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Infrared Sentry
If you have a doorway, passageway, window or
pathway – up to 25 metres wide or even more –
this nifty little project will stand guard for you.
If anyone dares intrude on its domain
it will scream long and loud!
There are many situations where we
would like to be warned if anyone is
present. Immediately, of course, we
think of security applications – intruder alarms, for example. We really
do need to know if someone has entered an area where they shouldn’t be.
Some form of detection and warning
is vital.
But there are other uses for a detection system, not necessarily used in
anger! The classic shop door buzzer
is a good example – if the shopkeeper
is busy or in the back office, he or she
might not notice a customer entering.
Customers don’t like being kept waiting . . . and then there are those who
might not be paying customers at all,
66 Silicon Chip
just waiting for an unattended counter.
Closer to home, you might like to
know if visitors are coming towards
your door long before they ring the
door bell. Advance warning will give
you the chance to quickly tidy the
room or perhaps turn the music down
so they’ll go away!
There are many other applications
but we’re sure you get the picture.
Having said all that, how are we
going to detect these intruders/customers/visitors/salesmen/etc?
We could use a passive infrared
Design by Branco Justic
Article by Ross Tester
de-tector or microwave sensor. While
very effective, they are not particularly easy to camouflage and most
really aren’t suitable for outside use.
Not only that, they are relatively
expensive
Under-carpet pressure mats have
been used for many years but these
have fallen out of favour, again mainly
due to cost but also because of their
propensity to be damaged, even under
carpet (stiletto heels were a real killer
– literally – for pressure mats!)
How about that good ol’ shop door
buzzer we mentioned earlier? Well,
until now we probably would have
dismissed this idea as well, because
of the rather high cost of such units.
But now there’s a build-it-yourself
alternative which is not only low in
cost, it’s rather more versatile than the
traditional light beam detector.
Most of the light beam detectors
we’ve seen have used a transmitter
and receiver housed in one unit, with
the light beam leaving the transmitter,
hitting a reflector and bouncing back
to the receiver. While effective, range
was somewhat curtailed by the fact
that the light had to travel twice the
distance.
This new design uses a separate
transmitter and receiver, both housed
in small (82 x 53 x 30mm) jiffy boxes.
The prototypes also had universal
mounting brackets attached to the
boxes but these could be regarded as
optional – mounting suits the application.
The circuits
While the circuit diagram of Fig.1
is shown as a complete system (transmitter and receiver) it really is two
independent components and we will
discuss it that way, starting with the
transmitter.
The heart of the transmitter is an
infrared light emitting diode, IRLED1.
Unlike a conventional LED, this produces no visible light when forward
biased. Therefore there is nothing
intruders can do to tell that there is
a beam of infrared light across their
path.
As a matter of interest, the old
smoke-across-the-beam trick you
often see in spy movies and the like
simply doesn’t work with infrared
light – unless, of course, there
is an element of visible light
(usually red ‘cause it looks good
on the screen) also in the beam.
There is no visible light at all
from this IR LED.
The infrared LED cannot be
constantly turned on otherwise
the detector in the receiver
would not work. It is pulsed
at about 38kHz. IC1a and IC1b
(two of the gates from a 4093
quad 2-input Schmitt NAND
gate) and their associated comThe transmitter PC board is tiny – this
ponents form an oscillator at
component layout and photograph will help
about 38kHz. You may wonder
you assemble it.
why two resistors (R2 & R3)
are specified: these set the oscillator
this resistor to 22Ω should increase
frequency and R3 allows tweaking if the range to more than 25 metres.
required. In practice, the system is
There is a trade-off, though, in
quite forgiving and adjustment is not
gaining extra range in this manner: a
needed. Still, it can be done.
transmitter power significantly greater
You will also note another oscilla- than that required for operation over
tor formed by R1, C1 and IC1d. This the range required may cause the beam
one runs at about 400Hz (again, not to be reflected around the room from
critical) and this “data stream” is im- other objects. It is possible that more
pressed on the 38kHz “carrier” by the than one beam path is formed and the
fourth gate in the chip, IC1c.
receiver may then not respond when
the required beam is cut.
Zener diode ZD2, transistor Q1
and associated components form a
There are other simple ways to
switch
ed constant current source
increase range – much more dramatwhich feeds the infrared LED, IRLED1. ically – which we will discuss shortly.
Therefore the LED is pulsing at 38kHz
Hang on a second! Why would you
modulated by 400Hz – which, of want a range to 25 metres or more
course, you cannot see unless your anyway? That’s one big window or
eyesight is the same as some birds!
doorway . . .
The peak current through the LED,
The reason is that this project can
set by R7, determines the range of the also be used as a perimeter alarm. With
overall system.
three small mirrors to reflect the beam
As supplied, with a value of 47Ω,
90°, you could go right around the
the range is about 17 metres. Reducing
wall of a small warehouse, storeroom,
Fig.1: both the transmitter and receiver are shown in this combined circuit diagram. The optional piezo buzzer is not
shown here but if used, simply connects to +12V and GND via the relay contacts.
