Infrared light tracking robot

[brownkr12]

I am helping my daughter create a science fair experiment investigating properties of visual vs infrared light. She is using the light-tracking bristlebot kit to create the basic robot that tracks visible light. Her hypothesis is that infrared light is "easier" to track because of its longer wavelength. She will use the tracking robot (and external light sources and obstacles) to compare the length of the path taken by the robot around obstacles using visible light to the path taken by the same robot using infrared light. So, after performing the visible light runs, we need to modify the robot to track infrared light.

I am hoping it is just a matter of swapping out the photoresistors for the equivalent infrared components, but I cannot find any detailed information on the characteristics of the photoresistors that came with the kit. Of course, we need the two sets of photoresistors to have the same sensitivity levels.

If such infrared photoresistors are available, can you point me in the right direction? Or if not, suggestions on how to otherwise accomplish this welcome. Thank you!

[brownkr12]

After a bit more research, I've found that common CdS photoresistors (LDRs) are not sensitive to IR. IR photoresistors can be made from PbS, but I have not found a source for such products online. Most examples I've found rely on photodiodes or phototransistors for IR detection.

If I were to use an IR diode or transistor, I'm thinking I would need to ensure the diode operated in the 0-3V range and adjust the potentiometers to set the trigger level. Since the diodes/transistors pass current when light is present (as opposed to the photoresistors which reduce current when light is present), I'd have to swap the left-to-right motor connections.

So, to narrow my questions a bit. Do you know of a source for PbS (or PbSe) LDRs?

If not, are there any other considerations or specs I need to take into account in selecting and installing a photodiode or phototransistor to convert this robot to IR tracking? Other ideas welcome of course!

[bfinio]

Hi brownkr12,

It looks like you are already more knowledgeable about circuits than our average parent. If it helps at all, this is the photoresistor included in the bristlebot kit, and the datasheet includes some info about the spectral response:

https://www.jameco.com/webapp/wcs/store ... llDownView

However, if you've already found out that CdS photoresistors aren't sensitive to IR, I'm not sure if that helps.

We have a line-following robot project:

https://www.sciencebuddies.org/science- ... wing-robot

that uses an IR emitter/detector:

https://www.jameco.com/webapp/wcs/store ... llDownView

which consists of an IR LED and an IR phototransistor. The robot works by aiming the sensor down at the ground to detect a black/white surface, but you could aim it "up" like with the light-tracking robot and aim your own IR source at the sensors (and leave the pins for the IR LED disconnected).

The problem is that this won't really be an apples-to-apples comparison because the response characteristics of the phototransistor and the photoresistor will be very different. In my experience working with the robots, the IR version is either on or off (the robot either stops or drives forward full speed) whereas the light-tracking version has a much smoother, more continuous response. That makes me wonder if it would be better to forego the photoresistor altogether, and compare an IR phototransistor to a visible light phototransistor instead. As for a PbS photoresistor - we source our electronic components from Jameco, and that didn't turn up any search results on their site. Didn't have any luck with Mouser or Digikey either and they have huge catalogs, so it doesn't look like they'll be easy to find.

Hope that helps,

Ben