e041046.pdf

READERS CIRCUITS

Burglar Alarm

With individual sensor signalling

Design by E. Martens

This is a circuit of a simple but effective burglar alarm, which shows the

status of each sensor with a separate LED. This allows you to see at a

glance whether any doors or windows aren’t closed properly before

switching the alarm on.

Main Specifications

– Maximum number of sensors that can be

connected: 8

– Monitoring and signalling of each sensor

for activation and cable continuity

–Tamper input

–Panic pushbutton

– Exit delay: 60 seconds

– Entry delay: 60 seconds

–Power supply: 230 V AC or backup battery

– LEDs indicate:

- alarm armed

- alarm activated

- tampering

- backup battery active

– Outputs: 2 (12 V DC , 500 mA)

– Alarm duration: 60 seconds

microcontroller provides all the func-

tionality of the burglar alarm. It also

takes care of (software) filtering of

the signals at the inputs. Only after

an input has remained unchanged

for 30 milliseconds, is this new sig-

nal level passed on for processing by

the microcontroller program.

The schematic for the burglar

alarm ( Figure 1 ) is simplicity itself

and only a small number of compo-

nents are required.

when the alarm is armed and after

the exit-delay has expired, the entry-

delay (60 seconds) will begin. Under

normal circumstances, the alarm will

be deactivated with key switch S1

during the entry-delay. In the event

of a burglary, the alarm will trigger

after the entry-delay has expired.

The entry-delay can only be inter-

rupted by switching the alarm off

with key switch S1.

The special ‘panic button’ S2, can

be used for those situations — what-

ever the reason may be — that

require the alarm signal to be acti-

vated. The alarm signal will be acti-

vated immediately, independent of

the armed or disarmed status of the

burglar alarm. The alarm will be

silenced after 60 seconds or after

pressing the panic button again.

Sensors

A maximum of 8 sensors can be con-

nected to the burglar alarm. These

can be found along the left side of

Figure 1 . These sensors need to

have their contacts closed when in

the inactive state (i.e. Normally

Closed). In addition, each sensor

needs to have its tamper connection

wired as well. A power supply volt-

age of +12 V DC is available for each

sensor at the corresponding wiring

terminals (K3 through K10).

Please note: Readers Circuits have not been tested or

post-engineered by the Elektor Electronics design laboratory.

Sabotage

The purpose of tamper input K11 is

to detect tampering attempts when

the alarm is armed. Possible scenar-

ios are opening of the burglar alarm

enclosure, the cutting of cables, etc.

The tamper input needs to be nor-

mally closed (NC). A number of NC-

contacts and wires can be con-

nected in series (the cables to the

alarm sensors, for example).

This article describes a design of a burglar

alarm that can monitor up to eight sensors.

The status of each individual sensor is indi-

cated with an LED. This LED shows whether

the sensor has been activated and whether

the wiring to the sensor is in order.

Obviously, this burglar alarm also has an

input to ‘arm’ the alarm, a tamper input and

a couple of outputs to control a siren and/or a

strobe light. The alarm is also ﬁtted with a so-

called ‘panic button’.

Operation

The alarm is switched on by opening

key switch S1, at which point the so-

called ‘exit-delay’ begins. This time

delay (60 seconds) allows you to

leave the protected area without

triggering the alarm. The alarm is

deactivated when key switch S1 is

closed.

If a sensor contact is opened

Circuit description

The burglar alarm is built around the

AT89C51 microcontroller from Atmel. This

Signalling

Eight LEDs (D10 through D17) indi-

cate the status of the corresponding

Elektor Electronics

1/2004

READERS CIRCUITS

DC output X

12Vdc max 500mA

100N

10uF

63V

K12

D10

D11

D12

D13

D14

D15

D16

D17

D18

D19

D20

D21

D22

IC3

BUZ11

R22

R23

R24

R25

R26

R27

R28

R29

R30

R31

R32

R33

R34

Vcc

RST

alarm

DC output Y

12 Vdc max 500mA

tamper

+12Vdc

AT89C51

K13

alarm

tamper

P10

P00

P01

P02

P03

P11

P12

P13

P14

P15

P16

P17

BUZ11

+12Vdc

P04

P05

P06

P07

alarm

tamper

+12Vdc

1N4001

P27

P26

P25

P30

P31

alarm

P24

P32

P33

P34

P35

P36

P37

tamper

P23

+12Vdc

DIV1

IC2

alarm

ALE/P

TR1

D1-D4

tamper

78 1 2

78 0 5

PSEN

P22

P21

315mA

+12Vdc

K11

1N4001

IC1

P20

XTAL1

10uF

63V

10uF

63V

12 MHz

XTAL2

330N

100N

alarm

GND

tamper

230V

+12Vdc

470uF

63V

15Vac

4x1N4001

1N4001

33pF

12Vdc

alarm

tamper

BT1

+12Vdc

alarm

tamper

+12Vdc

020422 - 11

K3-K10

Figure 1. The alarm consists mainly of a microcontroller, a number of sensor inputs and a set of indicator LEDs.

Outputs

sensors. When the alarm has been

activated, the LED of the sensor that

caused the alarm will light up, or

ﬂash in the event of a cable failure.

