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Hello all,

I have made this circuit (below) for laser detection in my work with photodiode in (X1-1 ; X1-2) and  Arduino. Everything works perfectly.

For technical reasons, I wish away the photodiode from the board (arround 1.5 m). But when I put a cable (02x0.4mm) greater than 30 cm between the photodiode and the board , transistors engage, as if there were laser detection.
I try to put variable resistors between the first transistor, but it has nothing do ...

any help would be welcome, as I am completely stuck

Br smiley-wink


* laser.jpg (58.23 KB, 793x443 - viewed 24 times.)

* laser2.jpg (93.49 KB, 500x385 - viewed 17 times.)
« Last Edit: February 08, 2014, 03:49:31 pm by azizth » Logged

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You need shielded coax cable to the diode, you are just picking up RFI.

[ well probably.  Another possibility is oscillation. ]
« Last Edit: February 08, 2014, 03:34:27 pm by MarkT » Logged

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The last thing you did is where you should start looking.
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Try some different valued capacitors across the input ex. .001  etc.
Also consider adding some decoupling for the power supply.
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You need shielded coax cable to the diode, you are just picking up RFI.

[ well probably.  Another possibility is oscillation. ]

Indeed, it is quite logical explanation for coax ...  I will try tomorow . What do you mean with oscillation?

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Try some different valued capacitors across the input ex. .001  etc.
Also consider adding some decoupling for the power supply.

I try 0-100K Ohm variable resistor nothing change..

FYI Voltage input is 12v, 4A. (Laser photodiode, support more than 40V)
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The last thing you did is where you should start looking.
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By the way thank you for the reasonable sized images, wish others would do so.
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You need shielded coax cable to the diode, you are just picking up RFI.

[ well probably.  Another possibility is oscillation. ]

Indeed, it is quite logical explanation for coax ...  I will try tomorow . What do you mean with oscillation?



You might have turned the circuit into an oscillator since it uses a feedback network
and the input cable adds capacitance changing the phase response of the feedback.
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I just test a Coax cable , no more problems .. Thank you!
You think a Cat.5 FTP network cable can also do the trick? Because I would put several photodiode in parallel.
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Unfortunatly, after some test, Coax cable is better but the system remains unstable smiley-confuse

The Circuit is very sensitive, because I only have to touch the sheath of the cable to trigger the transistors.

what  I need to change to reduce this sensitivity?
Maybe the first capacitor of 10nF to 1μF to filter more?

Thank you
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