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It is adequate for use with
things such as watches, small
engines, or any small mecha-
nisms that have a repeating ac-
tion. The use of LEDs keeps the
cost to a minimum, enables an
extremely simple circuit to be
used, and permits safe opera-
tion from a low voltage battery
supply.
Freeze the action with the second of our low-cost,
easy-build practical starter projects.
ther direction. This makes it
possible to closely analyze the
action and see precisely how
everything operates.
These days the word strobo-
scope probably conjures up im-
ages of high power flashing lights
at a disco rather than a scientific
instrument. However, the old
style stroboscope still lives on,
and units of this type can produce
some fascinating results
As many readers will no
doubt be aware, the scientific pur-
pose of a stroboscope is to
“freeze” moving machinery. The
basic idea is to synchronize the
flashing light of the stroboscope
with the machine so that the light
flashes at precisely the same
point in each cycle of the ma-
chine.
The flash of light must be
much brighter than the ambient
light level so that any onlookers
only see the machine during the
pulses of light. Because they only
see the machine at the same
point in each cycle it seems to be
stationary.
In fact, things become more
interesting if the stroboscope and
the machine are slightly out of
synchronization. With the light
flashing slightly later in each cy-
cle the machine appears as
though it is operating in slow mo-
tion. A lack of synchronization in
the opposite direction makes it
seem as though the machine is
going slowly in reverse!
By carefully adjusting the
flash-rate it is therefore possible
to move to any point in the oper-
ating cycle of the machine, and to
effectively make the machine op-
erate at the desired speed in ei-
BRIGHT LEDs
In order to “freeze” large
pieces of machinery it is neces-
sary to use a high power strobo-
scope. Such a device is not
necessarily very complicated,
but it requires the use of rela-
tively expensive flash tubes that
need high operating voltages.
Not a project that is usually con-
sidered suitable for beginners.
However, as the title sug-
gests, the LED Stroboscope
featured here is based on ultra-
bright light-emitting diodes
(LEDs) that provide compara-
tively small light levels and is a
low-budget project that is ideal
for the newcomer to electronics.
Consequently this unit must be
used in a darkened room and it
will only illuminate a small area.
DESIGN CONSIDER-
ATIONS
On the face of it, this appli-
cation requires nothing more
than a low frequency oscillator
driving one or more LEDs. In
practice the oscillator must pro-
vide very brief output pulses if
the desired action is to be pro-
vided. To be more precise, it is
the ratio of the on time of the
LEDs to the off time that is of
importance.
If the LEDs were simply to
be switched on for 50 per cent
of the time, the machine would
go through half a cycle during
the course of each flash of light,
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, July 1999 - www.epemag.com - 670
&RQVWUXFWLRQDO 3URMHFW
COMPONENTS
Resistors
R1, R2 15k (2 off)
R3 10k
R4 1k
R5 100k
R6, R7 4.7 ohms (2 off)
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Potentiometer
VR1 100k rotary carbon, linear
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Capacitors
C1 220u radial electrolytic, 16V
C2 470n polyester, 10mm lead
spacing
Semiconductors
D1 1N4148 signal diode
D2, D3 ultra-bright red LED
(see text) (2 off)
TR1 TIP121 or TIP122 npn power
Darlington transistor.
IC1 LF351N bi-FET opamp
Miscellaneous
S1 s.p.s.t. miniature toggle switch
B1 12V battery pack (8xAA size
cells in holder)
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Fig.1. Complete circuit diagram for the LED Stroboscope. The
two “strobe light” LEDs can be housed remotely from the main
unit (see photograph).
providing a very blurred image
to the users. Practical experi-
ments suggest that an off to on
ratio of at least 100 to 1 is
needed in order to obtain a rea-
sonably sharp “frozen” image.
This brings a second prob-
lem, which is a lack of bright-
ness from the LEDs when they
are only switched on for one per
cent of the time. A modern
ultra-bright LED will provide
good brightness from a current
of about 20mA, but pulsing it at
this current for one per cent of
the time this gives an average
drive current of only about
0‹2mA. This gives nothing more
than a faint glow from even the
most efficient of LEDs.
In order to overcome this,
the LEDs must be pulsed at a
much higher current than nor-
mal, and should ideally be
pulsed at about two amps (2000
milliamps). This gives an aver-
age current of 20mA and good
brightness.
Most LEDs are rated to take
continuous currents of up to
20mA or 50mA, but they can
withstand much higher currents
provided they are supplied for
short periods and the average
current consumption is within
the permitted maximum current.
The required high current
pulsed operation is therefore
acceptable provided the mini-
mum frequency is not made too
low, which would leave the
LEDs switched on for too long
during each pulse.
Multi-project PCB available from the
EPE Online store, code 7000932
(
www.epemag.com
); mediu
m size
metal or plastic case, size to choice
(see text); small box for LEDs
(optional); control knob; battery clip
(PP3 type); 8-pin DIL socket; multi-
strand connecting wire, solder pins,
solder, etc.
