IR detector system, biasing of photo diode

Phototransistors are horrible for that sort of job— too small, too noisy, not repeatable, for a start.

It’s not signal/background you care about, it’s signal/noise, specifically the shot noise of the sunlight.

An optical filter will help reject sunlight, and a bigger detector will help more. The real win is reducing the FOV with lenses as well as baffles, tubes, and so on.

Check out the Hamamatsu S6968–super good medicine.

Cheers

Phil Hobbs

You could drive the LED with a square wave, 10 KHz or whatever. The photodiode could have +DC on one end and the other end can hit a parallel LC to ground, resonant at 10K.

That takes out the sunlight DC component and adds bandpass filtering.

Just don't fry the photodiode in high light.

An optical bandpass filter would hugely improve things, take out most of the sunlight.

If the sunlight signal doesn't just saturate the signal chain, which wrecks the gain.

Bias doesn't really matter unless you are looking for avalanche multiplication of the charge carriers - if there there's enough bias to let you collect all of them. Single photon avalanche diodes are a different can of worms, but you should have plenty of photons, so why would you bother?

More bias does reduce the capacitance across the detection diode, making it a bit faster, but the reduction is a hyperbolic function of bias, so each extra volt makes progressively less difference.

In a vaguely similar sort of situation I got my mechanical colleagues to put a graphite liner inside the tube, and cut a screw thread into the liner. They thought I was nuts, until they skipped cutting the screw thread and I promptly complained about the loss of performance.

Hamamatsu do seem to have good products.

Yes, the classic solution was to operate PD into an inductor so DC didn’t overload.

I have tried to search for optical filters. Where would one get those?

Baffles and tubes, we can do ourselves, any guideline on the best surface of the inner tube?

Looks like a very big die. Is that the main reason to use that one, to get better sensitivity?

Ok, noted :-)

That's a very nice idea. The Q should not matter much, just as long as DC is removed.

The photodiode will still be subjected to the high ambient light, but the gain would be close to zero for the stage after. I would then still need to be sure the photodiode is never saturated by ambient light.

So adding a resistance in series with the diode?

I have looked into both ultrasonics and also magnetic coil pickup. I suggested ultrasound since we could do object detection as you mention, but the client really likes the IR concept, so need to follow that path for now.

Actually, wont a simple high pass filter work equally well?

Photo diode with bias -> capacitor to gain block....

Like this:

You could try Edmund Optics

Go one step further and make the indusctor part of a parallel resonant circuit tuned to the modulation frequency.

Yes, we have RTK GPS to position it within a cm at a location right in front of the docking, 2 meters away.

No, should be locked

That is fixed

Outdoors

That's also a solution we have been working on. The Worx Vision uses that

Yes, like Larkin wrote. Like this:

A photodiode won't saturate as long as it has a few volts of DC across it. It might melt if there's no current limiting.

The LC tank combines background light rejection and bandpass filtering and has high signal gain, with two parts.

I think there are photodiodes with colored plastic, essentially a cheap optical bandpass filter. Used in TV remote receivers.

The windows in TVs may be optical bandpass filters too. They work with very little signal from the remote, in high room light.

Right. A sonic scheme could measure the phase between two mikes to determine the direction of the source with high resolution. That could be an analog multiplier or a bit of code. The amplitudes would be useful too.

One could compute direction, distance, and velocity almost for free.

No problem with sunlight!

Cheap electret mikes have gain inside, a jfet or an IC, so there would be lots of signal to go straight into a uP ADC pin.

Google has lots of hits. One square foot of plastic-film BP stuff could be choped up into maybe 10,000 little filters.

Ask for samples!

At 10 KHz, gain is cheap.

To this I would add a trick. We know something very useful about the

So feed the amplified signal from the 10 KHz LC tank to a I+Q homodyne circuit, filter to pass signals from DC to a 10 Hz and compute the magnitude of the received signal - this is used for figuring out the direction to the docking station.

The phase of the received signal is discarded, as it is effectively random because the TX oscillator phase with respect to the RX oscillator phases is uncontrolled and unknown.

The advantage over a high-Q LC tank is that the resonant frequency of the tank need not be that precise.

Joe Gwinn

Terrible problems with acoustic reflections.