However
effective a domestic alarm system may be, it’s invariably better if it
never goes off, and the best way to ensure this is to make potential
burglars think the premises are occupied. Indeed, unless you own old
masters or objects of great value likely to attract ‘professional’
burglars, it has to be acknowledged that the majority of burglaries are
committed by ‘petty’ thieves who are going to be looking more than
anything else for simplicity and will prefer to break into homes whose
occupants are away.
Rather than simply not going on holiday –
which is also one solution to the problem (!) – we’re going to suggest
building this intelligent presence simulator which ought to put
potential burglars off, even if your home is subjected to close
scrutiny. Like all its counterparts, the proposed circuit turns one or
more lights on and off when the ambient light falls, but while many
devices are content to generate fixed timings, this one works using
randomly variable durations.
So
while other devices are very soon caught out simply by daily
observation (often from a car) because of their too-perfect regularity,
this one is much more credible due to the fact that its operating times
are irregular. The circuit is very simple, as we have employed a
microcontroller – a ‘little’ 12C508 from Microchip, which is more than
adequate for such an application. It is mains powered and uses
rudimentary voltage regulation by a zener diode.
A relay is used
to control the light(s); though this is less elegant than a triac
solution, it does avoid any interference from the mains reaching the
microcontroller, for example, during thunderstorms. We mustn’t forget
this project needs to work very reliably during our absence, whatever
happens. The ambient light level is measured by a conventional LDR
(light dependent resistor), and the lighting switching threshold is
adjustable via P1 to suit the characteristics and positioning of the
LDR.
Note that input GP4 of the PIC12C508 is not analogue, but
its logic switching threshold is very suitable for this kind of use. The
LED connected to GP1 indicates the circuit’s operating mode, selected
by grounding or not of GP2 or GP3 via override switch S1. So there are
three possible states: permanently off, permanently on, and automatic
mode, which is the one normally used. Given the software programmed into
the 12C508 (‘firmware’) and the need to generate very long delays so
as to arrive at lighting times or an hour or more, it has been
necessary to make the MCU operate at a vastly reduced clock frequency.
In
that case, a crystal-controlled clock is no longer suitable, so the
R-C network R5/C3 is used instead. For sure, such a clock source is
less stable than a crystal, but then in an application like this, that
may well be what we’re after as a degree of randomness is a design
target instead of a disadvantage. Our suggested PCB shown here takes
all the components for this project except of course for S1, S2, and
the LDR, which will need to be positioned on the front panel of the
case in order to sense the ambient light intensity.
The PCB has
been designed for a Finder relay capable of switching 10 A, which ought
to prove adequate for lighting your home, unless you live in a replica
of the Palace of Versailles. The program to be loaded into the 12C508
is available for free download from the Elektor website as file number
080231-11.zip or from the author’s own website: www.tavernier-c.com. On completion of the solder work the circuit should work immediately and can be checked by switching to manual mode.
The
relay should be released in the ‘off’ position and energized in the
‘on’ position. Then all that remains is to adjust the day/night
threshold by adjusting potentiometer P1. To do this, you can either use a
lot of patience, or else use a voltmeter – digital or analogue, but
the latter will need to be electronic so as to be high impedance –
connected between GP4 and ground. When the light level below which you
want the lighting to be allowed to come on is reached, adjust P1 to read
approximately 1.4 V on the voltmeter.
If this value cannot be
achieved, owing to the characteristics of your LDR, reduce or increase
R8 if necessary to achieve it (LDRs are known to have rather wide
production tolerances). Equipped with this inexpensive accessory, your
home of course hasn’t become an impregnable fortress, but at least it
ought to appear less attractive to burglars than houses that are plunged
into darkness for long periods of time, especially in the middle of
summer. (www.tavernier-c.com)
COMPONENTS LIST
Resistors
R1 = 1k 500mW
R2 = 4k7
R3 = 560R
R4,R6 = 10k
R5 = 7k5
R 7 = LDR
R8 = 470k to 1 M
P1 = 470k potentiometer
Capacitors
C1 = 470µF 25V
C2 = 10µF 25V
C3 = 1nF5
C4 = 10nF
Semiconductors
D1,D2 = 1N4004
D3 = diode zener 4V7 400 mW
LED1 = LED, red
D4 = 1N4148
T1 = BC547
IC1 = PIC12C508, programmed, see Downloads
Miscellaneous
RE1 = relay, 10A contact
S1 = 1-pole 3-way rotary switch
F1 = fuse 100 mA
TR1 = Mains transformer 2x9 V, 1.2 -3 VA
4 PCB terminal blocks, 5 mm lead pitch
5 solder pins
Downloads:
The PCB layout can be downloaded free from our website www.elektor.com; file # 080231-1.
