[SOLVED] Photodiode sensitivity vs distance

nacoolp said:

Hello all,

I am using a rotary laser light transmitter and a photo diode and its filter circuit to detect the former even in the sunlight. Now when I use the same at a distance up to 300 m the detection occurs perfectly. But beyond this distance, the diode hardly detects the light.

The detection circuit is attached along the post.

Thanks in advance.

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nacoolp said:

Thanks jiripolivka for your feedback.

First of all , I would apologize to all as there was a mistake in the circuit diagram I attached before. The photodiodes attached are reverse biased and not forward biased as shown in the circuit.

Also, I tried many different values of R1 and C1 to cope up with the filtering and biasing. And also, my photodiodes have been placed in a red coloured glass tube. But to no avail. I still think I must be having some problem with the Capacitor value because with distance the contact time of the laser light and the photodiode reduces and hence will not allow the capacitor to discharge completely and thereby not biasing the transistor. The transmitter rotates at a speed of 600 rpm. Hence, as per my calculation, the contact time of laser light and photodiode at 2 m is 23us and the same at 200 m is 397 ns and that at 400 m is 119 ns.

Is my assumption correct or its totally false? Kindly guide me through it as I am stuck to this problem.

Thanks and Regards.

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jiripolivka said:

It sounds strange to me that instead of light pulses generated by a mechanical interruptor you rotate the laser transmitter. Then, according to your calculation the problem is not in decreasing signal to noise ratio but in the narrow pulse width. This will really require readjusting the electronics.

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Also, as I can now see the circuit you use, several photodiodes in parallel means their capacitance may shunt the very narrow pulses. The PNP transistor should have a 100 MHz or more the limit frequency as ~100 ns pulse width means >10 MHz is needed to transmit. I cannot see the following electronics which may also need modification.
The basic problem is that your beam rotates. Can you try a simple rotating wheel interruptor? then there will be no problem with pulse length varying with distance.

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