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Vehicle Radio Ignition Control


This project, a vehicle radio ignition control from 1981,  shows how an ignition control system containing an RF key (generator) and RF recevier linked into the engine ignition system can be created.

Note: More modern systems use a microcontroller to control and monitor and control all aspects of the engine such as timing / rev / power etc. This project shows specifically the RF portion and so is useful for understand how this works.



Note also that the system described here is a fairly primitive implementation since there is no attempt to check the RF signal for noise which starts up the detection process. Also there is no attempt to filter out, detect or correct errors caused by noise. Another aspect is that the circuits are always "on" so the battery is always drained by an amount.

With todays microcontrollers it is easy to design a low power "hibernating" system. The number of codes is really too small (128) and could easily be increased.



Executive Summary of the Vehicle Radio Ignition Control

The engine ignition circuit on a motor vehicle is controlled by an electronic switch which is part of a receiver/controller unit on the vehicle. A separate self-contained transmitter unit generates a seven digit coded address at a certain baud rate which modulates an RF carrier of a certain frequency.

In the receiver/controller unit, if the baud rate and the carrier frequency of the transmitted signal are correct, the coded address is compared with a stored coded address. If they match, the electronic switch is closed, enabling the vehicle engine to be started. The usual ignition switch on the vehicle must be closed before the receiver/controller unit can be effective.

The receiver/controller unit remains on for a minute or two after the ignition switch is opened.

Background of the Vehicle Radio Ignition Control



Summary of the Vehicle Radio Ignition Control

This design relates to a coded control system for a vehicle engine ignition circuit i.e. an RF ignition control circuit.

Various anti-theft arrangements have been proposed heretofore to prevent the engine of an automotive vehicle from being started unless a coded signal, known by an authorized driver of the vehicle, has enabled the engine ignition circuit. In some of these prior proposals a code transmitter is wired into the engine ignition circuit while in others the code transmitter is a radio transmitter which broadcasts the coded signal to a receiver wired into the engine ignition circuit.

The present design is directed to a coded control system for a vehicle engine ignition system which uses a self-contained radio transmitter unit for broadcasting a coded signal to a receiver/controller unit on the vehicle which is wired into the vehicle's engine ignition circuit.

The preferred embodiment of the present coded control system has the following features which contribute to the security it provides against unauthorized starting of the vehicle engine:

(1) the coded signal required to enable the engine ignition circuit on the vehicle must be the correct single code out of 128 possibilities;

(2) the bits of the coded signal must be transmitted at a predetermined baud rate;

(3) the carrier frequency at which the transmitter unit operates must be at a particular frequency which corresponds to the frequency to which the receiver is tuned; and

(4) the vehicle ignition switch must be closed before the transmitter unit broadcasts the coded signal, otherwise the coded signal will not be effective to enable the engine ignition circuit.

An advantageous feature of the present control system is that, once it has enabled the engine ignition circuit, it keeps this circuit enabled for a short time interval after the vehicle driver has opened the ignition switch, so that if the driver recloses the ignition switch during this time interval the engine will restart immediately without the necessity of operating the code transmitter again.

Another advantageous feature of the present system is that, when the ignition switch is closed after being open for several minutes, the receiver/controller unit is reset to a starting condition in which it is ready to respond to the next transmitted coded signal.

A principal object of this design is to provide a novel and improved coded control system for the ignition circuit of a vehicle engine to prevent unauthorized starting of the vehicle.

Further objects and advantages of this design will be apparent from the following detailed description of a presently preferred embodiment which is shown schematically in the accompanying drawing.


Figure 1 : Is a schematic electrical circuit diagram of the coded transmitter unit in the present control system for the vehicle radio ignition control



Description of the Vehicle Radio Ignition Control

Referring to FIG. 1, the transmitter unit in the present system has an asynchronous transmitter 10 which receives a 7 bit coded address from an address source 11, which is adjustable to select any one of 128 different address codes. The asynchronous transmitter 10 is controlled by a baud rate oscillator 12 which determines the speed at which the transmitter produces discrete signal bits.

When the push-button switch 13 is closed, an internal battery 14 in the transmitter unit applies a command signal via line 15 to the asynchronous transmitter after a delay interval, determined by the R-C delay circuit 16, 17, long enough for the oscillator and other circuitry in the asynchronous transmitter 10 to stabilize.

This command signal loads the coded address into the asynchronous transmitter 10 and it produces, in sequence, a start bit, the seven bits which make up the coded address to be transmitted, and a stop bit. These bits are applied sequentially to a modulator and amplifier 18 at a predetermined baud rate under the control of oscillator 12.

A carrier frequency oscillator 19 is turned on when the push-button switch 13 is closed. This oscillator produces an RF carrier signal at a predetermined frequency which is applied to the modulator in block 18 and is modulated therein by the coded address coming from the asynchronous transmitter 10. The amplifier in block 18 delivers this modulated carrier to a broadcast antenna 20.

Preferably, the entire transmitter unit of FIG. 1 is enclosed in a small, portable housing which an authorized driver of the vehicle may carry on his person and hold in his hand.


Figure 2 : Is a schematic electrical circuit diagram of the receiver/controller unit and the engine ignition circuit which it controls for the vehicle radio ignition control

View larger image here.


Referring to FIG. 2, the receiver/controller unit which controls the engine ignition circuit includes an antenna 21 for receiving the signal broadcast by antenna 20. The received signal is applied to one input of a demodulator and amplifier 22. Another input to the demodulator in block 22 is provided by a carrier frequency oscillator 23, which operates at the same frequency as the carrier frequency oscillator 19 in the transmitter unit of FIG. 1.

