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[optoelectronics] how to protect a photodetector to avoid its saturation

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EugenioN:
Hi all!

I'm designing a measuring device for detecting the presence of some polluting elements mixed in a stream of dust. I need to illuminate the dust under test ( DUT  ;) ) with lots of energy in the near infrared region for a brief interval of time (~100us), then activate a photodetector to sense if the dust contains polluting elements that emit back some energy in the same near infrared region.
The DUT can travel fast (1-5m/s) on a conveyor belt and I assume I need to place the emitter and the detector close to each other to minimize the influence of DUT speed.

My problem is that the detector, a BPV11F phototransistor, is deeply saturated when the IR led is on, and it takes ages (~500u) to recover. During this time, the infinitesimally small light emitted back fades out, and I'm left with no signal at all.

The phototransistor is kept biased with emitter at -12V and collector fed to the inverting input of a fet opamp connected as a transimpedance amplifier with 47k feedback resistor. I used a TL072 because I have many of them in my drawers and there is plenty of slew rate.
The limit is the phototransistor itself, somehow blinded by the short emitted light. I've tried to help the phototransistor with resistors from base to emitter to drain the excess charges (220k, 1Meg, 2Meg), with slight improvement in speed but also a drop of gain, so no real help.
Also tried to short the base to emitter with an electronic SPST switch (DG449) when light is on, but this is not enough to shutter the phototransistor: when I open the short circuit from base to emitter the signal goes all the way up to full scale in a couple of microseconds with minor improvement in saturation recovery speed...

Decreasing the excitation led current will improve the phototransistor saturation recovery time, but also does not allow the glowing particles to be activated enough to be detected.

Mechanical shutters are a no go, way too slow and with a limited lifespan.
Any hints?

RoGeorge:
Put the transmitter and the receiver on the same side, such that the receiver can only see the reflected light.

If that is not enough, maybe use a brushless motor to rotate a disc with many slots in it, as a mechanical shutter.  If the speed of a rotating disk is not enough, try a rotating mirror with many faces and placed at a distance from the receiver.

S. Petrukhin:
Maybe to short-circuit the collector-emitter while the LED is on?

MasterT:
I'd think about "power control loop" - when LED driver is adjusted based on receiver signal strength. Same concepr used on mobile network, cell phone saves battery and prevents jamming for neighbours

S. Petrukhin:
We need a bright flash and then registration of the residual (fluorescent) glow, as far as I understand.
Are you trying to control the quality of grain?

Similar is used in our photo separators, it is expensive and complex equipment with a several Canon SLR cameras.

But I looked at the grain flow through these cameras (can monitor the picture in real time) and noticed that parasites and damage in the UV spectrum stand out very noticeably.
Maybe you should think about changing the pulse mode and controlling the live image?

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