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200 feet IR Beam

Posted: Tue Dec 20, 2005 2:40 pm
by Cirqmaster
I am building a horse racing timer, I need an IR beam to travel 200 feet.
I have been using commercially available optical sensors, but now I am trying to build my own. Can someone tell me some of the key points to getting distance.

number/pattern of LEDs etc...
Rusty Strauss

Re: 200 feet IR Beam

Posted: Tue Dec 20, 2005 7:21 pm
by jimandy
Wow. Is the track 200 feet wide? How many horses race on that thing?

Re: 200 feet IR Beam

Posted: Wed Dec 21, 2005 4:08 am
by Cirqmaster
It's actually a barrel racing Timer like you would see in a rodeo, so the beam has to span the entire width of the arena.

Mostly I'm just looking for some basic constants about distance. like
Which yealds better results a more powerful led, or more leds

How does frequecy affect distance?

Are some of the more common detectors better at detecting a weak signal then others?

Can 200 feet even be acheived without lenses?
Am I missing something obvious here?

Re: 200 feet IR Beam

Posted: Wed Dec 21, 2005 7:07 am
by jimandy
I can't say I would recommend a low power laser for obvious safety reasons, but they certainly traverse distances greater than 200 ft. and a lens system would be unecessary. Modulating an LED or Laser beam desensitizes the system from ambient light. The modulating frequency should not have any effect on the distance involved. The IR sensors used to detect a handheld remote usually operate in the range of 38-40Khz. I'm wondering though if dust in this rodeo environment might be a problem.

<small>[ December 21, 2005, 07:10 AM: Message edited by: jimandy ]</small>

Re: 200 feet IR Beam

Posted: Tue Jan 03, 2006 9:12 pm
by Sambuchi
hello, i like what jimandy talked about.. but i have another question.. will this be inside or out. this will determind the filter you may or may not need to recieve the transmision... may want to take a look into Sunlight & Ambient Light Filter Options

Re: 200 feet IR Beam

Posted: Wed Jan 04, 2006 3:16 am
by rshayes
Since you have a fixed path, it should be possible to use some sort of optical system. This will probably be enough to get the range you need. The system will have to be aligned to some extent.

Gallium Arsenide LEDs at 940 nm are a good match for silicon detectors. You should be able to get sometinmg in the milliwattt power range without too much trouble.

You want to modulate the source. This allows AC coupling to be used on the receiving end to reject daylight and other steady light sources. A modulation frequency in the 50 KHz range should be high enough to detect an obstructed beam within a few milliseconds.

An optical filter on the receiver that passes the transmitted wavelength will help a great deal by reducing the steady signals from daylight or incandescent lights.

A lens on the transmitter will concentrate most of the energy in the direction of the receiver. The LED itself will probably not have a narrow enough pattern, but a simple lens, such as a 3 inch magnifying glass should help a great deal. Don't bother with an achromatic or other multiple element lens. They are corrected for chromatic errors in the middle of the visible range, and their characteristics in the near infrared range are no better than a simple glass lens. Even antireflection coating is of dubious value. Since a LED is not a sharp light source to begin with, even a Fresnel lens may be usable.

You want a detector of moderate size to make aiming easier. A phototransistor will probably have a sensitive area that is so small that it will be difficult to align. It also won't receive much energy. Large area detectors, such as a solar cell, may have too much capacitance to work at your modulation frequency. Something in the vicinity of 2 to 5 mm might be a reasonable compromise.

If you use a 5 mm LED and a 6 inch focal length lens on the transmitter, the energy should fall in a spot about 6 feet in diameter at 200 feet. A 3 inch diameter receiving lens will intercept about 1/576 of the energy transmitted. With a 1 milliwatt LED, this will be around 2 microwatts. The sensitivity of a silicon detector will be around .6 amperes/watt, so the current from the photo diode will be about 1 microampere. The DC current due to stray light will probably be higher. You need to provide a low impedance load for the photodiode the minimize the effect of its junction capacitance. The first stage may have to operate at low gain to avoid being saturated by the DC current from the photocell. A single transistor amplifier with a feedback resistor from collector to base can be used as a simple transresistance amplifier. If the feedback resistor is 1K, the scale factor of the amplifier will be 1 volt/milliamp and the 1 microamp of received signal will become 1 millivolt at the amplifier output. This can be AC coupled, amplified up to the volt range (two or three stages, total gain over 1000), and rectified to provide your output signal. After filtering, a comparator can used to detect when the signal falls below a threshold level.

With a 6 inch focal length receiving lens, the transmitter will have to be within a 3 to 6 foot circle in order to be imaged on the detector. A smaller detector will make the alignment more difficult.

