This is only a preview of the September 2021 issue of Practical Electronics. You can view 0 of the 72 pages in the full issue. Articles in this series:
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Night
Keeper
Lighthouse
By Andrew Woodfield
The Night Keeper Lighthouse briefly lights up the darkness, to keep
children’s dreams from running aground on dangerous shores. This is an
excellent project for beginners; it’s easy to build, and you will learn several
important aspects of electronic circuit theory.
M
any readers will have children or even
grandchildren who from time to time gaze enquiringly at electronic parts and gizmos you’re working
with on the bench.
At moments like these, it’s useful to have a simple project
available to encourage the next generation to join in, have
fun and take up the hobby.
When my grandchildren were planning a visit recently,
I was asked if I could help the 8-year-old build ‘something
electronic’. Does this sound familiar?
Searching for a circuit suitable for children, it’s
essential that they can build it reasonably quickly,
before they lose interest. Equally, it should be useful
enough to gain parental approval.
I have had a blinking light circuit running on the
shelf above my workbench for several years. I built it
while testing some ideas for discrete high-efficiency
boost power supplies. The ‘rat’s nest’ of parts was
built on a scrap of prototype board. These days, I
use it for the occasional end-of-life 1.5V cell. It’s a
simple way to use up the very last whiff of energy
from such near-dead batteries.
Rather than just building a blinking light, I thought
I could make it a little more useful and exciting
with a few simple improvements. First, I designed
a printed circuit board (PCB) to make it easier for
children (and parents, grandparents or caregivers)
to build. That PCB allowed me to mimic a widely
recognisable object, and make it more attractive
and interesting.
It also suggested a few other applications, which
will be noted later.
This, then, is the Night Keeper Lighthouse.
Building it is well within the capabilities of
a bright 10-to 12-year-old, or perhaps even
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younger with some adult assistance. Since a soldering iron
is required, they will need close adult supervision and a
well-ventilated workspace. A kitchen table with a similar
clear workspace of about one square metre is perfect; cover
it with a cloth or some cardboard to protect the surface.
Circuit description
This simple and well-known oscillator circuit (shown in
Fig.1) consists of two transistors, a white LED, and a few
passive components. It brightly flashes the LED once every
second for many months from a single 1.5V cell. Even
a near-exhausted battery can power the LED for a
month or two.
The two transistors at the heart of the device operate as a highly efficient regenerative oscillator.
When power is first applied, the voltage on the base
of Q1 (Va) begins to rise slowly as the 10MΩ resistor
charges the 330nF capacitor from the battery. When
Va reaches about 0.6V, the base-emitter junction of
Q1, which acts much like a silicon diode, becomes
forward-biased and begins to conduct.
Meanwhile, the 10k
10kΩ resistor has quickly charged
100
the 100µF
capacitor to close to the battery voltage.
That’s about 1.5V for a new cell. This produces a
voltage across LED1 (Vc) very close to 1.5V. However,
LED1 cannot light up yet, because white LEDs need
more than 2.5V to operate.
As soon as Q1 begins to turn on, its increasing
base-emitter current causes its collector current to
rise still faster due to the transistor’s current gain
(beta or hFE) being greater than unity. In turn, this
results in Q2’s base-emitter junction starting to
conduct too. The instant Q2 begins to conduct,
voltage Vb starts to rise due to the current passing from Q2’s emitter to its collector.
Practical Electronics | September | 2021
3.5V
6.3mA
2.8V
5.4mA
2.1V
4.5mA
1.4V
3.6mA
0.7V
2.7mA
0V
SC NIGHT
Night
Kepper
KEEPER
1.8mA
-0.7V
Fig.1: the Night Keeper uses a two-transistor
oscillator to drive a charge pump based on the 100µF
electrolytic capacitor and the diode junction of white
LED1. Once per second or so, the point labelled ‘Vc’
will shoot up to around twice the battery voltage
(about 3V), providing enough voltage to light the LED
brightly for a few tens of milliseconds.
