Pulsed drive of white LEDs?

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.

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.

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

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.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

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

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
                     "If it doesn\'t fit, use a bigger hammer!"
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--- 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.

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.

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

Three questions:

1. What will be the distance between the emitter and receiver?
2. Where are you?
3. What does your schedule look like?

JF

--
forgot...

http://www.radioshack.com/product/index.jsp?productId=2049727&CAWELAID=37890884

JF

Lots of others have talked about using IR.

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 ! >---------------

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.

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.

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.

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

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

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.

I do have such parts in stock and in fact, I'd intended to use a

555 to drive the LEDs.

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

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.

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.

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.

Thanks.

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.

All noted. Thanks for the input.

=A0 =A0 =A0!

=A0!

Go with super bright blue LEDs for outdoors work. They can be seen over a very long distance because of the lack of bright blue objects in the environment.

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.

google drag race timing system

Many suppliers. Might give you some hints on how to set this up.

--
Joe Chisolm
Marble Falls, Tx.