The operation is as follows:

• Turn the key to the ignition position.
• Place a small magnet on a certain location of the dash board to enable a relay.
• Once enabled, the magnet can be removed.
• Turn the key to start the engine.
• The magnet activates a hidden reed switch that is located behind the dash board. It could also be located in the base of the drivers seat.

Disabling the starter is the least effective method. Disabling the ignition or EFI computer will allow the engine to turn
over but not start. Disabling the fuel pump will allow carbureted engines to start and run for a minute or so until the fuel
in the carb runs out. On my Celica with dual Weber DCOEs, I can actually drive quite a while before this happens. On EFI engines, the engine *may* start and run for a few seconds, until the fuel in the fuel rail loses pressure.

1. Automotive relay with a 12V coil and normally open contacts. The current rating of the relay contacts should be high enough to handle the current to the ignition coil, fuel pump, EFI computer, starter, or whatever item in the car you decide to disable. I like a relay with a rating of 10 amps minimum; 20 amps or 30 amps is even better. The relay coil resistance should be between 20 ohms and 200 ohms. These can be purchased at auto parts stores, or Radio Shack. It will be necessary to calculate the current required by the relay coil. This is equal to the battery voltage divided by the resistance of the relay coil. For example, with a relay coil resistance of 120 ohms, the required current will be:
   
   

   Battery Voltage	Relay Coil Resistance	Required Current
   14 volts	120 ohms	0.117 amps or 117 mA
   12 volts	120 ohms	0.100 amps or 100 mA
   8 volts	120 ohms	0.066 amps or 66 mA

   
   I chose 8 volts as the minimum battery voltage when starting the car. It could be lower in cold climates. It is important that the relay be able to maintain closed contacts when the battery voltage drops. Otherwise the car will NEVER start!
   
   Radio Shack sells an automotive relay (catalog number 275-226 for $5.99) with the following specs:
   single pole, single throw (SPST), contacts rated at 30 amps, 12 VDC coil, coil resistance = 66 ohms, coil current = 160 mA.
   This relay should work fine.
   
   
2. Silicon Controlled Rectifier (SCR), also called a Thyristor. I think they are available at Radio Shack. This is a three terminal device, and acts as a latching solid state switch. The three connections are the gate, anode, and cathode. It is turned ON by applying a voltage to the gate so that the "turn-on current" is exceeded. The device will remain ON as long as a minimum "holding current" flows from the anode to cathode.
   
   The voltage rating should be 100V or more. The current rating should be enough to handle the current to the relay coil (2 to 4 amps is enough). Choose a device with a power type package, for example a TO-220 case. This will be able to dissipate the heat and not require a heat sink.
   
   The "latching current" and "holding current" ratings must be less than the current required for the relay coil.
   
   See the calculations performed in #1 above. I like to pick an SCR with a "latching current" of 50 mA or less, a "holding current" of 25 mA or less, and a "turn-on current" of 25 mA max.
   
   Radio Shack sells an SCR (catalog number 276-1067 for $0.99) with the following specs: 6 amps, 200 volts, package = TO-220 case. This will probably work OK.
   
   
3. One resistor. The value should be chosen so that the minimum turn on current of the SCR is achieved. Assuming that the battery voltage is 12V when not cranking, a value of 470 ohms should allow enough current to trigger the SCR. The wattage rating should be 0.25 Watts or more. These are available at Radio Shack (catalog number 271-1317 for 0.25 W; or 271-1115 for 0.5 W).
   
   
4. One reed switch. This is a small switch that is activated by a magnet. The actual reed switch is enclosed in a sealed glass cylinder, with a terminal on each end. They are available from Radio Shack. They can also be obtained by disassembling the magnetic switches used in home security systems. The contact rating of the reed switch should be 100 mA or more. The terminal leads are sometimes difficult to solder to.
   
   NOTE: The reed switch must be the "normally open" type, which means that the contacts close only when the magnet is placed near the reed switch. The magnetic switches sold for home security systems are available in both "normally open" and "normally closed" configurations, with the "normally closed" type being the most common.
   
   Be sure to use the correct one! Radio Shack sells a normally open magnetic switch (catalog number 49-533) for $4.99.
   
   Another option would be to purchase a reed relay (Radio Shack catalog number 275-232 for $2.49) and remove the reed switch from the relay housing.
   
   NOTE: Other switches can be used in place of the reed switch. For example, a mercury tilt switch or a simple toggle switch. If a toggle switch is used, then the SCR (and maybe even the relay) can be eliminated since toggle switches can handle lots of current. However, then you are back to the old style kill switch, and you can trash this email.
   
   
5. One diode, something like a 1N4001, available from Radio Shack. This is rated at 50V and 1 amp. This is used to suppress the "inductive kickback" from the relay coil when the ignition is turned OFF.
   
   Radio Shack carries the 1N4001 diode for $0.49 for 2 (catalog number 276-1101). For more protection, use the 1N5402 (catalog number 276-1143 for $1.19 for 2) rated at 200V and 3 amps.

1. Connect a wire from the ignition switch "ignition position" to the anode of the
   SCR.
   
   
2. Connect a wire from the cathode of the SCR to one terminal of the relay coil.
   
   
3. Connect a wire from the other terminal of the relay coil to the chassis ground of the vehicle.
   
   
4. Connect the anode of the 1N4001 diode to the relay coil terminal that is connected to ground.
   
   
5. Connect the cathode of the 1N4001 diode to the other terminal of the relay coil.
   
   
6. Connect a wire from the gate of the SCR to one terminal of the resistor.
   
   
7. Connect a wire from the other terminal of the resistor to one terminal of the reed switch.
   
   
8. Connect a wire from the other terminal of the reed switch to the anode of the
   SCR.
   
   
9. Connect the two terminals on the relay that connect to the relay contacts in line with whatever device you decide to disable. This could be the fuel pump, ignition coil, power to the EFI computer, or starter relay.

Please forward any errors to:

Bryan Zublin
bzublin@gi.com

Rev 1.0 23MAY96: Initial release.
Rev 2.0 28MAY96: Added part numbers from Radio Shack and also clarified the requirements for the reed switch.