In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

earlitow — Wed, 23 Mar 2016 15:58:55 GMT

Thanks for the article.
Its well organized and very helpful.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Tue, 17 Feb 2015 16:25:52 GMT

I'm proud and happy to be of any assistance to such a hard worker!

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Fri, 13 Feb 2015 02:34:52 GMT

The friendly folks at Analog Devices are sampling these:

AD8015 datasheet and product info | 155 Mbps Transimpedance Amplifier | Transimpedance Amplifiers | Analog Devices

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Thu, 12 Feb 2015 20:49:11 GMT

They also make integrated sensors, this one can be had for $3-4, usd.

http://datasheet.eeworld.com.cn/pdf/TAOS/197253_TSL254R.pdf

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Wed, 11 Feb 2015 22:03:03 GMT

They call that 'star ground topology,' very good idea and practice.

The other reason to try a Norton amplifier is that your sensor is current-mode like the inputs of the Norton amp. This can give you nice-fast response. This is probably not the device to use as it is old and has probably been superseded with amps with better specs, but it gives the idea.

http://www.ti.com/lit/ds/symlink/lm3900.pdf

http://www.ti.com/lit/an/snoa653/snoa653.pdf

Here is the new device:

http://www.ti.com/lit/an/snoa666b/snoa666b.pdf

An analogy can be drawn between op amps vs. Norton amps and photovoltaic mode and photoconductive mode. Op amps are quieter and more accurate. Norton amps are faster.

Matching Zin//Zout can be quite rewarding. If you have a highZ sensor, the op-amp is the usual way to go. If you have a loZ sensor, the Norton amp can often be the ticket.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Wed, 11 Feb 2015 05:49:04 GMT

You should consider using a Norton operational amplifier in place (with appropriate rearrangement of the other circuit elements) of the op-amp. This would make 'nulling out' the quiescent current particularly easy.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Tue, 10 Feb 2015 04:12:17 GMT

There is some debate over photo-voltaic versus photo-conductive mode. Invariably, when I read an article about one or the other, the brandX comparator is a straw-man. You should vet photovoltaic mode experientially. The penultimately simple schematic is:[View:/resized-image/__size/382x207/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-3514529d-ac2f-40ea-b689-8f22f469bb95/5050.contentimage_5F00_178678.jpg:382:207]

For the CCS you should use a Wilson current mirror and a variable resistor. The device I suggest using is http://www.nxp.com/documents/data_sheet/BCM62B.pdf

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Tue, 10 Feb 2015 00:22:29 GMT

If I get the intent of this, adding an HPF, while greatly serving to stabilize the amplifier, is going to result in us getting information about the derivative dustiness rather than the absolute dustiness. If I don't fully get the project, impute it to my being on the outside and your being on the inside.

If you have a great range of possible signal, you may want to 'compress' it a bit using a log amp or an amp with an atan transfer function.

If you are using a psuedo-ground it probably doesn't have to split the supply evenly. Think about where you want to set the reference point.

You may need to trim the offset if your device has significant 'dark' or in our case 'gloaming' current. There are manifold ways to do this. One of the better ways is to feed your output into a two-port summing junction.

[View:/resized-image/__size/419x243/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-3514529d-ac2f-40ea-b689-8f22f469bb95/contentimage_5F00_178676.jpg:419:243]

Valid to shunt capacitor from tap to BoS or ToS.

[View:/resized-image/__size/420x384/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-3514529d-ac2f-40ea-b689-8f22f469bb95/contentimage_5F00_178677.jpg:420:384]

Yeah, a good place to take the output of the prior circuit would be the output of the op-amp. I just didn't wanna mess up the drawing.

It may or may not complicate your PS, but you can alter the DC bias of your circuit by injecting current (with a variable resistor from the supply, perhaps) into your inverting op-amp node. This could be used to offset the quiescent current from your sensor.

Other information that I should have included on the last diagram is that, neglecting saturation, the Zin that V1 sees is R1, and the Zin V2 sees is R2. Weighted summation.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Mon, 09 Feb 2015 21:02:03 GMT

[View:/resized-image/__size/272x312/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-3514529d-ac2f-40ea-b689-8f22f469bb95/contentimage_5F00_178675.jpg:272:312]

You might try this circuit. This has the attenuator, so you can get the gain you need with normal resistors. The intent of the resistor on the non-inverting input of the op-amp is to balance the currents on both sides of the mirror, eliminating one source of error. Here I use a green LED as the sensor. This will give you the optimal spectral insensitivity.

PI and T Pad Attenuator Calculator

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Mon, 09 Feb 2015 18:28:55 GMT

Yeah, I was wrong about the wavelength of that detector, slipped a decimal point, got distracted, sorry.

You are really at about 4500 angstroms in the green, right?

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Mon, 09 Feb 2015 18:18:30 GMT

As I understand it, our detector senses between lit and brightly lit, in this application.

You have chosen a voltage bias of zero here. In this context, zero is just another number. Forward and backward bias are other possibilities. I would back-bias. This would cause the depletion region of the detector to become wider. Here is the formula for a single, parallel plate capacitor.

The Greek bit has to do with the the dielectric constant of the material, which doesn't change for us. A for area, also constant for this discussion. Increasing the back-bias increases d, lessening C proportionally. This has the unwanted effect of increasing dark-current, but I don't think you care very much about that. Maybe a slight delta in noise, but you can probably afford the hit. This will almost certainly speed things up.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Mon, 09 Feb 2015 07:44:24 GMT

"One of often used photodiode amplifiers in physics laboratories is OP27 that has twice smaller input noise but requires bipolar power supply."

The OP27 likes to operate around thirty volts, but like all known op-amps, only has two supply terminals. Perhaps you intend to state that it doesn't swing quite all of the way down to V-, because it has a bipolar output stage. Despite the implication of Burr-Brown's DS for the OPA350, all op amps are 'single supply.' They may use a reference potential at the midpoint of the supply, but that is not a PS attach to the op-amp, just a point on the circuit.

Given the gains you are after, the neat thing about the OP27 is probably the on-board offset trim.

The reason why you can use a circuit with a single supply is because your input signal has one signum only. So you can group signal ground and supply minus as your output can get to 10mV of V-.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

D_Hersey — Mon, 09 Feb 2015 06:44:34 GMT

"It is not practical to use larger feedback resistor than 1 Meg as circuit becomes slow."

Try using an attenuator in the feedback path rather than a single resistor.

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

jw0752 — Sun, 08 Feb 2015 01:30:47 GMT

Hi Janis,

Thanks for the interesting and educational post. I will enjoy following your project.

John

RE: In-the-Air-Challenge: Laser-based dust counter using a photodiode, IoT connected

DAB — Sat, 07 Feb 2015 21:00:29 GMT

Nice design.

The more heat you can draw away from the laser diode the longer it will last.

Make sure you have plenty of air flow over the cooling fins. You might also consider using an air cooling system as an input to the laser.

If you can PWM the laser, then ;you get even more benefit in keeping it cool.

Are you doing any beam intensity measurements? It might pay you to keep tabs on the laser output to see if it degrades with time.

DAB