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With Constant Duty Cycle and
Constant Current Drive
Finally, white LED's are bright enough
to use in a practical stroboscope. This circuit can operate as a
bench-top stroboscope that, in conjunction with an oscilloscope or
frequency meter and bench top power supply can accurately measure
rotational speeds, or it can be operated hand-held from a single 9V
battery, using a marked scale to estimate rotational speeds. Its a nice
iinstrument to have on or around the workbench. Besides
that, its fun to use.
(Above) The stroboscope freezes a
piece of white tape on a fan blade that is rotating at 2616 RPM. Power is supplied and the strobe
frequency is monitored via the connector in the lower right corner.
Its bright enough to use without dimming the room lights and with the
aid of a frequency meter or oscilloscope, can measure rotational
frequency to within a few degrees per second. This LED stroboscope is a
fairly simple circuit for the similarly simple task of measuring the
speed of things that rotate. I wondered how fast some DC fans I had on
the bench were, and tried a quick-and-dirty LED driver, driven by
pulses from a function generator. I was pleased to see that, in
contrast to my earlier experience years before, the white LEDs I have
now, BUXC333 from BestHongKong.com, is bright enough to use with the
room lights on. I decided to go ahead and build up a small
stroboscope with one of them.
The circuit and an optional 9 volt radio battery are housed in a small
plastic box. A range switch selects the RPM range, and a knob
adjust the oscillator frequency. A white LED is mounted at the end of
about 60 cm of wire, so it can be aimed at whatever it is that I want
to illuminate with it, without affecting the orientation of the box.
When operating in the hand-held mode from its internal battery, the pot
is adjusted so that the LED flashes once per rotation of whatever is
being measured, and the revolutions per minute are estimated by reading
from the hand calibrated dial.
A headphone jack mounted in the box provides connections for an
external power supply, such as a bench supply or a "wall wart"
power supply and an pulse output so that the LED pulse frequency can be
accurately measured with a frequency meter or oscilloscope.
A note of caution: Be careful not to load the pulse output, since its
not buffered and loading it can stop the oscillator.
((Above) The comparitor on the
left is the oscillator. The comparitor on the right makes a
duty cycle pulse by comparing the sawtooth generated across
capacitor, Ct, to the DC level set by the 50k pot.
The circuit is composed of an oscillator, a pulse slicer, and a gated
constant current source. All of the circuits except the anode of the
LED receives power from an LM78L05 5 volt regulator.
The comparitor connected to pins 1,2, and 3 is connected as a 50% duty
cycle oscillator. This circuit's oscillation frequency is = 1/(1.4 R C)
where R is the sum of of the 10k fixed resistor and the 100k pot, and C
is the capacitance from pin 2 of the comparitor to ground. Multiplying
the frequency in Hertz by 60 gives RPM (Revolutions Per Minute), so the
formula becomes RPM= 43/(R C). Using this formula, you can choose
a value for the capacitor that will center the oscillator's range
around the frequency you are most interested in.
The second comparitor, the one using pins 5,6, and 7, slices the
sawtooth that appears across the oscillator capacitor. Whenever the
voltage of the sawtooth exceeds the voltage on the wiper of the 50k
pot, the comparitor's output is high. When the comparitor's output is
high, the constant current source made of the two transistors is on,
and the LED emits light.
A 2N4401 or similar should work
as well as a 2N2222.
Both have the same pinouts.
Whenever there is current available through the 3.3k resistor, the
upper 2N2222's current is limited to that which provides enough voltage
across the 10 Ohm emitter resistor to turn on the lower 2N2222.
This voltage is about 600 millivolts, so current through this resistor,
which is approximately the same as that of the LED is set to about 60
milliamps. That's twice the datasheet's 30 milliamps maximum current,
but well less than the 100 mailliamps peak pulse current. To reduce the
current, make the emitter resistor larger. To increase the current,
make the resistor smaller.
(Above) all of the circuitry,
plus the optional 9 volt batter fits snugly in a 5.5 cm x 6.5 cm plastic box.