The demodulator in block 22 recovers the incoming signal, including the coded address which modulated the transmitted carrier at the baud rate determined by oscillator 12.

This recovered signal is applied to one input of an asynchronous receiver 24, which has a second input from a baud rate oscillator 25 operating at the same rate as the baud rate oscillator 12 in the transmitter unit. The seven bits which make up the received coded address are applied via a seven line output port 26 from the asynchronous receiver 24 to one input port of an address comparator 27.

A second input to this comparator is a coded address stored in block 28, which also may be any one of the same 128 codes which the coded address source 11 in the transmitter may be adjusted to produce. The address comparator 27 compares the two coded address signals after receiving an "address available" signal via line 29 from the asynchronous receiver 24 after receipt of the stop bit which immediately followed the 7-bit address code in the signal broadcast by the transmitter.

If the two coded addresses are the same, the comparator 27 will produce an "address compares" signal on its output line 30. This signal sets a latch circuit 31 which closes an electronic switch 32.

The engine ignition circuit enclosed within the dashed-line box in FIG. 2 is shown in simplified form as including the vehicle battery 34, ignition switch 33, ignition coil 35, distributor 36 and spark plugs S. Not shown are the usual distributor points and condenser, connected in parallel with each other between the primary winding of the ignition coil 35 and ground. The secondary winding of the ignition coil is connected to the distributor 36.

In accordance with the present design, the electronic switch 32 is interposed between the ignition switch 33 and the ignition coil 35. An input line 37 to the electronic switch 32 is connected to the ignition switch 33, and an output line 32a from the electronic switch is connected to the primary of the ignition coil 35.

The electronic switch 32 closes, and consequently the engine ignition circuit is enabled, whenever the coded address received from the remote transmitter unit matches the coded address stored at 28 in the receiver, provided also that the following separate and independent conditions are met:

(1) the carrier frequency oscillator 19 in the transmitter unit operates at the same frequency as the carrier frequency oscillator 23 in the receiver/controller unit; and

(2) the baud rate oscillator 12 in the transmitter unit provides the same signalling speed as the baud rate oscillator 25 in the receiver/controller unit.

From FIG. 2 it will be apparent that the demodulator/amplifier 22, the carrier frequency oscillator 23, baud rate oscillator 25, asynchronous receiver 24, address comparator 27, and latch circuit 31 in the receiver/controller unit all are powered by the vehicle battery 34 when the ignition switch 33 is closed.

The power supply circuit includes lines 37 and 38 and a power storage network made up of a rectifier 39 in series with line 38 and a capacitor 40 connected between the cathode of rectifier 38 and the grounded negative terminal of the vehicle battery 34. The various components of the receiver/controller unit are turned on by closing the ignition switch 33.

After the ignition switch is opened, these components remain on for a brief interval, such as a minute or so, in case the driver closes the ignition switch 33 again during this interval. The capacitor 40 in the power storage network provides this time delay between the opening of the ignition switch 33 and the de-energization of the demodulator/amplifier 22, the carrier frequency oscillator 23, the baud rate oscillator 25, the asynchronous receiver 24, the address comparator 27 and the latch circuit 31.

A power on reset network, made up of a resistor 41 and a capacitor 42, determines the initial power-on reset state of the latch circuit 31. Resistor 41 is connected between the cathode of rectifier 39 in the power storage network and the reset terminal 43 of the latch circuit 31. Capacitor 42 is connected between this reset terminal and ground.

A normally-open manual switch 44 bypasses the entire remote control system. This bypass switch is connected between the ignition switch 33 and the ignition coil 35, so that when closed it puts the engine ignition entirely under the control of the ignition switch 33 and independent of the electronic switch 32.

Switch 44 would be used only if a malfunction occurs in the present coded control system or if for some other reason the user wants to bypass the coded control system altogether.

The lines 37 and 32a, connected respectively to the input and output of the electronic switch 32, may have special connectors to facilitate their connection in the engine ignition circuit.

Everything shown in FIG. 2 preferably is in or close to the engine compartment of the vehicle.

OPERATION

Assuming that the ignition switch 33 has been closed but the "address compares" signal has not appeared on line 30, the vehicle battery potential applied via resistor 41 to terminal 43 of the latch circuit 31 will reset this latch circuit to a condition in which it maintains the electronic switch 32 open, so that the engine ignition circuit is not enabled.

Also, this same potential is applied via line 24a to the asynchronous receiver 24 to remove any signals which may be present at its output terminals.

When the "address compares" signal does appear on line 30, in response to the operation of the transmitter unit of FIG. 1, as described, it sets the latch circuit 31 to the condition in which it closes the electronic switch 32 and thereby enables the engine ignition circuit.

From the foregoing description it will be apparent that the present coded control system incorporates the following practical safety features:

(1) the ignition switch 33 must be turned on before the transmitter unit (FIG. 1) is turned on; otherwise the coded control system will not start the engine;

(2) the coded address broadcast by the transmitter unit must be the correct one of 128 possible codes;

(3) the coded address broadcast by the transmitter unit must be on a carrier whose frequency matches that of the carrier frequency oscillator 23 in the receiver/controller unit (FIG. 2); and

(4) the bits of the coded address broadcast by the transmitter unit must occur sequentially at a baud rate which matches that of the baud rate oscillator 25 in the receiver/controller unit.


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