Pulsed drive of white LEDs?

[pimpom]

Has anyone tried driving ordinary white LEDs - the kind used in
cheap flashlights - with a pulsed current and done some kind of
evaluation of their performance? Any idea if they can cope with
frequencies of the order of a kHz?

I'm thinking of using them with photosensors with a pulsed drive
for discrimination against ambient light. It's for the drag
racing christmas tree system that I asked about a few days ago.
I've considered laser pointers and infrared devices, but a tight
schedule and possible problems with on-site alignment makes me
think that a pulsed white LED with a general-purpose photodiode
might be the best option.

I'm always open to alternative suggestions, but please remember
that quick procurement of anything but the most common
general-purpose devices is next to impossible from where I live.

 

[pimpom]

Has anyone tried driving ordinary white LEDs - the kind used in
cheap flashlights - with a pulsed current and done some kind of
evaluation of their performance? Any idea if they can cope with
frequencies of the order of a kHz?

I'm thinking of using them with photosensors with a pulsed
drive
for discrimination against ambient light. It's for the drag
racing christmas tree system that I asked about a few days ago.
I've considered laser pointers and infrared devices, but a
tight
schedule and possible problems with on-site alignment makes me
think that a pulsed white LED with a general-purpose photodiode
might be the best option.

I'm always open to alternative suggestions, but please remember
that quick procurement of anything but the most common
general-purpose devices is next to impossible from where I
live.

P.S.: I'll be doing some search on the net, but I'm really
pressed for time and will appreciate inputs from this group.

 

[RHRRC]

the problem with the leds used in cheap, and not so cheap, flashlights is
particularly the life of the leds.
By comparisonnthe rated life of flashlight some bulbs 50 hours (some higher
some lower) so the leds dont have much to compete with.

Infra red is much the better/cheaper/ easier.

Look at the (many) integrated IR receivers built for the job

As I stated:- better/cheaper/easier

 

[Nico Coesel]

Has anyone tried driving ordinary white LEDs - the kind used in
cheap flashlights - with a pulsed current and done some kind of
evaluation of their performance? Any idea if they can cope with
frequencies of the order of a kHz?

A few years ago I made a PWM for intensity control at a few kHz to
drive a white led.

 

[pimpom]

I'd imagine that if you want to save time that cheap laser
pointers
would be the go for the light source. With them you dont have
to
worrry about optics / focusing etc etc. Place the sensor in a
long
tube, painted matt black internally to damp ambient light etc.
Some
red plastic (for red LD's) would help eliminate unwanted light
also.

Thanks for your interest. The laser is attractive and I had
already considered using a blackened tube, but what made me
reject it for the time being is having to align the thin beam
precisely with the long narrow tube within a short time. The
event is to be held in an abandoned airfield almost an hour's
drive outside town. Everything will have to be set up on the day
of the race itself - four lasers and their receivers, the
christmas tree and control electronics.

 

[pimpom]

I recommend using infrared as well. Particularly if you're
going to
need to use it around electronic-ballast fluorescent lights,
which
are a disaster for IR remote controls. If the data rate is
sufficiently slow, just keep the operating frequency between 2
and 10
kHz, and filter heavily. (The switching frequency is around 40
kHz.)
If you use infrared-filtered PDs such as the BPY22NV, most of
that
crap goes away, but for higher data rates and better range, you
really have to go above 1 MHz, which starts to get difficult
with
nanoamp photocurrents and 15-pF capacitance. (Remember to
reverse-bias the BPV22NV if you want to go fast.)

I just did a glorified version of this for a customer in the
Far East.

The beam won't carry any data. It's to be a simple
blocked/not-blocked switching action. I thought of using a
pulsed drive to eliminate the effect of ambient light. The
receiver will have a loosely tuned circuit (perhaps an RC twin-T)
or just a high-pass filter.

 

[pimpom]

A few years ago I made a PWM for intensity control at a few kHz
to
drive a white led.

Thanks. There's no modulation involved with this application.
It's to act as a switch and driven with a pulses of constant
rate, width and amplitude. The main purpose of using pulse drive
is to enable filtering of the effects of ambient light.

 

[Spehro Pefhany]

Has anyone tried driving ordinary white LEDs - the kind used in
cheap flashlights - with a pulsed current and done some kind of
evaluation of their performance? Any idea if they can cope with
frequencies of the order of a kHz?

Yes, they certainly can (at least the ones I've worked with).


Best regards,
Spehro Pefhany

 

[_]

Thanks for your interest. The laser is attractive and I had
already considered using a blackened tube, but what made me
reject it for the time being is having to align the thin beam
precisely with the long narrow tube within a short time.

Cigarrette - blow smoke so you can see the laser, line it up and fix. Two
minutes.

