I am trying to wrap my mind around a design for a long light sensor bar. This is actually a laser sensor that needs to sense a moving laser beam continuously over a certain distance. I would need this sensor to be modular so I could make the sensor bar longer or shorter depending on the application. Think 2" x 6" pcb size that could have 30 connected together.

My thought is to design pcb that has a lot of phototransistors on it (thinking ALS-PT19) and tying the open collector outputs together. So as the laser hits the sensor at least one of the photo transistors is on and outputting a high signal. Then I would have connectors where I could plug a 2nd pcb (exactly the same) board in that would tie even more phototransistor outputs together. In the end there could be hundreds of the phototransistors tied together.

Basically, I don't care how many of the phototransistors are on. If even one phototransistor is on I want a high signal.

So does that make sense at all? Is there an issue with this design? Or would that work?

  • \$\begingroup\$ Tying the collectors of the phototransistors together sounds reasonable, provided you have the transistors set up so they only trigger from the laser. It will be difficult to filter out noise or spurious signals with so many tied together \$\endgroup\$ – C_Elegans Dec 3 '18 at 16:45
  • \$\begingroup\$ Yeah my thought would be to modulate the laser to try and help with the noise. \$\endgroup\$ – Breakout Dec 3 '18 at 16:49
  • \$\begingroup\$ Yeah, that would probably work. Good luck! \$\endgroup\$ – C_Elegans Dec 3 '18 at 16:52

I'm not sure you have thought through the problems in your design, and IMO you will have severe difficulties making a reliable working application.

  1. The ALS-PT19 is a broad range light detector with 450-650nm sensitivity. You have not said what Laser you intend to use, but it's going to be in the ambient light range so impacted by ambient light conditions.

  2. Since ambient light will be a problem (unless your application is in the dark) then you will need a filter on the front of your linear sensor array to eliminate wavelengths other than that of your laser.

  3. The Laser you use is important in terms of the sensitivity of the ALS sensor. Its best sensitivity is around 630nm, so your choice of laser is limited. The sensitivity curve is shown below:

enter image description here

In all probability the best choice may be to use a Green Laser (532nm) or Orange/Red (610nm). Unfortunately for this sensor the really common Red (647nm and 650nm ) Lasers are well down the response curve. Common choices of Lasers are shown below:

enter image description here

  1. The ALS sensor area is very limited, and so will be impacted by your Laser dot size on the sensor. You really need to design for the dot size to cover two sensors and this will set the number of ALS chips you need. If you don't cover at least one sensor then you will have gaps in your range, which I'm sure you don't want.
  2. You have to consider both width and length of you sensor. If you have a single row of sensors then you width capability will be low. You need to decide just what the width of sense track you need to decide if you need more than one row of sensors. You specify about 30*6" boards in the question, so does that mean you need a 15ft sensor track? If that is so, they you need to consider the amount of ambient light at your Laser wavelength, as even with a filter you will get some unwanted light activating ALL the sensors.

Lastly ...perhaps if you explain your application in more detail then others may be able to help you more. There are potentially other solutions such as cameras or TOF laser sensors on the Laser carriage that may be suitable.

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  • \$\begingroup\$ You might be able to reduce the number of detectors by running the laser through a frosted lens, if that's a concern. \$\endgroup\$ – Cristobol Polychronopolis Dec 3 '18 at 21:24
  • \$\begingroup\$ I think those are all good points. This is a laser maze (don't break the beam) game. It will be in very low light almost pitch black. So ambient light isn't a real issue. I will using a 532nm green laser and a white acrylic (or other material) diffuser. I'm very open to other phototransistors as well. I just had the ALS-PT19 and it seemed to give me better results than a photoresistor. So I was thinking about trying to go with that. \$\endgroup\$ – Breakout Dec 3 '18 at 22:19

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