Laser detecting from the distance 100 - 150 cm

Hi all,

I am planing to use 650 nm, 5 mW, 3V red laser diode and photodiode for my project. The laser should be detected from the distance of 100 - 150 cm. I am trying to achieve logic 1 (5 V) when laser detected and logic 0 (0 V) when laser is crossed. I may use op amp or ADC input of my microcontroller. What kind of photodiode should I use with minimum cost?

Thank you.

Once the laser cluster is collimated, although occur some dispersion along the path, the main issue seems concern to optical and mechanical scope, due the radiation felt at such distance is almost the same if sensor is placed closest.


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andre_teprom said:

Once the laser cluster is collimated, although occur some dispersion along the path, the main issue seems concern to optical and mechanical scope, due the radiation felt at such distance is almost the same if sensor is placed closest.


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Click to expand...

Thank you for the reply. What scares me the most is I do not know if I can obtain detectable voltage with sufficient current with the simplest photodiode (I do know the specifications but it is transperent and looks like simple IR LED) I did not buy the photodiodes and the laser diode yet. It takes 3 weeks for me to obtain these parts. It would be disastorous for me if the system did not work.

I´m not sure if I properly got the point, but as far I know, laser intensity is quite heavy, and even the weakest one, is able to deal with such application. To be honest, the issue seems to be oppositely how to protect it against so strong intensity


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I agree with andre_teprom. A 5mW laser should be easy to detect over much longer distances, maybe 1 Km or more if the optics are correct. Over short distances the problem is more likely to be that the laser is so intense it damages the sensor.

I don't think you will find a sensor that gives a 0V/5V output directly but a photodiode with an amplifier or a phototransistor should be able to do it easily. The biggest problem you will face is ambient light being detected and mistaken for the laser. There are three ways to protect against that:
1. shielding - preventing light falling on the sensor from any direction except the laser source,
2. optical filtering, for example if it's an IR laser, use IR filters in front of the detector to stop other wavelengths reaching it,
3. modulate the laser light to give it a distinctive characteristic that you can filter electronically. (as done with domestic IR remote controls)

Brian.

andre_teprom and betwixt, thank you very much for your replies. I will consider what you said.

Start with the output power of the laser as the maximum energy which can illuminate the photo-detector. Reduce this amount to take into consideration optical effects such as reflections, optical filtering and collecting area of the detector. Then, the photo-detector will have a specification called the "responsivity". Typically, this is in units of "Amperes per Watt" and is given for a range of wavelengths on a graph. For a silicon detector, a typical responsivity for green light is 0.8A/W. A red laser will have a lower responsivity. Look on the graph. Pick the wavelength of your laser. Then, multiply the optical power collected by the detector by the responsivity. This gives the resulting "photo-current" in the detector. From there, it is a matter of electronic design to convert the "photo-current" to the output. For example, let's say the maximum optical power is 5mW and 10% will be collected by the detector. This gives 0.5mW of optical power into the detector. From the responsivity graph, the laser is red and the responsivity is 0.5A/W. This gives a photocurrent of 0.5mWx0.5A/w = 0.25mA. One last point, the laser output will not be constant over time. For example, the output power may decrease as the laser ages. Take this into account in your design.

Detecting the blockage of a light beam can sometimes be as simple as using a "Schmitt Trigger Photo-IC". It has a digital output which can go right into a microcontroller. Search for this type of part.

Thank you, Analog Ground. The information is really helpful for me.

In case it comes to improvising, LED's can detect light.