 

[pimpom]

---
The usual way to do this sort of thing is with a pulsed
infrared
emitter (LED) and an infrared receiver module. The emitter is
pulsed
at the receiver's frequency and then pointed at the receiver
while the
receiver's output is being monitored. When the output changes
state
the beam from the emitter will be falling on the receiver. The
emitter is then rotated until the receiver's output changes
state
again, at which point the emitter beam will be just falling
short of
illuminating the receiver.

The emitter should then be rotated halfway between the two
limits,
where it'll be illuminating the receiver as strongly as it can,
and
fixed in position.

Now, when anything IR opaque breaks the beam the receiver's
output
will change state, and then change state again when the object
exits
the beam.

Thanks for the reply. I have not fully rejected the idea of IR.
The complicating factor is that the whole setup procedure,
including the christmas tree and control electronics, will have
to be done within a very limited time frame, outdoors, on the day
of the race itself. The path of detection has to be made narrow
(at emitter or receiver) because its purpose is to determine the
correct positioning of the racers before and right at the moment
of the start. Working within such constraints, I'm afraid using
an invisible IR will be a major hindrance. Four emitter-receiver
pairs are needed.

The emitter can be made from a 555, an IRLED, two resistors,
and two
capacitors, which should be available pretty much anywhere in
the
world, and the receiver module, which is available here from
Radio
Shack.

I do have such parts in stock and in fact, I'd intended to use a
555 to drive the LEDs.

The trick is to make sure you get an LED with an output
wavelength
which matches the receiver module's input.

I've already tested the compatibility of a flashlight LED with
reel sensor photodiodes from an old VCR.

Three questions:

1. What will be the distance between the emitter and receiver?

I'm not sure yet, but I'm guessing 8 to 15 feet. I'm thinking of
placing the four emitters between the two drag racers, two each
pointing to left and right.

2. Where are you?

In one of the remotest corners of eastern India. So suppliers
like Radioshack, Digikey, Farnell, etc. are out of the question.
The way things are done in India, ordering something even from
another Indian city is a cumbersome process.

3. What does your schedule look like?

The organisers have tentatively targetted the second week of
December for the race, but the date is not yet firmly fixed. I
have to make decisions quickly because I'll have to do everything
myself with materials I have or can get locally. No readymade
modules, no outsourcing service.

 

[pimpom]

Yes, they certainly can (at least the ones I've worked with).

Thanks.

 

[pimpom]

Lots of others have talked about using IR.

As I exlained in reply to another poster, my main worry about
using IR is that the detection path has to be quite narrow and
four beams have to be aligned outdoors within a limited amount of
time using an invisible beam. However, this does not mean that
I've rejected IR completely.

I will add that it is best to use a pure color LED (including
IR) and
not a white one because of two factors.

1 - Whatever wavelength LED you pick, you can filter the light
at the
detector to just that wavelength to improve your signal to
noise.

2 - The low cost white LED are internally a blue LED that is
pumping a
phosphor that makes a yellowish light. Some light is lost in
the
process and the speed of the LEDs switching may be decreased.

I would push the pulse rate of the drive up to a fairly high
frequency. If this is something where the light beam will be
interrupted and you have control over both ends I would also
make the
duty cycle something like 1:8 so that during its on time, the
LED is a
lot brighter than the average you can do.

At the receiver end consider a circuit like this:

R1 R2
IN---+--/\/\-------+----------/\/\--------+--- to low pass
! ! !
! X0 ---!-\ !
---O ! >---------------
<-----------!+/
GND----O 1/3
X1 4053


When R1=R2, this is the usual "Gain=+1 or Gain=-1" circuit.
If,
however, you make R2 7 times R1, the gains will be +1 and -7.
If this
ratio of gains is matched by the ratio of the times in the duty
cycle,
the average gain will still be zero. This means that a
constant light
source will be filtered out in the low pass. The desired light
will
see a gain of -7 in the circuit.

All noted. Thanks for the input.

 

[krw]

Thanks for the reply. I have not fully rejected the idea of IR.
The complicating factor is that the whole setup procedure,
including the christmas tree and control electronics, will have
to be done within a very limited time frame, outdoors, on the day
of the race itself. The path of detection has to be made narrow
(at emitter or receiver) because its purpose is to determine the
correct positioning of the racers before and right at the moment
of the start. Working within such constraints, I'm afraid using
an invisible IR will be a major hindrance. Four emitter-receiver
pairs are needed.

That doesn't mean the path has to be narrow at all. The only reason
for a narrow beam is S/N. Even if you had an omnidirectional (point)
source and detector only something directly in the path will interrupt
the signal. Noise may be an issue and the size of the detector and
source are important. Garage door opener safeties come into mind as
an example of your application. They're not very directional and easy
to set up but work well enough.

I certainly wouldn't try to use a narrow source and detector. You'll
never get that aligned right.

 

[pimpom]

That doesn't mean the path has to be narrow at all. The only
reason
for a narrow beam is S/N. Even if you had an omnidirectional
(point)
source and detector only something directly in the path will
interrupt
the signal. Noise may be an issue and the size of the detector
and
source are important. Garage door opener safeties come into
mind as
an example of your application. They're not very directional
and easy
to set up but work well enough.

