<VV> Reality

[Roger Gault]

The best way to do this is with a latching relay.  A pushbutton is mounted
in some un-noticeable spot where you can get to it with your left hand when
starting the car.  On my daughter's RX-7, I mounted a black pushbutton in
the steering column cover on the left side - it was practically invisible.
When you start the car, you must hold the button down.  This engages a
relay, which latches and stays engaged until you turn off the ignition.  The
other half of the double pole relay controls any number of anti-theft
things, electric fuel pump, horn, ignition, etc.



You'll need a (momentary contact) pushbutton switch and a 12Volt DPDT
(Double Pole / Double Throw) relay like Radio Shack #275-218.

For those of you not familiar with relays, these have two sets of contacts
and a coil.  Generally the relay contacts are labeled "C" for "Common", "NC"
for "Normally Closed", and "NO" for "Normally Open".  There should be two
sets of these, an "A" or "1" set and a "B" or "2" set.  In addition, there
should be two connections labled some form of "Coil".  (Naturally, the Radio
Shack relay has numbers, but the wiring diagram that comes with it is clear
enough).  When there is no voltage applied to the coil, the "C" terminals
are connected to their matching "NC" terminals.  When voltage is applied to
the coil, the "C" terminals are connected to their matching "NO" terminals.



"Latch" wiring instructions:

Connect the "C" terminal of set A to a wire that is powered in "Start" and
"Run".

Connect the "NO" terminal of set A to one of the relay coil connections.

Connect the other coil connection to ground.

Now connect one terminal of the pushbutton to the same "C" terminal used
above.

Connect the other pushbutton terminal to the "NO" terminal connected above.



So, now if the car is in Start or Run, there is 12V on the "C" terminal, but
no voltage on the relay coil.  When you push the button, it connects the 12V
to the relay coil.  This engages the relay, which connects the "C" terminal
to the "NO" terminal, and gives another path for the 12V to power the coil.
Release the pushbutton and the relay stays engaged until the ignition switch
is turned off.  The relay "latches".



Now, you have the "B" set of contacts to do what you like:



You could power an electric fuel pump through it.



You could connect a wire from the ignition (the one that goes to the
ignition coil) through the "NC" and "C" terminals to the horn relay so the
horn would sound if there was ignition power and the button had not been
pushed.



You could connect the "-" (points) side of the coil to ground through the
"NC" and "C" contacts so the points were shorted when the button hadn't been
pushed.



DO NOT connect the "+" side of the coil to ground through these contacts.
If someone hotwires your coil, the "hot wire" will be shorted to ground and
could cause a harness fire.



Roger



If this novel is too confusing to anybody, and you want to do this, let me
know and I'll draw up a diagram and e-mail it to you.




----- Original Message ----- 
From: <vairologist at juno.com>
To: <virtualvairs at corvair.org>
Sent: Wednesday, May 18, 2005 2:22 AM
Subject: <VV> Reality


> 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.
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