Detecting Lasers with photodiodes---Circuit layout?

[Paradox101]

For a certain science fair working model, what should I use to detect the collimated light of a HeNe laser(in the red;653nm if that is necessary)?After some surfing on the net, I found that I should use a 'photodiode'.
The desired result should be solely to detect whether the laser's light is falling on it or not.What electrical device should come into play if I was to employ it in this science project?
Note--- This project is actually similar to a burglar alarm, so upon detecting that an object is crossing the laser's beam,a buzzer should ring an alarm.

 

[mfb]

It depends on your setup.

If the laser is powerful enough, a simple "supply voltag, buzzer, photodiode, resistor" circuit could be sufficient. You probably want to amplify the signal with a transistor, however. How does the buzzer work?

 

[davenn]

For a certain science fair working model, what should I use to detect the collimated light of a HeNe laser(in the red;653nm if that is necessary)? After some surfing on the net, I found that I should use a 'photodiode'.
The desired result should be solely to detect whether the laser's light is falling on it or not. What electrical device should come into play if I was to employ it in this science project?
Note--- This project is actually similar to a burglar alarm, so upon detecting that an object is crossing the laser's beam, a buzzer should ring an alarm.

there's zillions of ideas on the net ...

http://www.bing.com/images/search?q=photodiode+light+detector+circuit&FORM=HDRSC2


Dave

 

[davenn]

you also need to consider some narrow band optical filtering if this is going to be used in daylight
as any ambient light will be picked up by the photodiode


Dave

 

[mfb]

you also need to consider some narrow band optical filtering if this is going to be used in daylight
as any ambient light will be picked up by the photodiode

Differential measurements (laser on/off) are an alternative, but I don't know if HeNe lasers can be switched on and off frequently without issues. Works fine with LEDs over short distances. It requires coupling the source and the receiver, of course. Which leads back to the point "we don't know the setup".

 

[davenn]

personally, I wouldn't even be considering a HeNe laser or even a laser diode UNLESS the distance between the source and detector was 10's of metres
HeNe lasers are dreadfully power hungry, run hot and have a very short life span ( when run continuously) compared to laser diode or IR LED
For anything up to around 10 - 20 metres, standard IR diodes ( focused) and use an appropriate IR receiver diode.
eg

these have a + and - supply and an output

This will make daytime use so much easier

Dave

 

[meBigGuy]

Not sure why you want to use visible red light. Is that just because it is what you have, or is it part of the demo?

For the basic design steps, you have to consider:
A. You need to deal with ambient light vs. laser intensity.
B. You need to deal with interfacing between the photodiode and whatever you want it to "trip".
C. You may want to consider advanced techniques to improve #1.

Assuming the laser is always on (#C is discussed later), you probably need to have a threshold calibration to set a threshold for the laser light level.
The laser needs to be bright enough to make a significant difference to the ambient light. This is essentially your signal to noise level.

A. There are several simple optical methods to help you out with ambient to laser ratio:
1. Get a filter for the laser frequency to put in front of the photo diode. <---- Probably Required
2. Put a neutral density filter in front of the laser to avoid saturating (overloading) your detector. <--- Kludge
3. Add a lens system that focuses a narrow area on the detector. <--- Good
4. Something as simple as a cardboard tube can reduce adjacent light sources <--- quick and simple

B. There are many different circuits ranging from very basic and insensitive, to very sensitive. The link from davenn shows they range from simple transistor based circuits to opamp based circuits. The most basic opamp principle for a photodiode is shown below. Measuring the voltage output from such a circuit would allow you to experiment with The ideas in A and give you some idea of what the final design needs. The value of Rf depends on the diode and light levels.

Essentially when light falls on a reverse biased photodiode, the leakage current increases and the opamp output voltage changes to supply the current.
This circuit is an over simplification to show the principle.

You may want to search for a module. Such as
http://www.lightinthebox.com/lm393-light-sensor-photosensitive-sensitivity-light-sensor-module-for-arduino-free-dupont-cables_p1023968.html <--- looks adequate!
https://www.amazon.com/dp/B00JRNN1FW/?tag=pfamazon01-20

Although, I didn't see schematics for either of those.

C. The next level of sophistication involves chopping or modulating the light at a particular frequency and building your detector to filter for that frequency. I think you need to understand all of the above and determine a true need before you head down the modulation path.

 

[Paradox101]

Not sure why you want to use visible red light. Is that just because it is what you have, or is it part of the demo?

Could you suggest any other laser,not necessarily in the visible spectrum but definitely in my low budget?

 

[davenn]

Could you suggest any other laser,not necessarily in the visible spectrum but definitely in my low budget?

reconsider what I said in my opening comments in post # 6


Is there really a good reason why you want to use a laser ?
if so use IR one instead
you still haven't commented over what sort of distance you want this to work


Dave

 

[meBigGuy]

Why a laser as opposed to an LED? You really have not explained what you are really trying to do, which makes it hard to focus on the correct advice.

 

[davenn]

Why a laser as opposed to an LED? You really have not explained what you are really trying to do, which makes it hard to focus on the correct advice.

Yes, this is what we have all been asking

 

[rbelli1]

Paradox, can you describe the experiment and its objectives? If the goal is for object detection then just about any other light producing technology from candle on up would be easier and more cost efficient than HeNe laser.

BoB