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Topic: Laser detection, Transistor problem... (Read 302 times) previous topic - next topic

azizth

Feb 08, 2014, 09:26 pm Last Edit: Feb 08, 2014, 09:49 pm by azizth Reason: 1
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 ;)

MarkT

#1
Feb 08, 2014, 09:32 pm Last Edit: Feb 08, 2014, 09:34 pm by MarkT Reason: 1
You need shielded coax cable to the diode, you are just picking up RFI.

[ well probably.  Another possibility is oscillation. ]
[ I won't respond to messages, use the forum please ]

LarryD

Try some different valued capacitors across the input ex. .001  etc.
Also consider adding some decoupling for the power supply.
The way you have it in your schematic isn't the same as how you have it wired up!

azizth


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?


azizth


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)

LarryD

By the way thank you for the reasonable sized images, wish others would do so.
The way you have it in your schematic isn't the same as how you have it wired up!

MarkT



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.
[ I won't respond to messages, use the forum please ]

azizth

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.

azizth

Unfortunatly, after some test, Coax cable is better but the system remains unstable :~

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 1?F to filter more?

Thank you

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