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central locking systems

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I am completely new to this and have almost no knowledge of electronics.
Can someone please explain to me me in simple terms how a central locking system works? And if it had to be done wirelessly how would that work?
Central locking for what?
I have a spare half hour or so, so will give a very brief overview from older automotive examples (Assuming this is what is required)

Single-point central locking:
The 'point of entry' (usually the driver's door) only has a simple 2-way (3 wire) switch assembly connected to the lock. The switch sends either a 'lock' or an 'unlock' signal to the main control unit, by connecting the 'common' wire to the intended 'signal' wire. The remaining doors have simple motors or solenoids and are supplied with power, over two wires, from the main control unit, for a sufficient duration to allow full travel from one state to the other. Operating any other door than the 'point of entry' has no influence upon the system and will only lock or unlock that particular door.

Multi-point central locking:
As the name implies, it is possible to lock and unlock the system from more than one point, so there has to be some form of feedback and motion control provided at each access point. This is done by combining a switch and motor assembly in one package. In one state, let's assume locked, all switches will report a 'locked' state to the control unit. The user operates the mechanical lock, which closes the 'common' and 'unlock' contacts within the switch, signalling to the main control unit that the system needs to be powered to achieve an unlocked state. Switch input signals are briefly ignored whilst power is supplied to the motors, to prevent any 'cycling'. The same two wires for the motor, or solenoid, are used to provide motion in either direction, the only factor that determines the direction is the polarity. Let's again assume a simple motor with two red and the other black. Connect the red to a positive supply and the black to a negative supply and the motor will turn one way. Reverse the connections and the motor will turn in the other direction.

The switching of polarity used to be done with relays, but is more commonly done with transistor bridges these days, however, we'll use relays in this example because it is simpler to understand.

A relay is, in simple terms, a type of switch which allows a small current to control a much higher current. Within the relay, is a small electromagnetic coil winding which, when energised by a small current, pulls a moving ferrous lever from connecting one set of contacts (the resting state) to another set of contacts (the active state). These usually hefty contacts can carry much more current and are particularly suitable for supplying a lot of power to such items as motors, bright lamps, heating elements, etc.

One relay alone is not sufficient for this example though. We actually need two relays. Remember that there are both 'resting' and 'active' states...

The relays are both connected identically in respect of positive and negative supply, and the motor wire. The 'resting' contacts are connected to negative, the movable contact is connected to one motor wire and the 'active' contact is connected to positive. The other relay is connected the same, but with the moving contact connected to the other motor wire. Since both motor wires are connected to the negative supply at rest, no current flows. Energise one of the relay coils and it's contacts will connect one of the wires to the positive supply, turning the motor in one direction. Energise the other relay coil and it's contacts will connect the other wire to the positive supply, turning the motor in the opposite direction. Energise both relay coils at once and both wires will be connected to positive...and no current will flow.

Think of sitting in a wheelchair with your hands on both wheels. You are connected to negative (ground) and pushing one wheel forward (positive) will turn you one way, pushing the other forward will turn you the other way. OK, pushing both wheels forward will provide a certain motion, but that does not apply in this example. ;)

Now that the basics are covered, the wireless part:

For the system to be controlled wirelessly, there has to be the basic underlying structure which still allows traditional (mechanical) access should the wireless system fail, plus additional electronic authentification of rights to remotely operate the system. There are so many different signals floating around in the air these days, on the same or very similar frequencies, which each require sole access to a particular system with the exclusion of all others.

Each particular system is generally designed to only respond to a particular string of information, once certain identifying tokens have been exchanged between the transmitter and receiver. In the past, static codes were used and were repeated. Sometimes 'copied' and re-transmitted even, for nefarious reasons. To counter this, complex algorithms were devised to produce small groups of expected valid sequences of cryptographic data, within the next few remote keypresses. Repeated keypresses out-of-range, such as that resulting from a child being 'entertained' with a bunch of keys, can have obviously negative results....and a trip to the dealer to report a faulty key.

Does this help?
Thanks a lot
that was really helpful to what i'm working on.
If i have any more problems i'll pop another thread up
Thanks again
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