Placing a black tube in front of the receiver will reject a large amount of stray light. You may need other baffles to insure that direct sunlight can't get into the receiver. This is most likely to be a problem around sunrise or sunset. Putting the receiver to the west (facing east) would help unless you are operating the system in the early morning.

Re: 200 feet IR Beam

Posted: Sun Jan 08, 2006 12:46 pm
by Chris Smith
The IR laser diodes will do the job and they are considered "eye safe".

No laser is actually Eye safe but what happens is if it hits the eye, it only affects part of the eye that doesn’t work any way, the IR sensitive part.

The eye can see IR but the brain doesn’t respond on a conscious level making it usless.

Hook your self up to a brain probe and you will see that the eye and brain do in actual fact respond on a subconscious level.

But You really have to stare at it for a long time before the migraine starts!

A 5 mw IR laser out of any CD Rom will go about 10 miles or more with the right collimating lens, and with a high gain sensor made out of a IR pick up and a Darlington transistor, you can set off your sensor at these distances easily and even further.

The best way to set up a trip sensor is to use the “missing pulse” method of modulating the light at a given frequency, then setting the sensor [using a missing pulse sensor] to receive this frequency to set off the trigger.

This stops stray light from hitting the sensor and interfering with the signal or trigger.

Also in day light use long tubes, painted black inside the tube, to avoid stray or incident light from striking the sensor and reducing its effectiveness or completely saturating it all together.

Place the pick up sensor in this tube and seal it at the back from all light.

The longer the tube, the better it is.

However at 200 feet, you probably wont need any gain from a Darlington and you should be able to use a straight photo sensor that is tuned to your IR frequency.

Re: 200 feet IR Beam

Posted: Fri Jan 13, 2006 12:56 am
by rshayes
As soon as you use a laser, OSHA becomes involved. This is especially true since you are using it in a public place. It doesn't take much energy to leave a scar on the retina, and the eye can still focus energy to some extent in the near infrared even if you can't see it. Think in terms of little kids looking directly into your transmitter to see what is inside.

The lasers that the military considers "eyesafe" operate at wavelengths longer than 1 micron. This means special material for both the laser and the receiver diode, and they will not be cheap. It also means that you won't be able to use a silicon camera as a viewer when you try to align the transmitted beam.

The lasers used in CD and DVD drives are probably the easiest to get. These will be in the 800 to 650 nm range, all of which will work with a silicon detector. The beam will probably have to be expanded to reduce the power density to levels that OSHA will tolerate. At 200 feet, it should be possible to keep most of the power in a spot a few inches in diameter. These wavelengths are also short enough to be viewable on some CCD cameras.

You may have alignment problems with a spot this small. It can probably be done, but it may be tedious. With a 200 foot optical lever arm, you might wind up with a moderately sensitive seismograph.

Re: 200 feet IR Beam

Posted: Fri Jan 13, 2006 5:25 am
by PlatinumT
If you go the laser route I would recommend using a retro reflector to ease the alignment process. I have designed a system using an IR laser diode and retro reflective material in a robotics application with success at +100 meters. If you would like details ask.

Re: 200 feet IR Beam

Posted: Sat Jan 14, 2006 3:03 pm
by Chris Smith

Eye Safe Lasers:

Definition: lasers emitting in a wavelength region with relatively low hazards for the human eye (for a given intensity level)

If you go the laser route consider all of the below. Some lasers are still not covered fully by the department of radiology or OSHA and there is ambiguity still left unresolved.

Such things as the common Store scanners have grey areas of operation and safety.

Also there is a antiquated statue involving general radiation of “non iodizing radiation” and many other laws and statutes that can muddy the water of understanding.

However, the common laser pointer isn’t covered well at all and it has up to 5 MW of power, and has very loose or non existing regulations on its sales and usage in public places.

Actually some states have better regulations regarding these devices than the Fed.

The biggest factor with any laser use is common sense and safety precautions, not simply the law.

Many laws are written and poorly observed.

Also if your laser signal is strong enough to do eye damage, or it’s the wrong frequency [below 800nm] but your optical knowledge is sufficient, the Concept of weakening the beam with optical spreading is prudent.

However, You do need a good knowledge on this subject as well as good optics to accomplish the task. Your lens frequency and laser frequency should match before you attempt this.

Class I laser product
No known biological hazard. The light is shielded from any possible viewing by a person and the laser system is interlocked to prevent the laser from being on when exposed. (large laser printers such as the DEC LPS-40 has a 10mW HeNe driving it which is a Class IIIb laser, but the printer is interlocked so as to prevent any contact with the exposed laser beam, hence the device produces no known biological hazard, even though the actual laser is Class IIIb. This would also apply to CD players and small laser printers, as they are Class I devices).