April 1999 67
Fig.2: use this PC board layout diagram in conjunction with the photograph
above to help assemble the receiver PC board.
Parts List
TRANSMITTER
1 PC board, 30 x 47mm*
1 plastic case, 82 x 53 x 30mm
1 swivel bracket
1 14-pin DIL IC socket
Semiconductors
1 infrared LED
1 4093 quad 2-input
Schmitt NAND gate
1 C8550 PNP signal transistor
1 4.7V 400mW zener diode
Resistors (5% 0.25W)
2 47kΩ
1 6.8kΩ
1 3.9kΩ
1 1kΩ
1 47Ω
Capacitors
1 100µF PC electrolytic
1 0.1µF polyester
1 .001µF polyester
RECEIVER
1 PC board, 52 x 47mm*
1 plastic case, 82 x 53 x 30mm
1 swivel bracket
1 infrared receiver module
1 12V PC relay, SPDT
1 12V piezo buzzer(optional)
Semiconductors
1 C8050 NPN signal transistor
1 C8550 PNP signal transistor
1 5.6V 400mW zener diode
1 GIG power diode
2 1N60 signal diodes
1 red LED
Resistors (5% 0.25W)
1 47kΩ
3 6.8kΩ
2 470Ω
Capacitors
2 100µF 16VW PC electrolytic
3 10µF 16VW PC electrolytic
* supplied as one board
68 Silicon Chip
office, etc to catch anyone breaking in
any window or door, or even through
the wall itself!
The receiver
Just as the heart of the transmitter is
one dedicated component, so the heart
of the receiver is RX1, a dedicated
infrared receiver module.
This module has just three connections – two for power and an output.
While ever a valid infrared signal (ie,
38kHz) is being received, the output
voltage remains low. Transistor Q2,
therefore, conducts. However, it’s
not just the 38kHz signal that’s being
received – the 38kHz is modulated by
the 400Hz signal. The module passes
this 400Hz signal which appears at
the collector of Q2.
Following Q2 is a voltage-doubling
rectifier circuit (D1 & 2, C5 & 6) which
converts the 400Hz AC signal to DC.
LED2 is then forward biased, not
only lighting itself in the process but
supplying bias for Q3. Q3 conducts,
pulling in the relay.
But why go to all this trouble of
transmitting 400Hz along with the
carrier, then detecting it, rectifying
it and so on? Why not simply detect
the 38kHz carrier? After all, it is just
as surely cut by someone walking
through it as a modulated 38kHz
carrier?
The reason is twofold. The first
problem is that a savvy intruder, once
they knew what type of infrared detector you were using, could possibly
bypass the system by simply firing
a beam from just about any infrared
remote controller (probably the one
they knocked off from the house next
door!). If the system didn’t have to do
any signal handling, it would probably
react to any infrared signal, regardless
of encoding.
Second, and a little more downto-earth, is that the system could
be prone to either electrical or even
light-induced noise if operated in a
simple mode. As it is, the circuitry
is quite good at rejecting noise and is
quite reliable.
OK, that’s what happens when the
receiver is receiving. What happens
when someone cuts the beam?
Very little! The output of the infrared module goes high, cutting off Q2.
Therefore there is no signal at Q2’s
collector, so LED2 and Q3 lose their
bias. When that happens, the relay
drops out.
To ensure no harm is done to Q3
or other semiconductors, a diode is
connected across the relay coil. When
Q3 stops conducting and the field
around the relay coil collapses, a quite
high voltage spike can be induced
in the coil, with opposite polarity to
the voltage which powered the coil
originally. This forward biases D3,
effectively shorting the coil.
In the prototype, a low voltage
piezo buzzer was glued into the case
and connected between the positive
and negative supply with the appropriate relay contacts in series. This is
reminiscent of the shop door buzzers
of old – the buzzer sounds when ever
anyone cuts the beam. If you walk
slowly enough through the beam, it
actually sounds twice. Guess why?
Oh, come on, it’s not that hard . . .
Of course, you don’t need to fit a
buzzer. You can wire the relay contacts
to do just about anything you want to
(short of setting off a man trap, because
that’s illegal). Just remember that the
contacts of the relay aren’t rated for
mains voltages, so you should limit
your circuitry to low voltage and
reasonably low currents.
Construction
Construction is very simple but, as
always, check the PC board first for
any etching defects (rare, but they do
happen).
Next, you’re going to have to separate the receiver and transmitter
PC boards. For economy, both are
supplied on the one board but the
cut mark is clearly shown. Use a
fine-toothed hacksaw and be sure to
protect the PC pattern from damage if
you grip the board in a vyce.
It’s up to you which board you
assemble first. All component positions are clearly marked but take
This photo shows the
method of mounting the
transmitter PC board in
its jiffy box. The board
snaps into place on lugs
moulded into the box
walls with no screws or
nuts needed. Holes must
be first drilled in the
case for the IR LED and
also the power leads.