I didn't get your point at first, but I do now. You correctly
pointed out that it's only the line of sight between source and
receiver that matters - well, up to a point. Some care will still
be needed to limit the angle of detection for two reasons. One is
that reflections could cause problems. The other reason is that
there will be two virtual lines in parallel working
independently, only 7 inches apart over a distance of several
feet. Cross detection is unacceptable.

I suppose I could pulse the two beams at different frequencies
and filter the signals at the receivers, but that adds complexity
and could result in unforeseen problems..

I certainly wouldn't try to use a narrow source and detector.
You'll
never get that aligned right.

Correct. It's why I rejected the laser option, at least for the
upcoming race. The only sensor with an area large enough to ease
alignment would be an LDR, but it's too slow. Anyway, I doubt if
I can get one an inch wide or larger in a short time.

 

[pimpom]

Assuming you have a cable between emitter and receiver I'd do
two
things...

Square wave modulate the emitter and synchronous detect at the
receiver.

Then add a DC loop at the receiver to null out the ambient
light.

That's an idea. I'd intended to place the four emitters as
unidirectional transmitters between the two racers, battery
powered without any electrical connection to the receiving units.
Running a cable to the middle of the track from the side, even
behind the starting line, might be a problem. But it's worth
thinking about.

 

[pimpom]

Use a pulsed drive, for sure, but also an IR filtered
photodiode.
Otherwise you get blown out of the water by the shot noise of
detected
daylight and the 40 kHz-1 MHz junk from electronic ballast
fluorescents. It's mildly inconvenient not to be able to see
what
you're doing, but a webcam can see 850 nm just fine.

Or do it in the visible, if you don't need much range. It'll
be
cooler looking, for sure, but it'll crap out much sooner with
distance.

One thing I'm pretty sure of is that there won't be any
fluorescent ballasts spewing EMI. The venue is an abandoned
airfield and the starting line will be quite some distance from
the old terminal building. There won't be any electrical supply
at that point even if there's one in the building, unless we
string a long power cable. This is why I decided to power
everything with batteries, including the christmas tree lights.

Thanks for the webcam idea. I still haven't completetely
abandoned the IR route yet.

 

[pimpom]

I'm busy on a project right now, so I can't try it myself right
now,
but I'm contemplating how to view the photocell-controlled
street
light across the street from me and use it to control my yard
lights.

(I'm down in a notch, between three hills, so ambient light is
a poor
controller. But the street light is about thirty feet up with
a photo
sensor with a better sky-view than I have ;-)

I made a trial batch of street light controllers for my state's
electricity board nearly 20 years ago. Control was from
distribution points rather than at each lamp post. The problem
was that practically the whole state, including the capital, is
made up of small jagged hills. Daylight varies from point to
point and the controllers had to be set individually. They worked
fine when set properly, but the engineers left the setup to
unskilled workers who had no idea what they were doing. That
turned the experiment into a failure.

 

[Baron]

That's an idea. I'd intended to place the four emitters as
unidirectional transmitters between the two racers, battery
powered without any electrical connection to the receiving units.
Running a cable to the middle of the track from the side, even
behind the starting line, might be a problem. But it's worth
thinking about.

Would not red laser pointers work.

 

[krw]

I didn't get your point at first, but I do now. You correctly
pointed out that it's only the line of sight between source and
receiver that matters - well, up to a point. Some care will still
be needed to limit the angle of detection for two reasons. One is
that reflections could cause problems. The other reason is that
there will be two virtual lines in parallel working
independently, only 7 inches apart over a distance of several
feet. Cross detection is unacceptable.

You can limit the view with a simple shield. I used "Omnidirectional"
just to try to get the point across.

I suppose I could pulse the two beams at different frequencies
and filter the signals at the receivers, but that adds complexity
and could result in unforeseen problems..

Even simpler, shut down one transmitter when the opposite receiver is
in use. Alternate at some sufficiently high rate.

Correct. It's why I rejected the laser option, at least for the
upcoming race. The only sensor with an area large enough to ease
alignment would be an LDR, but it's too slow. Anyway, I doubt if
I can get one an inch wide or larger in a short time.

A laser pointer would be simple to point accurately (kinda the purpose
of the thing . Get the interest of all the neighbor cats too. ;-)

 

[ehsjr]

pimpom wrote:

I didn't get your point at first, but I do now. You correctly
pointed out that it's only the line of sight between source and
receiver that matters - well, up to a point. Some care will still
be needed to limit the angle of detection for two reasons. One is
that reflections could cause problems. The other reason is that
there will be two virtual lines in parallel working
independently, only 7 inches apart over a distance of several
feet. Cross detection is unacceptable.

To avoid cross detection, you can do this:

emitter1------->detector1

detector2<------emitter2

Each detector can "see" only its own emitter.

Ed

<snip>