0.9mA
-1.4V
0mA
-2.1V
-0.9mA
-2.8V
-1.8mA
-3.5V
-4.2V
4.8s
5.1s
5.4s
5.7s
6.0s
6.3s
6.6s
6.9s
7.2s
7.5s
-2.7mA
7.8s
Q2 amplifies Q1’s collector current still further, as a Fig.2: this simulation shows how the voltages at Va (cyan), Vb
result of its own current gain. The increasing voltage (green) and Vc (red) in Fig.1 change over time. Va ramps up, and
Vb causes Va to rise in ‘lock-step’ as the rise is coupled then all three voltages suddenly shoot up, at which point the
through the 330nF capacitor. This triggers a swift current through LED1 (blue) spikes, until the voltages drop and
‘avalanche’ effect through Q1 and Q2, causing them the process begins again.
to both switch on fully as a result of their combined
current gain.
330nF capacitor takes to charge from −1V to about 0.6V
Consequently, the voltage at Vb rises suddenly and via the 10MΩ resistor.
abruptly up to the full battery voltage, around 1.5V with
Note that while the parts list suggests BC54x and BC55x
a new cell. Since Vb is now suddenly at 1.5V, Vc rises in types, you could also use a 2N3904, 2N2222 or 2SC1815
‘lock-step’ via the 100µF capacitor to give about 3V at Vc. for the NPN transistor; and a 2N3906, 2N2907 or 2SA1015
This is enough to forward-bias LED1, lighting it up. The for the PNP. Almost any pair of NPN and PNP transistors
charge stored in the 100µF capacitor is then dumped into will work, but keep in mind that pinouts can vary.
LED1, giving a brief bright flash of light.
This process is demonstrated in the simulation traces Construction
shown in Fig.2. Va is shown in cyan, Vb in green and Vc in If all of the parts are ready to hand, the Night Keeper
red. The current through LED1 is in blue. You can see that Lighthouse should take about an hour or so to build. Exall three voltages rise rapidly at the same time, coinciding pect younger children to take longer. Splitting the build
with the spike in LED1’s current.
into two parts, fitting the resistors and capacitors in one
While LED1 is lit, the 330nF capacitor keeps Q1 switched brief session and the remaining parts in a second, makes
on and in doing so, discharges through its base-emitter construction easier and suits the shorter attention spans
junction. It manages to keep Q1 on for about 30ms. How- of young children much better.
ever, as soon as Va falls below 0.6V, Q1 begins to turn off.
Completing the project with the addition of the battery
This causes Q2 to abruptly turn off too. The result is Vb holder and base could be managed in a brief third session.
suddenly falls from 1.5V to 0V. Va, via the 330nF capacitor,
The Night Keeper Lighthouse is built on a PCB coded
then drops from 0.5V to −1V.
08110201, which measures 64 × 91mm and is available
It goes negative because, just before Q1 and Q2 switch from the PE PCB Service. Before starting, snap or cut off the
off, Va is at around 0.5V while Vb is about 1.5V. So when circular base from the side of the lighthouse, and file or sand
Vb drops to 0V, that is coupled through the capacitor and both edges smooth. It’s a good idea to score along the cut
0.5V – 1.5V = −1V.
line before snapping it. To do that, run a sharp knife along
At this point, the entire cycle begins again. The result is the line joining the small ‘mouse bite’ holes several times.
a very efficient regenerative oscillator which produces a
Set the base aside for now, then refer to the PCB overlay
brief, but bright flash from the white LED about once every diagram (Fig.3) and construction guide (Fig.4) to see which
second or two. This is largely determined by the time the parts need to go where. All of the parts, except for the battery
Parts list – Night Keeper Lighthouse
1 PCB, code 08110201, 64 x 91mm, from the PE PCB Service
1 BC547, BC548 or BC549 NPN transistor
[Jaycar ZT2154 or Altronics Z1042]
1 BC557, BC558 or BC559 PNP transistor
[Jaycar ZT2164 or Altronics Z1055]
1 5mm white high-brightness LED
[Altronics Z0876E or Jaycar ZD0190]
1 100µF 16V electrolytic capacitor
[Jaycar RE6130 or Altronics R5123]
Practical Electronics | September | 2021
1 330nF MKT, ceramic or greencap capacitor
(code 0.33, 330n or 334)
1 PCB-mount AA or AAA cell holder [AA: Altronics S5029 or
Jaycar PH9203; AAA: Altronics S5051; Jaycar PH9261]
Glue or double-sided foam tape to fix cell holder to back of
main PCB
Resistors (all 1/4W, 1% or 5%) (see overleaf for colour codes)
1 10MΩ
1 10kΩ
2 1kΩ
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To join the two PCBs together, first
‘tack’ them with solder and then
run a bead of solder along the
tinned copper tracks on the PCB. It
won’t let go in a hurry!