Note that these photos
show early prototypes.
care when placing any polarised
components. There are a couple of
side-by-side components which are
opposite-way-around to each other.
Also make sure you don’t mistake the
power diode, small signal diodes and
zener diode.
It is possible, though difficult, to
insert the infrared detector module
the wrong way around. Pinouts are
marked on the circuit and on the PC
board. To be safe, we would leave the
detector until all other components
are inserted and soldered in.
Testing
If you’re going to use the piezo
buzzer, we strongly suggest you leave
it until the very last thing, or at least
heavily muffle it! It’s very annoying
to have it going off all the time while
setting it up.
Testing is probably easiest carried
out before mounting the assembled
PC boards in their jiffy boxes.
Connect a 12V supply (a battery
is fine) to both the transmitter and
receiver boards and aim one at the
other. You should hear the relay click
in when they are aimed at each other
and drop out when you turn either
one away.
If that happens, you can proceed to
mount the boards in their cases. The
photographs give a good idea of how
this was done. You may have other
ideas, particularly if you have a specific location in mind which requires
some ingenuity!
If they don’t work? One board at a
time, carefully check your soldering
(especially bridges between close
contacts) and component placement/
orientation.
If all appears OK, check voltages.
The supply to the IR receiver module
(pin 2) should be about +5.6V (plus
or minus a tad). On the transmitter
board, the easiest voltage check (after
the supply, that is) is the voltage across
ZD2 – about 4.7V.
If basic voltages appear OK, check
the output voltage from pin 1 of the
receiver module. With the transmitter
firing, it should be about +2.5V. With
no transmitter, it should be about
+5.6V.
If these voltages are OK, the error
is further down the track – possibly
Q2 or Q3 are inserted the wrong way
around (though that’s hard because
the orientation is shown on the PC
board overlay).
Perhaps D1 or D2 are back-to-front?
If you suspect the relay, that can be
checked by carefully shorting Q3’s
collector and emitter. It should pull in.
Mounting the boards
Even if you buy the complete kit, the
jiffy boxes supplied
will not be drilled.
The boxes are actually
used upside-down
– the lid of the box
becomes the base.
You will need to drill
holes in the bottom of
the receiver box for
the infrared receiver
module, the signal
LED and the piezo
(if used). The power
supply wires, along with any external
connection wires, can emerge through
suitable holes drilled in the box lid.
Similarly, the transmitter will need
a hole for the IR LED and a pair in the
lid for the power wires.
The prototype boxes also had swivel mounting brackets attached to the
base (ie, the box lid) to make mounting and aiming much simpler. At the
price, we think they’re good value.
Mounting the system
Assuming you’ve used the jiffy boxes and swivel brackets, all you need
to do is determine which aperture
you want to protect with this system,
mount the units so that they face each
other – and that could be it.
When you apply power there
should be a brief squeak from the
piezo buzzer and the system will sit
there until the beam is broken, at
which time the buzzer should squark
its head off!
If you haven’t used the jiffy boxes
and brackets, you’ll need to work
out a method of mounting. But it’s
straightforward – as long as the IR
LED points to the IR receiver (and as
long as they’re not too far apart) the
system should work.
By the way, when protecting a passageway or similar access route, it’s
normal to mount the system down
April 1999 69
Left: the receiver PC
board mounted in its
case (it actually screws
to the lid which
becomes the base!)
This is shown fitted
with the optional piezo
buzzer, glued into the
bottom of the case.
Right: using the
optional swivel
bracket makes
mounting and aiming
both the transmitter
and receiver a lot
easier.
low (to catch anyone crawling)
but not so low as to have pets
or other small animals set it off.
Increasing the range
We mentioned before a range
of up to 17m should be possible
with the units as described, or
25m if R7 in the transmitter is
reduced to 22Ω.
This is a pretty handy sort
of range, you’d agree. But wait,
there’s more!
If fitted with simple optics,
the range can be dramatically
increased. A simple glass lens
placed at the focal point of the
IR receiver module will give
you double, triple and even
more range. The same thing
applies to a lens at the focal
point of the IR diode.
Alternatively, using a parabolic reflector will also give
an amazing increase in range.
In this case, the IR LED and the
IR receiver are turned around
t
Shop soiled bu
!
HALF PRICE
to face into the reflectors and are
mounted at their focal point. Aiming
becomes a little more tricky over
longer ranges but it can be done.
Finally, while the system is relatively free from the effects of ambient light, any system such as this is
usually improved with the used of
internally blackened tubes.
Neither length nor diameter are really
important. If you’re looking for very
cheap tubes, try toilet roll holders. SC
Where To Buy The Parts
Parts for the Infrared Sentry are
available from Oatley Electronics. The
PC board(s) and all on-board components
with the exception of the relay are
$17.00 while the relay and buzzer are
each priced at $3.00
To complete the project, a set of
two jiffy boxes, complete with swivel
brackets and labels, is available for
$6.00. Oatley Electronics’ phone number
is (02) 9584 3563; fax (02) 9584 3561;
email oatley<at>world.net
14 Model Railway Projects
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