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photodetector circuit

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electro98221

New Member
Hi,
I am trying to build a circuit which can detect modulated laser light from a standard red laser. However everything I have tried so far has not been sensitive enough to pick up the modulated signal from any significant distances, I would like it to be able to detect from at least a metre away. The laser light is being modulated at 16MHz. Ambient light has also been a big problem. Ive search around a bit but I haven't found anything suitable. I would appreciate your help.

Thanks.
 

I²R

New Member
What's a "standard" red laser? And are you trying to detect the beam directly (which should be trivial) or reflected light? You should also probably tell us what you're tried so far which hasn't worked, so we don't duplicate your efforts.
 

electro98221

New Member
Thanks for the reply. Its a red laser with 1.5mw output. I am trying to detect the reflected light. I need to pick up the 16MHz sinewave so that I can process it at a later stage. I will be measuring the phaseshift using the heterodyne technique. I have tried simple circuits with a photodiode in photoconductive mode, similar to here https://www.electro-tech-online.com/custompdfs/2008/03/Photodiode20Typical20Circuits.pdf
I have also tried a photodiode in series with a LC instead of a resistor but just ended up with a oscillator circuit. If I connect more than one photodiode in an array, would that help improve things?
 

electro98221

New Member
I am trying to measure distance using phase shift, using the high frequency should give me good resolutions. I was hoping to be able to measure distances up to 5 metres or so but now I realise because I have inexperience with receiver circuits I will be lucky to get 1 metre.
 

I²R

New Member
Use a short-focus converging (convex) lens with the photodiode at the focal point. The larger the lens diameter, the greater the light gathering power and therefor the better your range. To find the focal point, simply aim the lens at a distant bright light source and move a piece of white card or paper back and forth behind it until the light source is in focus.
 

ronsimpson

Well-Known Member
Most Helpful Member
What photo detector are you using? Did you check to see if it will work at 16mhz. That is very fast for a low light detector.
 
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