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Discussion Starter · #1 ·
The attached jpeg file is a schematic for a self made, passive arming automotive theft deterrent system that qualifies for the full 10% insurance discount. I once installed this on a car and had it checked by Allstate (since it wasn’t a “purchased alarm) it passed they gave me the 10%.

If you total up the parts list it comes up less that $15, a pretty good deal. No alarm here, just an automatic way of stopping someone from starting and driving off with your car, even if they have the key. And only you know how to starting the car because you install the device, as creatively as you desire. It also has a valet bypass feature so with the flip of a switch it’s as if the thing isn’t there at all.

K1 is the main relay. It’s coil is powered from switched +12 volts so every time you turn your car off, the relay opens breaking the connection to whatever you hook it up to. Interrupt the coil B+ line, or the electric fuel pump power…whatever suits you, be creative.

S1 is the hidden “enable” switch. A very small normally open pushbutton switch, which is pressed once when you key the car on and the +12 switched voltage is on. This pulls in the relay and self latches the coil as long as the +12 volts switched is present. Turn the car off, +12 volts switched disappears and the coil opens protecting your car, automatically. Be creative where you hide the enable switch, I put it somewhere I can use my left hand, since my right hand is turning the ignition key. Even if someone has the key and tries to start the car, they’ll have no luck unless they know where the hidden pushbutton switch is.

S2 is a toggle valet bypass switch. Flip the toggle switch to the closed position and there is always a coil ground path, bypassing the theft deterrent feature. Just make sure you hide this switch too.

I usually put the two switches on long, very thin “twisted pair” (spin two wires in your electric drill) wires allowing me to get real creative where the switches get mounted. Just thought I’d share a very cheap but effective way to protect our beloved Opels, without the expense of a full blown alarm. Happy Opeling!
 

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Opeler
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138 Posts
Passive Alarm

Thanks jimsky!

I showed your diagram to a friend who knows electronics. He described your diagram as a simple "latch" circuit and would work great for older cars.

Thanks for the tip!

Richard
 

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Discussion Starter · #4 ·
I kind of like the fact that all the parts needed to make this were around when our cars were made, no fancy electronics. An electronic chirping alarm in a classic car seems out of place.

For the truely paranoid who are afraid that this might break and leave them stranded... the ultimate bypass for this device.

Take a simple in-line fuse holder and connect it across the contacts of the relay going to the coil or fuel pump you are interupting. Take the fuse out of the fuseholder and toss it in the glovebox. Without the fuse, it's like it's not really there.

If anything ever craps out and you can't start the car, put the fuse in the fuse holder. It connects the two "interupt" points together, totally bypassing the broken device allowing you to start your car.

I've put these devices in just about all the cars I've owned (old and new alike) and never had one die on me. I've never added the fuse holder bypass, but I've had a few friends who have.
 

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4ZUA787
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665 Posts
lo-jack all the way

i say if ur gonna go and put an alaram in ur car u get a lo-jack system because as uve just said u can just take a lead and run it from the positive on the battery to the coil and drive away, and if it does have a siren they would just snip the wires to the siren.a lo-jack is the only way to go on any car older then about 1995 anything newer has all that fancy gizmo junk in it. thats my opinion. also with the lo-jack u can go on the net and watch ur car so when ur kids say there borowing the car u new were there going.its a nice feature to have.
 
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