1. Home
  2. Hobby Electronics Basics
  3. reflected laser light detector circuit

reflected laser light detector circuit

I'm trying to make a battery-operated, microcontroller-controlled, short-range, laser range-finder. It needs to be very light (weight-wise).

Does anyone know of a reliable method of detecting reflected green laser light from about 50 feet from a grassy surface in an outdoor environment? The grass seems to reflect a sizeable amount of light back to the source (at least by eyballing it). It seems a detector should be able to differentiate the presence or absence of the laser light. The green laser light seems to reflect better than red on a grassy field. I have a very small, light-weight green laser module which is suitable to my application but now I need to develop a reflected light detection system.

I have tried a phototransistor (Panasonic PNZ121S0R) but it didn't seem to pick up any changes at about 10 feet onto a fairly glossy hardwood surface (wood panelling). When I try the laser directly into the phototransistor over about 10 feet distance, it worked very well, though. I had the collector pulled up to 4.5VDC (3 AA cells) and the emitter grounded. The collector showed about 4.5VDC with no laser light and about 0.15VDC with direct laser light. With the reflection, though, there was no change at the collector from the 4.5VDC. I verified the detector was aligned with the reflected light. I even tried a small straw as an ambient light reducer but no change.

I then tried a photoresistor. It seems to be a little more responsive. With just an ohmmeter across the leads, it showed a quiescent resistance of about 100K and when it sees the reflected light, it drops to about 85K or

90K. This was inside in a flourescent light environment, though. I did use a guide tube (straw) with the photoresistor and it helped significantly in reducing the ambient light. I thought I could use the photoresistor in a comparator circuit to set a range of operation.

Any thoughts or experience would be appreciated.

Thanks.

Dave

Reply to
starfire
Loading thread data ...

you might want to show the circuit you're using that forms the detector.

I'm guessing that you're not modulating the laser to a specific frequency and thus you are not, passing it through a BP filter.

formatting link
"

Reply to
Jamie

There's really not much to the circuit I used. I just used a phototransitor with the collector tied to a 10K resistor and then to +4.5VDC. The emiter was tied to ground. The device had a small bubble lens integrated onto it to gather light and focus it on the base. I monitored the output from the collector relative to ground.

I did some more Google searches for reflected light detection circuits, etc., and saw several references to modulated systems including the lase driver and the photo detection portion. The laser module I have is integrated and potted. I tried looking at the variations in power supply voltage to see if I could determine the modulation frequency. As near as I can tell, its about 100KHz.

Dave

Reply to
starfire 

--
Look at the phototransistor\'s collector with your scope when it\'s
being illuminated directly by the laser and you should be able to see
whether your laser\'s output is being modulated and, if it is, at what
frequency.

JF
Reply to
John Fields

Excellent suggestion...

I tried that and when the laser hits the phototransistor, the collector goes to a logic ground and there is no acitivity on it at all. It stays at ground (~0.15VDC). If I start to veer away with the laser, the output starts back up toward +4.5VDC. There is no activity at the collector other than a fairly stable low level when illuminated with the laser.

Dave

Reply to
starfire

Am I missing something here?

You are using an IR sensitive (800nM) Phototransistor to detect a Green (~500nM) Laser signal ??? Not very efficient ! That would explain the Direct 10 ft. response but no response to the scattered diffuse signal.

I would expect a green Optical filter would reduce interference but you need a GREEN sensitive detector to detect a monochromatic green signal.

Yukio YANO

Reply to
Yukio YANO

Good point. I just grabbed a phototransitor out of the drawer for a quick test and didn't really look at the spectral response. According to the datasheet, it should still have about 30% efficiency at the green frequencies, though. I would have thought it would have showed some response. I did try it with a green filter (525nm) in front with no difference, also.

Can you suggest a suitable green-sensitive phototransistor for this type of application? All my Google searches turned up nothing suitable.

Thanks.

Dave

Reply to
starfire

"starfire"

** You must be dreaming.

A real wet one.

..... Phil

Reply to
Phil Allison

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.