Tune that dial - Index of

22
Luminous house number
During the dark autumn and winter months in
particular you’ll fi nd that ordinary house numbers
are more diffi cult to read, especially if your home
is further back from the road. If you want to avoid
that your family, friends and deliverymen drive
past your home, you should build this electronic
version. At the same time you can show the world
that you’re an avid electronics hobbyist! It can
easily be put together during a rainy weekend.
For a change this isn’t a hi-tech circuit, but instead is
quite straightforward and it comes in useful too. House
numbers can sometimes be diffi cult to read. In this
article we show you how to build a luminous version
that uses large 7-segment displays made by Kingbright,
which have digits with a height of 57 mm. There are
of course other manufactures that produce similar
displays and it is not essential that you use the same
displays that we’ve used here. You do have to keep
an eye on the number of LEDs used to make up each
segment (7 in total) and on the forward voltage drop
when the segments are lit, so that you don’t exceed the
maximum current through the LEDS, but more on this
later.
The required segments of the display are connected
to the supply via resistors. When you connect all
segments you obtain the digit 8, for a 0, 6 or 9 one of
the segments is disconnected and a 1 for example only
needs two segments lit. In this way you can display all
of the digits from 0 to 9. Each segment in the display
consists of a number of LEDs connected in series (in
our module this is 4). In the display used by us the
anodes of the fi rst LED in every segment are connected
together, hence the term ‘common anode display’.
The common anode obviously has to be connected to
the positive supply. The ‘free’ ends of the segments
that have to be lit are connected via a resistor to the
negative supply (ground). The current through the
segments and hence their brightness is dependent on
the value of the resistors.
For the power supply we can use a doorbell
transformer, which is often already present and which
generally supplies 8 Volts AC. The alternating voltage
of the doorbell transformer is rectifi ed so that the
peak current through the LEDs doesn’t exceed their
limits. The rectifi er circuit consists of a standard bridge
rectifi er followed by a smoothing capacitor. These
can supply the right voltage for a one or two-digit
house number. For house numbers of three digits or
Electronics inside out !
more you will have to experiment with the resistors
to fi nd what values give the correct current through
the segments. But for house numbers like 88, 80 or
90 (many segments will be lit) this also applies. For
every extra segment that is lit the current consumption
increases, which causes the supply voltage to drop by
an amount dependent on the quality of the doorbell
transformer. It is therefore diffi cult to give an exact
fi gure for the value of the current limiting resistors
in the circuit; the 47 Ω mentioned here should be
taken as a guideline to give a current of about 22 mA
(which is a safe value for the type of display we used)
when an 8 Volt doorbell transformer is used. If you
want to be sure that you’re not exceeding the current
limit you should place an ammeter in series with the
supply and divide the measured current by the number
of segments that are lit. If the result is greater than
22 mA you should increase the values of the resistors,
and reduce their value if the result is less than 22 mA.
To determine what current fl ows through a segment
you could of course measure the voltage across the
resistor connected to it and use Ohm’s law to calculate
the current.
For the installation it’s best to split the circuit into
two parts: the bridge rectifi er (B1) and the smoothing
capacitor (C1) near the transformer and the display
and resistors in a weather proof box next to the front
door. When your house number consists of two digits
you should build two display modules (shown in the
circuit diagram inside the dotted lines). The display
section should be connected to the power supply
section using a twin-cored cable such as loudspeaker
cable. Try to use a cable with reasonably thick cores,
especially if a lot of segments are lit. For ‘88’ the
current requirement is 0.3 A!
The house number also has to be legible in strong
sunlight. To achieve a better contrast you can place
a piece of transparent plastic (with the same colour
as the LEDs) in front of the display, or you could stick
transparent tape over the segments (using the right
colour). In the latter case you should only cover the
active segments so that you can still see the number
without an applied voltage.
The table below shows you which resistors need to be
soldered to produce each digit:
0 R1,R2,R3,R4,R5,R6
1 R2,R3
2 R1,R2,R4,R5,R7
3 R1,R2,R3,R4,R7
4 R2,R3,R6,R7
5 R1,R3,R4,R6,R7
6 R1,R3,R4,R5,R6,R7
7 R1,R2,R3
8 R1,R2,R3,R4,R5,R6,R7
9 R1,R2,R3,R4,R6,R7
8V
B1
B80C1500
i-TRIXX collection - 12/2006
C1
1000µ
16V