A Simple Alarm

OPERATION OF A SIMPLE ALARM SYSTEM 

To understand how modern alarm systems operate let us first take the simplest bell circuit and then improve on this, step by step, until we arrive at a satisfactory system for protecting a premises. Before we start though let me state two important facts:

(1)      All intruder alarm systems work on a 12 volt DC supply.

(2)      All detection devices are simple switches, they are either open or closed.
Below we see a simple bell circuit, the same circuit we would use for a door bell. Nothing complicated just close the switch and the bell rings.

SWITCH CLOSED TO RING BELL

 

However this wouldn’t be much good as an alarm system since if someone cut the wire then closing the switch is not going to ring the bell. So what about using a circuit where the switch is closed and must be opened to ring the bell. This would certainly get over the wire cutting problem. Over we see such a circuit. For simplicity we will assume that all the electronics necessary to have a circuit that will ring the bell when a switch is opened are contained in the black box on the right. (This will later become our control panel)

 

SWITCHES MUST BE OPEN TO RING BELL

 

So for this example all the switches must be closed, if any one of these is opened then the bell will ring. We also have the Advantage in that if the cable is cut to any of the detector switches then this will also cause an activation.

What we need now is to be able to switch the circuit on when we need it and off when we don’t. We also need to be able to test the circuit before switching on to make sure all detector switches are closed and the cable is intact. Below is such an arrangement.

Note: we will assume the battery is housed in the “black box”.

 

 THE SYSTEM IS SWITCHED TO TEST BEFORE BEING TURNED ON. A “FAULT” LIGHT IS ILLUMINATED TO SHOW A DETECTOR IS NOT CLOSED.

 

		Detector circuit
		Open		Closed
	Off
	Test	Fault Light		Clear Light
	On	Bell Rings
THIS TABLE SHOWS WHAT OCCURS IF THE DETECTION CIRCUIT IS OPEN OR CLOSED IN EACH OF THE THREE SETTING POSITIONS

 

 

 

 

 

 

 

 

 

So now we have a system whereby we can switch it “on” or “off”. We can also switch it to “test” before turning on and check to see if any detector switches are open or the cable is damaged. When in “test” two lights (LEDs) are used to give a visual indication to the operator of the state of the detection circuit. If a detector is open or the cable broken then a “fault” light will illuminate. If everything is ok then a “clear” light will illuminate and the system can be switched on.

 

We have now established something close to a practical alarm system. However we still have one fundamental problem and that is how do we protect the system during the day when it is switched off?

The answer is to have another circuit which is not switched off during the day and can be used to protect the cables and to prevent tampering with the system. We will call this circuit a “tamper circuit” (sometimes called a 24 hour circuit as it is active at all times).