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Photo Diodes to Detect Laser Reflections

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lbc

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Can anyone suggest a source for some photo-diodes that I can use to detect reflections from the cheap, toy laser pointer pens (650nm). I'd like to use these laser pens and photo-diodes for science fair projects for my grand kids.

Any suggestions will be sincerely appreciated.

LBC
 





lbc ---
There are no TOY lasers. That's first.
Looking for reflections from this 650 nM laser will require a good photo diode plus a 650 nM pass filter (likely $50) plus the circuits to amplify the signal. Remember that what we call White light contains some light in the 680 to 650 nM spectrum and background "noise" will need to be controlled.
What are you attempting to demonstrate?

HMike
 
Thanks, HMike. I was thinking we could maybe make a simple distant measurement device -for instance bounce the laser off a distant wall, or maybe try bouncing off water or do some reflection/refraction through water. Right now just trying to get ready for the updoming science fair season with my grandkids. Where can I get a 650nm filter and some good photo diodes? What type of brand or type of photodiodes would you recommend?

LBC
 
With a "toy" laser, detecting the signature of its reflection from a distant wall in ambient lighting might be akin to detecting a candle's output from 100' in broad daylight. The filters can be found at Edmond's Optics and the photodiode --- Radio Shack may have something as a starting point.
Look under the "welcome" thread at the 7th grade project discussion.

HMike
 
Thanks, HMike. I will give Edmond's Optics a look see. I happened to see a laser distance measuring tool at Lowe's a while back. It sold for around $90 and used an "eye-safe" laser. The box said it could measure distances from 2 inches to 130 feet. I thought that might be something we could try to duplicate. I might go ahead and buy it and take it apart to see what is in it.

Regards,
LBC
 
lbc---
Most of the distance measuring tools are sonic with a laser pointer. Look at it first and determine if it is truly an optical rangefinder. My optical rangefinders use IR for the beam.

HMike
 
Again, thanks, HMike. I will look and ask questions before I buy one. I am surprised that IR would have a long range. How far do they sense?

lbc
 
If you are trying to detect a laser beam...you may be better off
modulating the Laser and detecting it with a demodulating circuit.
That would block a lot of the ambient light and you may not need
a 650nm filter...

To use a laser as a distance measuring device will require a very precise
high frequency oscillator circuit as a timing reference..

It would be easier to use an ultrasonic xmitter/rcvr for distance measuring
since the timing oscillator frequency would be a lot lower...


Jerry
 
Last edited:
Laserbee has stated what I was reluctant to bring up. I used the word "signature" in an above post to imply what Laserbee has said.
He is correct but it raises the level of complexity to level 4 !!!

HMike
 
Agreed Mike....^^^^

Simple circuits will give you simple results....:cool:


Jerry
 
Thanks, Lasersbee and HMike. I guess as a start I would want to just detect the laser or its reflection. Modulating one of the cheap pointers might be an interesting or imposing project. Maybe I could come up with some way to pulse it on and off. If I use a demodulating circuit wouldn't I still need some form of photo detector?

I have been looking on the web for sources of photodiodes and I am surprized at the cost for diodes in the 650nm range. I have seen a low cost of around $12 each but digikey and some other sites are quoting $120 to $400. I don't think I want to play with a component over $10 each especially with their sensitivity to static.

Regards,
LBC
 
photodiodes can be really cheap, and with a lick-in amplifier, you can get a very good SNR. Even with a simple electronic circuit I could detect a bright LED pulsed at 25hz about 19 meters away, with a bare photodiode and no lenses.
Are you measuring reflections that are completely diffuse or are you measuring a direct beam?
 
Wow, Bluefan, that sounds like what we would like to try. I am not sure exactly what you mean by "completely diffuse" but I notice if I point the laser dot at styrofoam cup and angle the cup toward a white sheet of paper I can see the paper has red light covering it (not a dot like when I reflect off a mirror). What kind of amplifier and simple circuit did you use. My original intent was to use the Walmart pointers as the laser source but I can probably figure out how to put the laser diode on the output side of a pulse generator circuit. At this point just being able to detect some reflection with some home made circuitry would be a good starting project. Then maybe we can get to motion detectors and other applications. Can you recommend a source of cheap photodiodes? I am not sure why some photodiodes (at 650nm) cost over $100 and some cost under $10 but I suspect there is a good reason.

Many thanks, for the info. It looks like it is doable.

Regards,
LBC
 
Be sure to use the trigger output for the laser to trigger the sensor to keep it in phase.

HMike
 
LBC,

It really depends on what you'd want to measure. If you just want to measure how much light of the laser falls onto the sensor, you can try the following:
Keychain bullet-style pointer are very simple lasers with no real regulation, just 3 batteries, 1 resistor and 1 laser diode. The casing is very thin and could almost be cut by ordinariy scissors. You are left with the diode on the pcb with the resistor and switch glued to a shiny pieve of plastic that holds the collimating lens and conducts the current.
The switch should be bypassed, solder a wire across the terminals of the switch. The spring is the negative lead, the shiny case around the LD and lens is the positive "lead". These pointers should take about 4.5v or lower, but 5v is too much and the LD can emit up to 10mW before it dies.

You need to make a circuit that pulses the laser diode. A function generator can be used for testing, but mind that the voltage may not become negative. A pulse generator works fine. Simple oscillators based on a single opamp exist too. Let the diode pulse at say 1khz, that is high enough for most uses.
The easiest way would be to connect the photodiode to an active filter, this takes a single opamp and filters and amplifies the signal. A bandpass filter around 1khz should filter practically any signal not belonging to the laser.
Phase sensitive detection is harder, but a decent filter would get you quite far without. Simple silicon photo diodes should only be a few $, only very fast, large or IR photodiodes should be more expensive.
 
Thanks, Bluefan. We have already played with taking the pointers apart. It has been over 30 years since I played with opamps but if I remember they are pretty easy to plug and play with. I can probably find an idiots guide to using them. I do remember building a bandpass filter with them. I found some reasonable priced photo diodes (650nm) at Hamamatsu.

Thanks to everyone for their help and advice. I will get back and report on progress as we stumble through this high tech jungle. Looks like we will learn some good stuff.

Regards,
LBC
 
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