See also the
SHOP TALK Page!
CIRCUIT OPERATION
The full circuit diagram for
the LED Stroboscope appears
in Fig.1. The basis of the circuit
is a (more or less) conventional
oscillator circuit based on IC1.
This is a form of relaxation os-
cillator and it operates by first
charging capacitor C2, and then
discharging it.
Integrated circuit IC1 is an
operational amplifier (opamp),
but it is used here as a voltage
comparator. Its output (pin 6)
goes high when the inverting
input (pin 2) is at a lower volt-
age than the non-inverting input
(pin 3). Reversing the relative
states of the two inputs results
Approx. Cost
Guidance Only
(Excluding Batts & Case)
$15
in the output going low.
Resistors R1 and R2 form a
potential divider that biases the
non-inverting input of IC1 to
half the supply potential, but the
coupling through resistor R3
and potentiometer VR1 to the
output of IC1 modifies this po-
tential. When the output of IC1
is high the bias potential is
pulled higher, and when it is low
the bias is taken lower. The
amount of change depends on
the setting of VR1, and be-
comes greater as the resistance
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, July 1999 - www.epemag.com - 671
&RQVWUXFWLRQDO 3URMHFW
of VR1 is reduced.
Initially there is no charge
on capacitor C2 and the output
of IC1 goes high. Capacitor C2
then charges from the output of
IC1 via resistor R5 and the
much lower resistance path pro-
vided by R4 and diode D1. The
low resistance of R4 results in a
rapid increase in the charge po-
tential until it goes above the
bias voltage at the non-inverting
input. The output of IC1 then
goes low, and capacitor C2
starts to discharge via resistor
R5.
tiometer VR1, which is shown
configured as a variable resistor.
Conventionally, the frequency
of an RC oscillator is controlled
by varying the resistance in the
timing circuit, but that would be
difficult in this case as there are
two resistors in the timing circuit
(R4 and R5). Simply varying the
value of R5 would produce sub-
stantial changes in the mark-
space ratio of the output signal.
Altering the value of the positive
feedback resistance gives the re-
quired changes in frequency with-
out significantly altering the out-
put waveform.
With VR1 at a high value
there is little change in the bias
potential at the non-inverting in-
put when IC1’s output changes
state. The charge on capacitor C2
therefore has to change by only a
small amount to move from one
threshold to the other, and this
takes relatively little time.
With VR1 set at minimum re-
sistance the charge and dis-
charge threshold voltages are
pulled several volts apart, greatly
lengthening the charge/discharge
cycle. The flash rate can be var-
ied from approximately 17 to 100
per second. This corresponds to
rotation speeds from about 1000
RPM to 6000 RPM.
CURRENT DRIVER
Only output currents of a
few milliamps can be provided
by IC1, and a large amount of
amplification is needed to pro-
vide the LEDs (D2 and D3) with
suitably high drive currents.
This is provided by TR1, which
is a Darlington power transistor
used as an emitter follower
buffer stage.
A Darlington transistor is re-
ally two transistors connected so
that the output current of one de-
vice drives the input of the sec-
ond. This effectively gives a su-
per high gain transistor having a
current gain equal to the product
of the current gains of the individ-
ual transistors. The current gain of
TR1 is typically several thousand,
and this enables it to provide out-
put currents of a few amps.
A current of a little over one
amp is driven through each
LED, which produces an aver-
age current of about 10mA per
LED. This is high enough to
give good brightness but low
enough to avoid operating the
LEDs close to the point where
they are in serious danger of
being destroyed. The average
current consumption of the cir-
cuit as a whole is about 23mA.
There is no discharge path
through resistor R4 and diode
D1 because D1 blocks any flow
of current in this direction. Ca-
pacitor C2 therefore discharges
at a relatively slow rate through
R5 alone.
The waveform produced
across C2 is a form of sawtooth
wave (Fig.2a). More impor-
tantly, the output of IC1 pro-
duces brief positive pulses, as
in Fig.2b. In fact, the pulses are
even briefer than those shown
in Fig.2b due to the massive
difference in the values of R4
and R5. This gives the required
mark-space ratio of about 1 to
100.
STROBE RATE
For a stroboscope to be of
any practical value, it must be
possible to vary the output fre-
quency over a reasonable span.
This is the purpose of poten-
Fig.2. (a) Example waveform at pin 2 of
IC1 and (b) at pin 6 of IC1.
Component layout on the multi-project PCB.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, July 1999 - www.epemag.com - 672
&RQVWUXFWLRQDO 3URMHFW
CONSTRUCTION
The EPE and EPE Online
multi-project printed circuit
board (PCB) forms the basis of
this project, and it utilizes the
component layout, copper foil
master and wiring shown in
Fig.3. This board is available
from the EPE Online Store
(code 7000932) at
www.epemag.com
Although this proje
ct is ex-
tremely simple, the usual warn-
ings about the multi-project
PCB have to be repeated here.
