1
\$\begingroup\$

While I did find a little information in searching your site I am still unsure what to use in my application. I want to detect the (red) laser light hitting a screen or target. My questing is what is a good device to use, I read that photo diode is faster than a photo transistor. Can any one give me any part numbers to try?

Application Simple laser target, when the training pistol laser hits the target I need a fast detection. I will experiment with the final output later in this project, to start with I might just turn on a light. I also have realized that other indoor lighting may need to be filtered, I have knowledge of filters, I will later research this.

\$\endgroup\$

closed as too broad by PeterJ, brhans, Daniel Grillo, Dave Tweed Jan 12 '16 at 17:19

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • 2
    \$\begingroup\$ Reaction speed maximum? Spot size of laser? Divergence angle of laser? Distance to target? Window size of detector? Wavelength of laser light? \$\endgroup\$ – Andy aka Jan 5 '16 at 12:37
  • 1
    \$\begingroup\$ You'll find that the detection is easier if the laser is modulated with a known waveform. Otherwise, it will be difficult to distinguish from bright sunlight. \$\endgroup\$ – Simon B Jan 5 '16 at 13:21
  • \$\begingroup\$ As these things go, I don't think you really need fast detection. \$\endgroup\$ – Scott Seidman Jan 5 '16 at 13:40
  • \$\begingroup\$ This is a valid question. Too bad was closed. \$\endgroup\$ – WeGoToMars Dec 11 '18 at 18:03
  • \$\begingroup\$ I had a 5mW laser diode shine in an infrared receiver (photo-diode used in a TV set) and I got 90mV. If I expose the LED to normal light I only get 1-2 mV. \$\endgroup\$ – WeGoToMars Dec 11 '18 at 20:34
2
\$\begingroup\$

I assume the 'training pistol' is hand held.

If it is, you likely only need detection in several milliseconds, or more.

Almost all photodiodes and phototransistors are fast enough by a factor of 100x, or more. So, as Scott Seidman says, fast detection is not likely an issue.

I'd recommend starting with a visible light phototransistor. They are easy to use as detectors, and are cheap, i.e. under $1. (e.g. Vishay TEPT4400). It is over 90% sensitive at about 630nm. These have a wide enough range of wavelengths that they will still pick up infrared light, so use an optical filter, to reduce the intensity of other light sources too.

To avoid confusing the detection of the laser with sunlight, or some other light source, the laser needs to be modulated. This needs to be done in such a way that it is easy to discriminate the laser signal from other light sources.

Simple hand-held laser's can be modulated at about 5kHz, which is well away from mains-powered light modulation (50, 60, 100 or 120Hz), and Infrared remote control (36kHz or higher). Depending on your level of skill, you might build the filter using 'pure electronics', or you might use a small microcontroller.

\$\endgroup\$
1
\$\begingroup\$

LEDs also work as photo diodes - just not characterized as such on the datasheet. An LED with its spectral peak close to that of your RED laser will naturally ignore other broad spectrum light sources, and if the LED lens is RED (not clear) then it has a built in optical filter. Finally, the LED viewing angle (field of view as a photodiode) can be reduced to match that of the maximum angle of incidence of the laser.

Here is one with a peak wavelength very close to the 635nm red laser and a 30 deg FOV.

https://www.digikey.com/product-detail/en/ALMD-EG3D-VX002/516-2162-1-ND/2389846

As others have stated, modulating (psudo random pulsing of the laser) will also aid in detection. With modulation and the appropriate signal processing you won't need an optical filter unless the red component of the ambient light is so bright that it approaches saturation (and the modulation is lost).

Finally I'll add, if used indoors, then be aware the white LEDs and florescent lights emit in spectral bands and often flicker at 120Hz. You might want to choose a laser color that is different from their peak wavelengths.

\$\endgroup\$
0
\$\begingroup\$

600nm - 680nm filter on photodiode(or phototransistor or thermistor) what you need I suspect. However, they are neither cheap nor as fast as you need it to be I guess. Thats because of the filter of course. They are in limited production usually, I remember that we had a quote for 10 photodiode with 880 nm filter from heimann 35€ for each.

And you are right, most of the photodiodes responds to unwanted lights unless they have filter on. However, you can always specialize your application to satisfy your needs such as adding some modulation, arranging the receipt sensitivity and gain etc..

Have a look at this page.

You will see some photodiodes with specific response ranges. But they are limited to 10 - 100 MHz ranges so I am guessing they do not have less than a few hundreds of nanoseconds response time.

edit:

particularly this one looks like what you need if you care for your budget. I hope I could help.

\$\endgroup\$
  • \$\begingroup\$ I finally had time to dig out the old breadboard and work on this and what I wound up using was two LDR resistors in parallel and a 741 op amp. I made a test target (white paper can) the laser is reflected and the inside of the paper can turns bright pink. Thanks for the comments and the links. \$\endgroup\$ – hnash01 Jan 18 '16 at 10:31

Not the answer you're looking for? Browse other questions tagged or ask your own question.