Home > Circuits > Car Imobiliser
Modified:20:35, 22 October 2013

This project was inspired by a friend who wanted a combination imobilier to disable the ignition on some MGs he was renovating. The idea was simply to wire the imobiliser in series with the ignition supply from the standard key switch in the car.

To enable the ignition, just type in the PIN code and the ignition would come on. To indicate the status of the imobiliser, a 5mm red/green LED was used and mounted just above the keypad which was a standard 12 button type.

This project can be used for anything that runs from 9 - 12 volts and can have an extra switch in line with its supply - as long as it doesn't exceed 3 amps.

NOTE: It is strongly recommended that this is only used on older cars, pre-ECU types. Things were much simpler in those days...




The circuit is very basic as the PIC (IC1) performs all the complex functions. RG1 provides 5volts for the IC with C1 and C2 providing stability for the regulator. C2 is a tantalum type and provides more than enough smoothing for the IC.

The relay is driven by Q1 through R3 to provide the extra current for the coil with D2 suppressing any EMF generated by it when it switches off.

The ignition input from the keyswitch in the car is buffered by Q2. This is needed because the PIC runs at 5v and the car ignition could be 12-13.5v. R4/5 switch the transistor on pulling the collector low and providing a low signal to pin 1 of the IC. This buffering is better than just a potential divider as it also provides some isolation and protection to the IC from possible hazardous spikes in the car wiring.

The buzzer provides audible confirmation of key presses and operation - it even plays tunes

  • When ignition switch is turned on/off
  • When the unit is enabled or disabled
  • When SET mode entered or new code stored

- well why not? It's easy with a PIC. They were created using the MUSIC FACTORY software and then stored on the chip.


The unit starts up in disabled mode - the ignition +Out is disabled. The LED pulses RED.When the iginition is turned on and off, the unit plays a short tune to acknowledge it, but +Out will stay off.

The code can be up to 15 digits. As the code is typed in the LED flashes faster, to show mid-entry. If there is a pause of longer than 4 seconds between presses, the unit aborts the entry and beeps.
Press # to accept the entry.
The correct entry will change the LED from RED to GREEN or vice versa and activates/deactivate the relay and +Out.

The unit must be enabled to change the code (Green). Press the * key and the LED turns red. Enter a new code up to 15 digits. Press * again to confirm, # to cancel. Any delay greater than 4 seconds aborts the change.

If the ignition is off and the unit still enabled (+Out on) for longer than approx 60 seconds, the unit will automatically switch to disable mode, switching off +Out.




Ign Sw. In


Permanent Connection

With Ignition

Ign Out

Ign Sw. In

Ign Out

Ign Sw. In


Generally car keyswitches have 4 common positions
1 - Off
2 - Accessories
3 - Ignition
4 - Start.

TB2 connects in series with the Ignition connection from the keyswitch of the car. You can simply cut the wire from that point (or disconnect and add an extra wire) and insert the end from the keyswitch into the lower terminal of TB2. The other end connects to the top terminal. So effectively you are putting an extra switch (relay) in series with the ignition.


There are 2 ways to connect up the imobiliser to the supply (TB1).

Permanent - the imobiliser is always on.
Connect the top terminal of TB1 to a permanent live and the lower one to earth or zero volts. This method means the LED is always flashing.

With Ignition - imobiliser comes on with the ignition -
Connect the top terminal of TB1 to the same Ignition connection from the keyswitch and the lower one to earth or zero volts. This method saves power, but the AUTOSET feature will not work since when the unit is on so is the ignition.
(Note: The Ignition Switch input is connected to 2 points in this mode)

PCB size: 56mm x 43mm
RG1 = 78L05
BZ1 = 5v PCB mount piezo transducer
RL1 = 12v PCB sub-miniature relay (6A contacts)


SIL1 connects to the keypad and the LED using a header and housing connector. The keypad and LED are connected using rainbow wire for neatness.

The connections are shown right. Connectors are labelled 1-7 on the keypad and 1-9 on the PCB starting from the left.

The last 2 connections, 8 and 9 from the PCB are connected to the LED which is mounted above the keypad.



To prevent wires breaking off from repeated bending, the cable was fixed to the rear of the keypad with a resin glue.

The LED was attached loosly at the top of the keypad so it could be repositioned if needed.

Pins - 1 . . . . . 7
Pins - 1 .. .. .. .. .. .. .. 9
1 = Col 2
2 = Row 1
3 = Col 1
4 = Row 4
5 = Col 3
6 = Row 3
7 = Row 2
1 = Row 2
2 = Row 3
3 = Col 3
4 = Row 4
5 = Col 1
6 = Row 1
7 = Col 2
8 = LED Anode (Red)
9 = LED Cathode (Green)


- Circuit Diagram

- PCB Layout - will need resizing

A pre-programmed PIC of the Imobiliser
is now available from our shop

Designed and Written by Phil Townshend 2010
Back to Circuits page...

www.edutek.ltd.uk - Working Electronics For Students & Teachers