Simple car theft prevention

[audioguru]

Hi Zach,
I've corrected your schematic, your CAD got the PNP upside-down again.
I added a resistor from the collector of the transistor to the gate of the SCR to limit current. The transistor saturates (or tries to saturate) but the gate of an SCR is just a 0.7V diode to ground. Lots of smoke without the current-limiting resistor! :lol: The current-limiting resistor could be put in series with the transistor's emitter instead, to lengthen the transistor's on time.
Hey! Why have a cap? Why have a transistor?
I also added an important reversed diode across the relay coil to arrest its very high voltage spike when it is switched off.
Hey! What turns-off the SCR? Disconnect the power?

When you push the button, an unlimited current will flow through the switch contacts to charge the cap. If they survive the extremely high current, a small pit will be eaten out of them each time. The contacts might actually weld themselves closed without some current-limiting. You could add a small-value resistor in series with the switch to limit the current. :lol:

 

[dingo]

If you want a simple but effective way:

Open circuit your fuel pump and short or open circuit (depending on how it is wired) your fuel guage so it shows empty.

No matter how good your security system it can always be bypassed. I doubt your car thief will look further than the fuel guage if he thinks you are out of petrol.

 

[zachtheterrible]

Thanx audio, always nice to have someone perfect my circuit :lol:

I wouldve installed this thing by now, but there is something extremely weird about this relay: on BOTH sides of the relay, i measure ground! Even when I start the truck!

 

[audioguru]

Hi Zach,
Maybe the relay's coil is damaged and is shorting to ground.

Your relay driver circuit that I corrected has a big timing cap and a transistor. What are they for? A momentary switch connected to +12V and a resistor to the gate of the SCR will turn it on forever. About the only way to turn off an SCR is to disconnect it or short it for a moment. :lol:

 

[zachtheterrible]

Well, the way that it is supposed to work is that the relay only has current when the truck is on, keeping the SCR on. When the truck turns off, the current to the relay, and therefore SCR cuts.

The capacitor and resistor allow the SCR to pass current for a while, allowing me to start the truck. After the current has drained, and the truck has been started, the SCR keeps the relay going.

The reason for all of this is that all I have to do is press that little pushbutton switch before I turn on my truck and don't have to worry about turning it off when I turn my truck off. The circuit does it automatically.

 

[audioguru]

Good, your corrected relay driver circuit will work fine. :lol:

Maybe the relay's coil already has one wire connected to ground, and the other wire measures the very low resistance of its coil to ground. Therefore this end of the relay's coil must be connected to +12V to activate it. :?:

 

[zachtheterrible]

I would say that is probably true, but when I start the truck, I still measure ground on both sides. its soo FRUSTRATING!!

 

[George L.]

hello,

istead of using a switch, would it be possible to somehow charge the car, its mostly metal, so that when a theirf touches it, they get a painful shock, like a taser for a car. could this work?

thanks,

George

 

[Nigel Goodwin]

hello,

istead of using a switch, would it be possible to somehow charge the car, its mostly metal, so that when a theirf touches it, they get a painful shock, like a taser for a car. could this work?

It would work in that it would earn you a speedy trip to jail, charged with either murder (premeditated) or attempted murder.

Presumably this isn't what Zach is looking for?.

I was under the impression that Zach had already said that his car includes an immobiliser built in the key? (as mine does!). So why is he trying to add an extra one?.

 

[audioguru]

George,
If you charge the truck with a high voltage and the tires aren't wet, how many thieves have bare hooves (feet) and are standing in a puddle? :?:

It takes two connections to a voltage source to pass current, or an extremely high voltage to arc.

I think the capacitance to ground of a truck is so low that it wouldn't hurt anyone, maybe just cause a cop to drop his donut! :lol: :lol:

 

[George L.]

ok, thanks...I was just wondering if it would work

I saw a car that shocked people in a movie, very unrealsitic movie! :lol:

thanks,

George L.

 

[zachtheterrible]

James bond? 8)

Can't say that the thought didnt cross my mind . . . but then i thought better of it :lol:

I don't think Ill be putting the circuit on my truck, but I am really curious as to the electrical system and why it will not work.