Class II laser products
Power up to 1 milliwatt. These lasers are not considered a optically dangerous device as the eye reflex will prevent any occular damage. (I.E. when the eye is hit with a bright light, the eye lid will automatically blink or the person will turn thier head so as to remove the bright light. This is called the reflex action or time. Class II lasers won't cause eye damage in this time period. Still, one wouldn't want to look at it for an extended period of time.) Caution labels (yellow) should be placed on the laser equipment. No known skin exposure hazard exist and no fire hazard exist.

Class IIIa laser products
Power output between 1 milliwatt and 5 milliwatt. These lasers can produce spot blindness under the right conditions and other possible eye injuries. Products that have a Class IIIa laser should have a laser emission indicator to tell when the laser is in operation. They should also have a Danger label and output aperature label attatched to the laser and/or equipment. A key operated power switch SHOULD be used to prevent unauthorized use. No known skin hazard of fire hazard exist.

Class IIIb laser products
Power output from 5 milliwatts to 500 milliwatts. These lasers are considered a definate eye hazard, particularly at the higher power levels, which WILL cause eye damage. These lasers MUST have a key switch to prevent unathorized use, a laser emission indicator, a 3 to 5 second time delay after power is applied to allow the operator to move away from the beam path and a mechanical shutter to turn the beam off during use. Skin may be burned at the higher levels of power output as well as the flash point of some materials which could catch fire. (I have seen 250mW argons set a piece of red paper on fire in less than 2 seconds exposure time !) A red DANGER label and aperature label MUST be affixed to the laser.

Class IV laser products
Power output >500 milliwatts. These CAN and WILL cause eye damage. The Class IV range CAN and WILL cause materials to burn on contact as well as skin and clothing to burn. These laser systems MUST have:
A key lockout switch to prevent unauthorized use Inter-locks to prevent the system from being used with the protective covers off Emission indicators to show that the laser is in use Mechanical shutters to block the beam Red DANGER labels and aperature labels affixed to the laser
The reflected beam should be considered as dangerous as the primary beam. (again, I have seen a 1,000 watt CO2 laser blast a hole through a piece of steel, so imagine what it would do to your eye !)
Registration of laser systems
Any laser system that has a power output of greater than 5 milliwatts MUST be registered with the FDA and Center for Devices and Radiological Health if it has an exposed beam, such as for entertainment (I.E. Laser light shows) or for medical use (such as surgery) where someone other than the operator may come in contact with it. (this is called a 'varience' and I have filled them out and submitted them and they ARE a royal pain in the backside !)

<small>[ January 14, 2006, 03:05 PM: Message edited by: Chris Smith ]</small>

Re: 200 feet IR Beam

Posted: Thu Jan 19, 2006 7:01 pm
by jollyrgr
An idea....

I like the idea from PlatinumT. Use a laser pointer and reflector. A red diode can be used as a detector and a laser pointer as the source. Point the diode and the laser in the same direction and aim it at a red or white bicycle reflector. Place the reflector at the bottom of a PVC tube to minimize someone looking at it and getting "flashed" by the laser. Since it is in a tube, a small mirror like those found in grooming/makeup kits can be used. Put the reflector or mirror in one tube and the laser and the diode detector circuit inside a tube. Using a mirror will likely give longer distances. Using a refector makes alignment easier. If you want to get real fancy, you can make a corner reflector. Take three mirros and form one corner of a cube with the reflective surfaces on the INSIDE of the cube. Any light shined into the corner reflector will be reflected back toward the source. You will be trying to aim right into where the three mirrors meet for best results.

As far as a detector circuit, one can be built from scratch using a few transistors or OP amps, caps, and resistors for an amp circuit. The laser source can be a simple $5 laser pointer driven by a regulated 5VDC. But what I described can be bought in kit form for under $30 Go to

Look up their kit LTS1. This is a LASER TRIP SENSOR device. This kit will provide the laser pointer, the detector circuit, and even the PVC tube for the emitter/detector.

You want 200 feet, this is not a problem for this kit. It will work well over 500 yards (1500 feet). With a more powerful laser pointer you can get even more distance.

The kit's manual (here: ... s/LTS1.pdf ) suggests on page 20 to use FIRST SURFACE MIRRORS. While this is probably good for longer distances, 400 Ft (from the pointer back to the diode) should not be a problem using a common mirror.

Mount the mirror tube and the emitter/detector circuit tube on adjustable stands of some sort. Using a visible red laser will make alignment a piece of cake.

With the way I described doing things the laser and detector are in the same location. Thus the light must make a round trip so you get half the distance the laser travels. This kit allows you to separate the detector and emitter and put them in different locations. This will give you longer distances but at the added cost of a second power supply.

You mentioned using IR beams. If you want to do this you can sub an IR laser module and IR LED in this kit.

<small>[ January 19, 2006, 07:04 PM: Message edited by: Jolly Roger ]</small>