<VV> Reality

[Tony Underwood]

At 12:22 hours 05/18/2005, you wrote:
>  Smitty says:  Way back in the 70s someone put a simple circuit in our
>news letter to prevent simple car theft.  Nothing is going to protect it
>if someone loads it on a flatbed with the tires sliding but this seemed
>pretty slick to me at the time.  It utilized a switch under the dash for
>the owner to operate.  In the Norm position it would allow a start with
>the key as usual.  In the safe position it placed a dead short across the
>positive terminal of the coil.  If someone tried to hot wire it they
>would encounter a shorted circuit.  If the owner forgot the switch and
>tried to start it , it would blow a fuse provided for that purpose to
>remind him to form a new habit of putting the switch in the Norm
>position.
>         I wonder how hard it would be to get one of our electrically
>minded to draw the circuit so that if the switch is in the safe position
>and someone tries to hot wire it, it would blow the horn rather than
>being shorted.  A circuit like that would be worth printing in the
>Communique.
>  _____________________________________________




I know a quick and easy way to do it, using a wire spliced into the wire 
harness under the dash and an add-on relay, wired to the horn relay.   Of 
course if you didn't flip the switch back, soon as you turned on the 
ignition the horn would sound.

All it takes is one wire from the IGN terminal of the ignition switch, run 
to the field of a $4.99 Bosch GP relay (available at most auto parts 
stores) with a hidden toggle switch in-line to the ignition switch.   The 
Bosch relay is wired so that when energized, it grounds the downhill side 
of the horn relay (wire which goes to the horn button).   Any time voltage 
appears on the coil + terminal, wether it's from the ignition switch or a 
jumper wire to the coil, the horn sounds until that hidden switch is turned 
off or the 12 volts applied to the coil is removed.   Parts cost is less 
than ten bucks total and that's if you have to buy the wire too.   And yes, 
the relay will function with the resistor wire in-line with its field when 
12 volts is applied to the coil, backfeeding through the resistor wire.

We used to wire up cheap & dirty alarms this way at the shop I worked for 
Back When.   Another relay wired up in a similar fashion to the door jamb 
switches would sound the horn if the door was opened.   A DPST switch 
controlled both of the relays.   $4.99 at Radio shack for the DPST switch 
vs $2.99 for the SPST  switch to control just one relay.

Those Bosch relays were used for boatloads of alarm installations, were 
laying around the shop like scattered rice at a wedding.    I picked up 
about a dozen or so off the floor during my time there, would toss them 
across the lot into the back seat of the ragtop, still have about 9 in a 
box in my workroom at the house.   They're about the size of a Whitman's 
Sampler chocolate.   Black plastic, with a mounting tab on one side.   12 
volt field (of course) and able to conduct a dozen or so amps.   Cheap way 
to awaken the neighbors at 2 AM.    This also works with a door jamb switch 
added to the rear deck lid and wired to the rest of the jamb switches, did 
it to my '65 ragtop.   My switches are wired to a standard remote operated 
car alarm, so when I open the door I don't make racket until I find and 
flip the switch.   However, the horn works as well... and if you wanna get 
really tacky, you can spend a few bucks more and get a latching relay which 
self-energizes and will continue to sound after being tripped, until you 
either trip another hidden pushbutton switch or you yank a battery 
cable.   And if you have a backup battery wired up in the trunk, not even 
yanking the regular battery cables off will work to stop the noise (cheap 
lawn and garden battery in the trunk works well).    Hide the 2nd battery 
cable under the car, attached to the starter BAT terminal just like the 
regular battery cable, nobody is the wiser.

Of course, this only works if someone tries to gain access to the car to 
hotwire it.   A "claw foot" wrecker or a rollback with a winch that will 
drag the car up onto the deck will both make off with the car and nobody 
will worry about starting it...  which is where a motion detector alarm is 
needed.



tony..