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Keypad engine immobiliser

There's no need for a hidden "kill" switch.

By John Clarke

For good protection against car thieves the Engine Immobiliser described last month works well but you do need a concealed switch to operate it and this can be a drawback. Using a keypad to enable the Immobiliser is much more elegant. The design uses a standard 12-button keypad, labelled from 0 to 9 plus asterisk (*) and crosshatch (#) keys.

Four buttons must be pressed in the correct sequence before you turn on the ignition. The car can then be started in the normal way.

You can program in any 4-digit code, including the "*" and "#" buttons, by means of links on the PC board. This means that you can set the combination to say, #123, 1223 or whatever.

You cannot trick the keypad circuit into disarming the Immobiliser by pressing all keys at once, by disconnecting the battery and reconnecting again or any other jiggery-pokery. The code must be entered in the correct sequence. If you enter the wrong code, you can start again by pressing any key which is not used in the code sequence, followed by the correct code sequence.

The Immobiliser is armed by pressing any key which is not used in the combination code.

A LED flashes to indicate when the Immobiliser is active and it goes out when the correct code is entered.

The keypad can only be used when the ignition is turned off. It does not respond to any buttons when the ignition is switched on. This means that you can only arm the Immobiliser once you have switched off the engine. Similarly, to disable the Immobiliser, you must enter the correct code before switching on the ignition.

The reason for this approach is so that the Immobiliser cannot be activated by the keypad when the car is in motion; if this happened the car could possibly be stopped in a dangerous situation if any of the keypad buttons was inadvertently touched.

As with the basic Engine Immobiliser described last month, the keypad version becomes active when power to the ignition is switched on, provided it has already been armed. If the ignition is off, the Immobiliser circuit is off and the only current drain from the battery is that drawn by the keypad circuit itself. This draws about 6mA which should not be a problem for the car battery.

Main Features
  • Keypad operation to restore normal ignition.
  • 4-digit code entry.
  • Any of 12 keys can be used for the code.
  • Any order, sequence or duplication of code is allowed.
  • LED flashes when ignition is disabled.
  • LED is off when correct code entered to enable normal ignition.
  • Keypad disabled when power to ignition is switched on.
  • Normal ignition cannot be restored by disconnecting and reconnecting battery supply.
  • System is armed by pressing a key when the ignition is off (which is not part of the code).
  • Can be used in unarmed mode by not pressing a key.

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