When the alarm is armed, the

LED ‘alarm armed’ (D18) will flash

during the exit-delay. After the exit-

delay, the LED will light continu-

ously. D18 turns off, of course, when

the alarm is disarmed.

The LED ‘alarm triggered’ (D19)

flashes during the entry-delay and

will turn on continuously once an

actual alarm has been generated.

D19 turns off only when the alarm is

switched off with key switch S1.

When an alarm has taken place, it

can be determined afterwards

which sensor (or tamper input)

caused the alarm to trigger.

The LED ‘tamper’ (D20) lights up

when the tamper input (K11) is

opened. This LED will also continue

to be on until the alarm is switched

off.

Finally, the LED ‘battery opera-

tion’ (D22) indicates that lead-acid

battery BT1 has taken over the

power supply for the burglar alarm.

age is subsequently reduced to about 12 V

by diode D7. Voltage regulator IC2 in turn

changes this 12 V into a stabilised 5 V power

supply voltage.

In the event that the mains voltage at K1

disappears, the 12-V lead-acid battery con-

nected to K2 will immediately take over the

power supply for the circuit. The battery is

continually being charged via resistor R1 and

diode D6, when the mains voltage is present.

D7 and D8 prevent the charging current from

ﬂowing in the wrong direction.

The burglar alarm is provided, from

the viewpoint of reliability, with two

separate outputs (K12 and K13). Both

outputs are controlled by a BUZ11

(T1 and T2) and can switch up to

500 mA at 12 V. This is more than

enough for all common signal sources

such as strobe lights and sirens. If

more power is required or a signal

source with a different voltage needs

to be controlled, then a 12-V relay can

be connected directly to the alarm

output and it in turn can then switch

the signal source on and off.

Software

The software that is required for the alarm is

of a relatively simple design. The ﬂow chart

shown in Figure 2 illustrates this. This really

shows a kind of logical summary of all the

things that have been described above.

After the reset-phase of the microcon-

troller, all the LEDs are switched on for a

period of two seconds. This allows for a quick

lamp test to check if all the LEDs are func-

tional and are connected correctly.

In the source code for the software, the

values for the various timers can easily be

adjusted to suit your own requirements. It

concerns the following timers:

Power supply

The circuit is provided with its own

mains power supply. It follows the

usual design of transformer, bridge

rectifier (D1 through D4) and filter

capacitor (C1) and generates an

input voltage of about 18 V for volt-

age regulator IC1. With the addition

of diode D5 in the ground connec-

tion, the output voltage if IC1

amounts to about 12.65 V. This volt-

1/2004

Elektor Electronics

READERS CIRCUITS

Construction perils

It is recommended to build the bur-

glar alarm into two separate enclo-

sures, as illustrated in the sketch of

Figure 3 .

The larger enclosure contains the

more important part of the circuit,

including the power supply and

lead-acid battery, but excluding the

controls and indicators. This enclo-

sure needs to be mounted in a place

that is relatively difficult to access.

This enclosure actually has only

cables going in and out of it. When

building the mains power supply,

keep the relevant safety require-

ments in mind and provide sufficient

isolation and a good strain relief

(grommet) for the mains cable.

The enclosure for the LEDs and

switches needs to be mounted in an

easily accessible place, of course.

Because of the way the circuit is

designed, damaging this enclosure

will have no inﬂuence on the correct

operation of the burglar alarm (the

key switch to operate the alarm is

normally closed). It is also recom-

mended to include this enclosure in

the tamper circuit.

To prevent the burglar alarm from

being easily circumvented without

this being detected, it is necessary

that the cables to the sensors are

located in such a way that they are

difficult to access. These cables can

also be included in the tamper cir-

cuit.

A practical application of the tam-

per circuit is shown in Figure 3 . A

switch (normally closed) is also

included in the tamper loop. This

switch needs to open in the event

that the enclosure is opened or van-

dalised.

If not all sensor inputs are being

used, the alarm contact and tamper

contact of each unused sensor needs

to be shorted out.

PANIC

button pressed?

YES

panic period

active?

activate panic period

YES

panic timer

elapsed?

(>60 seconds)

panic period

active?

YES

end panic period

alarm timer

elapsed?

(>60 seconds?)

YES

alarm period

active?

end alarm period

alarm

switched on for

<60 seconds?

ALARM

switch on?

YES

entry period

elapsed

(<20 seconds?)

end alarm period

alarm contact

open?

tamper contact

open?

sabotage contact

open?

end entry period

YES

alarm period

or panic period

active?

activate entry period

activate alarm period

YES

activate outputs

020422 - 12

Figure 2. This ﬂowchart makes it obvious how the software has been designed.

timer

variable

default (s)

Incidentally, the software for the bur-

glar alarm (ref. no. 020422-11 ) can be

downloaded free of charge from

the www.elektor-electronics.co.uk

website.

panic timer

PANVAL

exit-delay timer

UITVAL

entry-delay timer

INLVAL

output X active

ALXVAL

output Y active

ALYVAL

output X

PCB

output Y

siren

lead-acid

battery

signal lamp

sabotage

(020422-1)

sabotage loop

CONTROL

INDICATORS

020422- 13

Figure 3. A practical implementation of the burglar alarm separates the control and indication

from the other parts.

Elektor Electronics

1/2004

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