The red/black wire exiting the hole in the box cover connects
the LED to the circuitry.
I had a pretty good idea of what I was
going to use as a circuit, so I went ahead and modified the box -
cutting the holes and mounting the frequency control pot, the on/off
switch, range switch, and I/O jack, and cutting the circuit board to
size before prototyping the circuit. If I had built the circuit first,
I think I would have used a larger box. The small parts were mounted on
a small piece of pad-per-hole phenolic circuit board and the leads were
bent toward one another and soldered together. This layout is not
critical, but you should keep the leads to pins 2 and 3 of the
comparitor as short as practical since noise pickup on these leads can
affect the oscillator.
After you've got it all connected together test the circuit using an
inexpensive red LED rather than an expensive white LED because if the
constant current source doesn't work, or if the duty cycle pot is
turned the wrong way, you run the risk of "frying" the LED.
Set the duty cycle pot by aiming the LED at a rotating target,
preferably one with a timing mark of some kind on it, like the small
fan blade with a piece of white tape shown in the photograph at the top
of this page, and adjust the duty cycle pot for the best tradeoff
between apparent brightness and sharpness. A large duty cycle will
smear the image. A very short duty cycle will look dim. The tradeoff is
yours to make.
You can also use an oscilloscope to set duty cycle by probing pin 7 of
the comparitor and adjusting the positive going pule to the desired
duty cycle. I recommend 10% as a starting point.
You can calibrate the frequency adjust knob if you want. I put a piece
of white cardboard on the front of the box and drew a scale on it with
a ball-point pen. Using a frequency meter to monitor the oscillator
output of the headphone jack, I made marks at dial positions
corresponding to 500, 750, and 1000 RPM on the slow scale, and at 300,
3000, 4000, 5000, and 6,000 RPM on the fast scale. With this
scale, I can estimate revolutions per minute when operating in the
battery powered hand held mode.
Given that this circuit can drive LEDs with a forward voltage drop of
up to about 1 volt less than the battery voltage, it will also drive an
Ultra Violet (UV) LED. An ultraviolet stobeoscope in conjunction with
florescent paint might be more useful in a room with the lights on than
a while LED stroboscope.
I tried something similar using my one and only UV
LED and luminescent paint, and found it to work in principle. I painted
a dot of luminescent
paint onto an electric fan and turned it on. The paint glows
yellowish-green for a long time after being exposed to the UV. When I
have the UV strobe timing set correctly, a
bright yellowis-green ring appears, tracing the orbit of the
dot. The ring glows and fades as the phase of the fan drifts with
respect to the phase of the LED strobe. With this setup, one could find
the rotational frequency of the target by adjusting the strobe
frequency for maximum brightness of the glowing ring.
Circuit Board Version and Astonishing Demonstration Movie
Guillermo Cardarelli, of Argentina, made a fine looking strobe with the
circuit board in the pictures below, and then captured the action in a
Windows Medial Player movie. Guillermo reports that his photographs and
the movie were taken with his Canon A95 camera. The video was edited
with ArcSoft VideoImpression 2, which came with the camera. The printed
circuit board design was created with PCB Wizard 3.
The component side of Guillermo's
board is shown above.
Above is the copper side of Guillermo's
Guillermo used external pots to
control both the frequency and the duty cycle. Other wires
lead to the on/off switch, the
fast/slow switch, and a connector for the 9 volt battery.
HERE or on the picture above to see or
download an amazing
Windows Media Player movie showing Guillermo's strobe in action.
About 600 kB.
Guillermo's stobe (above) used the same circuit as the origingal single
but it drives seven LEDs in parallel. This gave him the same amount of
light as the single LED strobe,
but with the advantage of the illumination being more even.
Danger of Eye
Damage From Visible Light Emitting Diodes
Except where noted, contents ©2005, ©2006 Richard Cappels All
Rights Reserved. http://www.projects.cappels.org/
Printed circuit board, photograph of the printed circuit board, photo
of seven LED stroboscope and
movie of the stoboscope in action, copyright ©2006 Guillermo
First posted in August, 2005. Last update
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