I'm trying to build a kill switch in my car so that when I turn the ignition, a button also has to be held down. When the power goes through, the button can be let go and the system stays closed until the car shuts down, then you have to repeat the process. I came up with this diagram:

https://imgur.com/a/UOysWMc enter image description here

What kind of parts does it take to build this? I drew logic gates but I have no idea how that translates to real components. Is a button and relay from an electronics store enough? I'm worried about 1) splitting the ignition wire like that and 2) how much power actually loops back, would things over heat?

  • \$\begingroup\$ Does your car use a starter motor to turn over the engine? Because, if it does, all you need to do is buy a momentary switch capable of handling the required current to the starter motor and wire it in series. (Or use a relay and a smaller momentary switch.) \$\endgroup\$ – jonk Aug 21 '18 at 7:24
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    \$\begingroup\$ @jonk cranking current is pretty high. I wouldn't want to put my thumb on that switch. \$\endgroup\$ – Oliver Aug 21 '18 at 7:59
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    \$\begingroup\$ Starters in all cars (except BMC mini's) is switched by a solenoid. The key current is only a few amps. \$\endgroup\$ – Henry Crun Aug 21 '18 at 8:21
  • \$\begingroup\$ I've put my thumb on a starter switch, (it was the back end of the starter pull in a morris 1100). the engine cranked. nothing exciting happened. \$\endgroup\$ – Jasen Aug 21 '18 at 9:12
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    \$\begingroup\$ Starter motors are used with both gas and diesel engines - they crank the engine. Both types of engine do their own compressing of the air or air/fuel mixture. You stop a gas engine by turning off the electric ignition system. You stop a deisel by stopping fuel delivery from the fuel injection pump. \$\endgroup\$ – Peter Bennett Aug 21 '18 at 17:19

Typically you use a relay wired so that the live side of the switch powers the relay coil. the button used should be capable of powering the whole switched load


simulate this circuit – Schematic created using CircuitLab

by adding a diode the load can be isolated fronm the button allowing the use of a smaller button that only needs to pass the relay coil current (about 0.1A)

  • \$\begingroup\$ Thank you. I saw the first pic and thought OK I have no idea what's going on this looks legit, after understanding the diode that makes even more sense. Thank you \$\endgroup\$ – Hugh Myron Aug 21 '18 at 12:44
  • \$\begingroup\$ So you also mention automotive relays, would you recommend those over one I get in a Fry's or Micro center? \$\endgroup\$ – Hugh Myron Aug 21 '18 at 12:58
  • \$\begingroup\$ the quick connect terminals and mounting tab on automotive relays mean you can use them without using a PCB or an enclosure \$\endgroup\$ – Jasen Aug 22 '18 at 10:43

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