Bend the legs of each resistor in turn with a pair of fine
needle-nose pliers or a bending jig, so they neatly fit through
the holes for each component in the PCB. Insert them, one
by one, in turn, spreading the wire leads apart slightly to
hold them in place. They can be fitted either way around.
Turn the PCB over and solder both leads to the pads. Then
trim off the leads flush with the solder joint using a pair
of sharp side-cutters.
Next, fit the 330nF capacitor. It may be either a mylar,
MKT or ceramic type. Then install the electrolytic capacitor,
and solder and trim the leads in the same manner. Make
sure that the longer lead of the electrolytic goes into the pad
marked ‘+’ on the PCB. The striped side of the can should
be opposite the ‘+’ symbol.
Now it’s time to fit the two transistors. Q1 is an NPN
transistor while Q2 is a PNP transistor. Each transistor
must be fitted in the correct location. They are generally not
pushed right down on the PCB, but rather, left with leads
sticking out by about 5-10mm. This distance is not critical.
You will probably find it helpful to spread the three leads
of each transistor slightly apart before inserting them into
the PCB, making sure the flat face is oriented as shown.
Once you have pushed the leads through the PCB, spread
them apart a little more on that side to hold them in place
before inverting the PCB to solder them to the PCB. Again,
trim the leads once soldering is completed.
Now mount the white LED at the top of the board. It has
a slight flat edge on one side. The LED should be inserted
so this matches the shape printed on the PCB overlay for
+
Fig.3: the PCB is made of two parts, the lighthouse itself and
its round base, complete with dangerous rocks! Snap or cut
them apart
before fitting the components where shown here.
Ratherthan attaching the cell holder via wire leads (as
shown here, which you could do), we instead recommend
mounting the holder on the back of the board.
holder, mount on the top side (the side with the component
outlines and part numbers), with their leads soldered on
the opposite side. The battery holder is mounted the other
way around, and that should be done last.
Begin by fitting the four resistors, which can be identified by the coloured bands as shown. 1% resistors usually
have five bands, while 5% resistors typically have four.
Both possibilities are shown.
LED1
White LED
Align flat on LED with PCB overlay
10k resistor, 5% or 1%
Brown - Black - Orange - Gold
or
Brown - Black - Black - Red - Brown
10M resistor, 5% or 1%
Brown - Black - Blue - Gold or
Brown - Blk - Blk - Green - Brown
BATT+
100F electrolytic ‘can’ capacitor
Align longer lead with PCB +
(Stripe on opposite side from +)
1k resistor, 5% or 1%
Brown - Black - Red - Gold
or
Brown - Black - Black - Brown - Brown
+
Q2
BC557 (PNP)
Align shape with PCB overlay
557
330nF MKT capacitor
Fit this capacitor either way
Q1
BC547 (NPN)
Align shape with PCB overlay
1k resistor, 5% or 1%
Brown - Black - Red - Gold
or
Brown - Black - Black - Brown - Brown
547
Fig.4: in case it isn’t clear from Fig.3 which part goes
where on the board, here is what each component looks
like. Just follow the arrow to see where it goes. You can
match up the part orientations to the drawings, too; the
five components where orientation matters are LED1, Q1,
Q2, the electrolytic (can-shaped) capacitor and the battery
holder. The rest don’t care which way around they go.
