[SOLVED] Photodiode sensitivity vs distance

[nacoolp]

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.

 

[jiripolivka]

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.

I cannot see any details but if the system works up to 300 meters and then it stops, I would suggest:

- check the optics adjustment (focus), add a tubular screen to the photo diode to reduce ambient
- check the filter circuit and photo diode bias. For low-level signals, they both may need a fine tuning.

 

[nacoolp]

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.

 

[jiripolivka]

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.

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.

- - - Updated - - -

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.

 

[nacoolp]

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.

- - - Updated - - -

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.

I need a rotating laser light source as i want detection to be omnidirectional. And my source has certain limitation to move at a speed of 300 - 1200 rpm. For normal 600 rpm, it will be having 10 Hz frequency. As soon as the photodiodes move away from the source, the frequency remains same. The change occurs with the contact time of the photodiode and laser light source.

So I have some doubts whether the intensity of the light is playing a role or discharging of capacitor or any other....??

 

[nacoolp]

Thank you all for your help...

The problem was with my circuitry ahead and it was due to a minor voltage drop due to a resistor. All is working properly now. But thanks for the support.