Unlike an ordinary custom
printed circuit board, the multi-
board has numerous “leftover”
holes that tend to confuse
things slightly when fitting the
components. Take extra care to
avoid misplaced components
when building this board and
double-check the completed
board very carefully for errors.
In all other respects con-
struction of the board is largely
straightforward. The LF351N
used for IC1 is not static-
sensitive, but it is still advisable
to use a holder for this device.
Fitting TR1 is slightly awk-
ward because the leadouts of
the device do not match up
properly with the board layout.
Things would be much easier if
the base (b) and collector (c)
terminals of the Darlington tran-
sistor were the other way round.
In order to fit the device into this
layout it is necessary to cross
over the base and collector
leads, but this is not difficult
provided TR1 is given the orien-
tation shown in Fig.3.
The pinout wires of TR1 can
be fitted with short pieces of
PVC sleeving to ensure that
there are no accidental short-
circuits. Fig.4 shows the leadout
configuration for TR1, and
should help you to avoid errors
when connecting this compo-
nent.
Although the TIP122 is a
power transistor it only operates
at very low average power lev-
els in this circuit, and so no
heatsink is required. A TIP122
is used for TR1 on
the prototype, but
some suppliers
stock the TIP121
instead, and this is
equally suitable.
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Fig.4. Pinout details for the
TIP121/122 Darlington power
transistor.
LEDs
Virtually any
medium size plastic
or metal box should accommo-
date this project, but bear in
mind that the battery pack con-
Fig.3. PCB layout copper foil master and interwiring details.
Check component positions as not all holes are used.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, July 1999 - www.epemag.com - 673
&RQVWUXFWLRQDO 3URMHFW
finished unit before connecting
the battery and switching on be-
cause mistakes could easily re-
sult in the LEDs being fed con-
tinuously with a high current.
This would destroy them in a
fraction of a second.
When initially testing the
circuit it is not a bad idea to
connect a 220 ohm resistor in
the cathode (k) connection to
the LEDs. This will limit the cur-
rent to a safe level in the event
of a fault.
With control VR1 set at
maximum resistance (fully
counter-clockwise) the LEDs
should flash at a low enough
rate for the pulsing action to be
seen. Advancing VR1 in a
clockwise direction will soon in-
crease the operating frequency
to the point where the flashing is
too fast to be perceived.
A quick way of checking
that the LEDs are still strobing is
to simply wave them around in
the air. This should draw a sort
of dotted line of light in the air.
cycle, etc., but the “frozen” im-
age will be rather blurred. Opti-
mum results are produced at
the highest flash rate that gives
a single “frozen” image.
The maximum rate of
100Hz (6000 RPM) might not be
sufficient for some small pieces
of machinery. The maximum
output frequency can be in-
creased to about 300Hz (18,000
RPM) by increasing the value of
VR1 to 220 kilohms, but accu-
rate adjustment of the flash rate
will then be more tricky. Using a
value higher than 220k is not
recommended as it could result
in the oscillator stalling.
sisting of eight AA size cells is
fairly bulky. Remember to allow
for this factor when selecting
the case. Mount the printed cir-
cuit board on the base panel of
the case using either plastic
stand-offs or 6BA bolts plus
spacers.
The two “strobe” LEDs can
be mounted on the front panel
of the main case, but the unit is
easier to use if they are fitted in
a separate much smaller case.
The LEDs are then connected to
the main unit via a piece of
three-way cable about 0‹5 to two
meters long.
The current flow to the
LEDs is not sufficient to warrant
any form of heavy-duty cable.
Twin-screened cable or a three-
way lead peeled from a piece of
ribbon cable will suffice. The
rear panels of both cases must
be drilled with holes to take the
cable, and if metal cases are
used the holes should be fitted
with PVC grommets to protect
the cable.
Practically any ultra-bright
LEDs should work well in this
circuit, and the higher their effi-
ciency the brighter the pulses of
light produced. However, even-
ness of illumination is also im-
portant in this application, and it
is worth experimenting with a
few LEDs to find the ones that
give the best results.
In general, larger LEDs
seem to give more even beams
of light, but the latest 5mm di-
ameter types give the highest
light levels. The 3mm types do
not seem to give high enough
light output levels to be of use
in this application.
NEXT MONTHS
STARTER PROJECT - 3
IN USE
Although this unit is only
suitable for use with small
items of machinery it is still
essential to use it with due
care so that accidents are
avoided.
Most ultra-bright LEDs
produce fairly tight beams of
light, so it should not be neces-
sary to use the unit at very
close ranges.
Finding the right setting for
frequency control VR1 is largely
a matter of trial and error. If the
required “freezing” is obtained,
but with the machine in two or
more positions simultaneously,
the flash rate is too high result-
ing in more than one flash per
cycle of the machine. The
“freezing” effect will be obtained
if the stroboscope is set to flash
on every other cycle, every third
FREEZER ALARM
Don’t miss out!
TESTING
Once the small amount of
hard wiring has been added the
unit is ready for testing. It is es-
sential to thoroughly check the
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, July 1999 - www.epemag.com - 674
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