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BATT-
AA or AAA Cell Holder
Glue or double-sided tape to the OTHER
(copper) side of the PCB
+ (red) lead goes near LED1
- (black) lead goes to ‘Batt-’
Practical Electronics | September | 2021
Here’s a side-on view showing the two
boards soldered together and the battery
holder in position. OK, we cheated a bit: we
found that the stiff tinned wire was sufficient
to hold it in place without glue or tape.
the LED. The longer anode lead will be
on the opposite side to the flat.
Carefully check that all of the parts
are correctly located, and that all of the
component leads have been soldered
and trimmed. Check also that there are
no solder splashes which would cause
short circuits.
The battery holder can then be mounted on the back of the PCB. A standard AA
cell holder is sufficiently large that the
end of the battery holder allows the lighthouse to sit it on the edge of a shelf or a book, as
shown in the photo. The battery provides an ideal weight
to hold the lighthouse vertical, useful for tight corners of
a bedroom or office.
The wire tails of some battery holders will fit precisely
into the holes provided on the PCB. The positive (+) lead
should go into the hole nearest the top of the PCB, adjacent
to the LED. Other battery holder leads may need to be bent
slightly to fit. Use a pair of needle-nosed pliers to bend the
wires gently into the appropriate shape to fit neatly.
Ideally, space the battery holder off the conductor-side
of the PCB by about 3mm. This provides enough space to
solder the two wire connections of the battery holder to
the correct pads on the rear of the PCB.
Attaching the base
Alternatively, the circular base PCB can be added. This
features a ‘rock-like’ overlay to add to the overall effect, and
allows the Night Keeper to be placed on a flat surface. This
part of the build may require additional adult assistance
to complete – two hands to hold everything in the right
place, the other two to apply solder and the soldering iron.
Begin by briefly soldering two small ‘blobs’ of solder at
each end of the lower tinned edge of the lighthouse PCB.
Place this on the tinned strip located on the upper surface
of the circular base PCB. The main PCB should be approximately central and vertical on top of the base.
Touch the soldering iron to the two ‘blobs’ of solder to
‘tack’ the two boards together.
Repeat this if necessary, reapplying the soldering iron
briefly to each tacking point while adjusting the main PCB
slightly, until the main board is precisely vertical and
centred on the base.
Then apply further blobs of solder with the iron along
the join, keeping the two boards in their final position.
Finally, run the soldering iron down the tack seam to
smooth the join and tidy its appearance.
Operation
Have you noticed that there’s no power switch? The circuit uses such a tiny current, a switch is unnecessary. The
battery life in use is similar to that of the shelf-life of the
battery. A new non-alkaline AA battery can run the Night
Keeper for over a year.
Hopefully, the faces of the new builders will light up
as brightly as the Night Keeper just as soon as they insert
the battery. As soon as the battery is inserted, the circuit
will start to blink.
Note that you could use a AAA battery holder and cell
instead of the AA type. In that case, you can expect the cell
to last closer to six months. The battery life you achieve
Practical Electronics | September | 2021
You don’t have to solder the main PCB
to the base: the weight of the AA battery
holder will ensure it stays in place
‘hanging’ over the edge of a bookshelf.
will vary depending on the battery type
(heavy-duty, alkaline...) and on its condition when first inserted (new, slightly
used or near-exhausted).
Using the Lighthouse
The Night Keeper makes a useful bright
night-light for children. But keep in
mind that flashing lights can disturb
sleep, especially if they’re aimed at
one’s face.
Also, because of the brightness of
some high-efficiency white LEDs, the
Night Keeper should not be placed where the LED will shine
directly into any young and especially sensitive eyes. It’s
preferable to locate the Night Keeper so that the LED light
shines slightly upwards or at right-angles, perhaps onto
an adjacent wall. Such arrangements are generally more
effective for use as a night light anyway.
Older constructors may find, as I did, that the Night
Keeper can be useful for locating things in the night, for
children and adults alike.
Suitably mounted near a door, a light switch or placed on
a shelf, it can help guide your way to a location or around
furniture in the depths of the darkest of nights. Just like a
real lighthouse!
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
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