The source code and .hex files for this project are available free on www.elektor.com; file # 080231-11.zip.
effective a domestic alarm system may be, it’s invariably better if it
never goes off, and the best way to ensure this is to make potential
burglars think the premises are occupied. Indeed, unless you own old
masters or objects of great value likely to attract ‘professional’
burglars, it has to be acknowledged that the majority of burglaries are
committed by ‘petty’ thieves who are going to be looking more than
anything else for simplicity and will prefer to break into homes whose
occupants are away.
Rather than simply not going on holiday –
which is also one solution to the problem (!) – we’re going to suggest
building this intelligent presence simulator which ought to put
potential burglars off, even if your home is subjected to close
scrutiny. Like all its counterparts, the proposed circuit turns one or
more lights on and off when the ambient light falls, but while many
devices are content to generate fixed timings, this one works using
randomly variable durations.
while other devices are very soon caught out simply by daily
observation (often from a car) because of their too-perfect regularity,
this one is much more credible due to the fact that its operating times
are irregular. The circuit is very simple, as we have employed a
microcontroller – a ‘little’ 12C508 from Microchip, which is more than
adequate for such an application. It is mains powered and uses
rudimentary voltage regulation by a zener diode.
A relay is used
to control the light(s); though this is less elegant than a triac
solution, it does avoid any interference from the mains reaching the
microcontroller, for example, during thunderstorms. We mustn’t forget
this project needs to work very reliably during our absence, whatever
happens. The ambient light level is measured by a conventional LDR
(light dependent resistor), and the lighting switching threshold is
adjustable via P1 to suit the characteristics and positioning of the
LDR.
Note that input GP4 of the PIC12C508 is not analogue, but
its logic switching threshold is very suitable for this kind of use. The
LED connected to GP1 indicates the circuit’s operating mode, selected
by grounding or not of GP2 or GP3 via override switch S1. So there are
three possible states: permanently off, permanently on, and automatic
mode, which is the one normally used. Given the software programmed into
the 12C508 (‘firmware’) and the need to generate very long delays so
as to arrive at lighting times or an hour or more, it has been
necessary to make the MCU operate at a vastly reduced clock frequency.
In
that case, a crystal-controlled clock is no longer suitable, so the
R-C network R5/C3 is used instead. For sure, such a clock source is
less stable than a crystal, but then in an application like this, that
may well be what we’re after as a degree of randomness is a design
target instead of a disadvantage. Our suggested PCB shown here takes
all the components for this project except of course for S1, S2, and
the LDR, which will need to be positioned on the front panel of the
case in order to sense the ambient light intensity.
The PCB has
been designed for a Finder relay capable of switching 10 A, which ought
to prove adequate for lighting your home, unless you live in a replica
of the Palace of Versailles. The program to be loaded into the 12C508
is available for free download from the Elektor website as file number
080231-11.zip or from the author’s own website: www.tavernier-c.com. On completion of the solder work the circuit should work immediately and can be checked by switching to manual mode.
The
relay should be released in the ‘off’ position and energized in the
‘on’ position. Then all that remains is to adjust the day/night
threshold by adjusting potentiometer P1. To do this, you can either use a
lot of patience, or else use a voltmeter – digital or analogue, but
the latter will need to be electronic so as to be high impedance –
connected between GP4 and ground. When the light level below which you
want the lighting to be allowed to come on is reached, adjust P1 to read
approximately 1.4 V on the voltmeter.
If this value cannot be
achieved, owing to the characteristics of your LDR, reduce or increase
R8 if necessary to achieve it (LDRs are known to have rather wide
production tolerances). Equipped with this inexpensive accessory, your
home of course hasn’t become an impregnable fortress, but at least it
ought to appear less attractive to burglars than houses that are plunged
into darkness for long periods of time, especially in the middle of
summer. (www.tavernier-c.com)
COMPONENTS LIST
Resistors
R1 = 1k 500mW
R2 = 4k7
R3 = 560R
R4,R6 = 10k
R5 = 7k5
R 7 = LDR
R8 = 470k to 1 M
P1 = 470k potentiometer
Capacitors
C1 = 470µF 25V
C2 = 10µF 25V
C3 = 1nF5
C4 = 10nF
Semiconductors
D1,D2 = 1N4004
D3 = diode zener 4V7 400 mW
LED1 = LED, red
D4 = 1N4148
T1 = BC547
IC1 = PIC12C508, programmed, see Downloads
Miscellaneous
RE1 = relay, 10A contact
S1 = 1-pole 3-way rotary switch
F1 = fuse 100 mA
TR1 = Mains transformer 2x9 V, 1.2 -3 VA
4 PCB terminal blocks, 5 mm lead pitch
5 solder pins
Downloads:
The PCB layout can be downloaded free from our website www.elektor.com; file # 080231-1.
The source code and .hex files for this project are available free on www.elektor.com; file # 080231-11.zip.
