohms       0.117 amps or 117 milli amps (mA)
12 volts    120 ohms       0.100 amps or 100 milli amps (mA)
8 volts     120 ohms       0.066 amps or 66 milli amps (mA)

I chose 8 volts as the minimum battery voltage when starting the car. 
It
could be lower in cold climates.  It is important that the relay be able
to
maintain closed contacts when the battery voltage drops.  Otherwise the
car
will NEVER start!

Radio Shack sells an automotive relay (catalog number 275-226 for $5.99) 
with the following specs:  single pole, single throw (SPST), contacts
rated 
at 30 amps, 12 VDC coil, coil resistance = 66 ohms, coil current = 160
mA. 
 This relay should work fine.


Silicon Controlled Rectifier (SCR), also called a Thyristor.  I think
they are available at Radio Shack.  This is a three terminal device, and
acts as a latching solid state switch.  The three connections are the
gate,
anode, and cathode.  It is turned ON by applying a voltage to the gate
so
that the "turn-on current" is exceeded.  The device will remain ON as
long
as a minimum "holding current" flows from the anode to cathode.

The voltage rating should be 100V or more.  The current rating should be
enough to handle the current to the relay coil (2 to 4 amps is enough).
 Choose a device with a power type package, for example a TO-220 case. 
This 
will be able to dissipate the heat and not require a heat sink.

The "latching current" and "holding current" ratings must be less than
the
current required for the relay coil.  See the calculations performed in
#1
above.  I like to pick an SCR with a "latching current" of 50 mA or
less, a
"holding current" of 25 mA or less, and a "turn-on current" of 25 mA
max.

Radio Shack sells an SCR (catalog number 276-1067 for $0.99) with the 
following specs:  6 amps, 200 volts, package = TO-220 case.  This will 
probably work OK.


One resistor.  The value should be chosen so that the minimum turn on
current of the SCR is achieved.  Assuming that the battery voltage is
12V
when not cranking, a value of 470 ohms should allow enough current to
trigger the SCR.  The wattage rating should be 0.25 Watts or more. 
These 
are
available at Radio Shack (catalog number 271-1317 for 0.25 W; or
271-1115 
for 0.5 W).


One reed switch.  This is a small switch that is activated by a magnet.
 The actual reed switch is enclosed in a sealed glass cylinder, with a
terminal on each end.  They are available from Radio Shack.  They can
also
be obtained by disassembling the magnetic switches used in home security
systems.  The contact rating of the reed switch should be 100 mA or
more.
 The terminal leads are sometimes difficult to solder to.

NOTE:  The reed switch must be the "normally open" type, which means
that 
the contacts close only when the magnet is placed near the reed switch. 
The 
magnetic switches sold for home security systems are available in both 
"normally open" and "normally closed" configurations, with the "normally 
closed" type being the most common.  Be sure to use the correct one! 
Radio 
Shack sells a normally open magnetic switch (catalog number 49-533) for 
$4.99.

Another option would be to purchase a reed relay (Radio Shack catalog
number 
275-232 for $2.49) and remove the reed switch from the relay housing.

NOTE:  Other switches can be used in place of the reed switch.  For
example, 
a mercury tilt switch or a simple toggle switch.  If a toggle switch is
used, then the SCR (and maybe even the relay) can be eliminated since
toggle 
switches can handle lots of current.  However, then you are back to the
old
style kill switch, and you can trash this email.

One diode, something like a 1N4001, available from Radio Shack.  This is
rated at 50V and 1 amp.  This is used to suppress the "inductive
kickback"
from the relay coil when the ignition is turned OFF.

Radio Shack carries the 1N4001 diode for $0.49 for 2 (catalog number
276-1101).  For more protection, use the 1N5402 (catalog number 276-1143
for 
$1.19 for 2) rated at 200V and 3 amps.

Wiring Instructions:

1. Connect a wire from the ignition switch "ignition position" to the
anode
of the SCR.
2. Connect a wire from the cathode of the SCR to one terminal of the
relay
coil.
3. Connect a wire from the other terminal of the relay coil to the
chassis
ground of the vehicle.
4. Connect the anode of the 1N4001 diode to the relay coil terminal that
is 
connected to ground.
5. Connect the cathode of the 1N4001 diode to the other terminal of the
relay coil.
6. Connect a wire from the gate of the SCR to one terminal of the
resistor.
7. Connect a wire from the other terminal of the resistor to one
terminal
of the reed switch.
8. Connect a wire from the other terminal of the reed switch to the
anode
of the SCR.
9. Connect the two terminals on the relay that connect to the relay
contacts in line with whatever device you decide to disable.  This could
be
the fuel pump, ignition coil, power to the EFI computer, or starter
relay.


Installation Instructions:

The most reliable connections will be achieved by soldering all
connections. 

 Be sure to insulate all connections with electrical tape (and also
shrink
tubing if available).  The SCR, relay, diode, and resistor should be
mounted 
in a location that is somewhat hard to get to, so that it can not be
easily
bypassed.  Mount the reed switch in a location so that it can be
activated
by a magnet (behind dash board, etc).

Please forward any errors to:

Bryan Zublin
bzublin@gi.com
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