How to use multiple photo diodes as one? - Page 1
Forum section: electronics / projects

author: Chriss
author_rank: Frequent Contributor
timestamp: April 26, 2018, 11:14:43 am
content: Hi!I have to make some sort of IR photo sensor and connect the sensor to the uC.The 4pcs photo receivers are photo diodes this type: SFH 203 FA.The photo diodes should be connected to only one I/O pin on a uC.I have two problems here:1.Is there some sort of logic gate which have multiple inputs in my case 4 inputs ) and if one of the photo diodes is triggered theoutput will go Hi or LOW? no matter what state.But, the triggering should be exclusive.I mean, it can be a situation does in a fraction of time, the same object could hit the second photo diode and also trigger them, butin this case, the output should not react, or so to say, the output should not change again the state because it is the sameobject in the photo sensor. ( this is the photo senor part of a seeder machine ).2.What would be your suggestion of the design, how to connect these 4 photo diodes so it will use only one I/O pin of the uC?If it is possible I would save I/O pins for some other thinks.And If there is no other option than I have to use 24 I/O pins...For now I'm just collect ideas, infos and thinking about the concept.Thank you for any suggestion or info.My best regards.

author: Rerouter
author_rank: Super Contributor
timestamp: April 26, 2018, 11:26:25 am
content: Your question needs some refinement, but to simplify things, no retriggering within a certain time frame is easy as a software problem, having a period where you ignore that pins input after a valid trigger would likely be the solution thereFor the 4 diodes, all going high to low, its going to come down to your diode leakage a little, but I imagine either a fixed pullup resistor or a pin liftup, with all 4 diodes connected anode to ground, cathode to input,You may need an op amp to amplify this difference, and having it act as a differentiator may improve noise resistance, still connected to 1 pin, just some glue inbetween,

author: Chriss
author_rank: Frequent Contributor
timestamp: April 26, 2018, 11:32:41 am
content: I thought also to make the first problem on the software side as you mentioned.To implement a waiting time of 10-20ms, but I not sure would this work correct...This will be definitive an easy task.

author: madires
author_rank: Super Contributor
timestamp: April 26, 2018, 12:58:24 pm
content: Maybe a transistor/opamp/comparator (with adjustable trigger level?) for each photo diode and 4 diodes to combine the signals for the MCU.

author: C
author_rank: Super Contributor
timestamp: April 26, 2018, 02:05:17 pm
content: QuoteDepends some on how you arrange the photo sensors, but I can see a seed effecting two photo sensors at same time.Treating each sensor as separate could miss this case.If you think of each photo sensor as a current source, then with no seed you have a combined current. Any blockage will change this current.Start with a resistor connected to each sensor. The four resistors connected to one resistor in series.Think a comparator sensing voltage change across the one resistor is what you want.Only reason for the four resistors is to protect each photo sensor. This is photo voltage protection and might not be needed.C

author: Brumby
author_rank: Unknown
timestamp: April 29, 2018, 06:48:42 am
content: It might be better to step back a bit and rather ask for opinions as to how to handle a scenario you understand, is to give us a more descriptive view of the physical setup.For example - how are the photodiodes positioned? Are they meant to be detecting the same seed - or up to 4 seeds?These are things that will make a huge difference in us being able to ask intelligent questions and offer you practical solutions. Sometimes the best solutions can be extremely simple ... so simple, in fact, that they may not be immediately obvious.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 29, 2018, 06:42:37 pm
content: Here is a block diagram from scratch how it works on real seeder machines:And here is a picture of the sensor on a seeder machine:I set up my sensor on my bench, based on the pictures above just with 3 ir diodes and photo sensitive diodes.I use 74HC14 scmitt trigger ic to get a clear signal from the photo sensitive diodes.When I drop a corn seed through the optical channel it is sensed by the photo sensitive diodes.I use my oscilloscope for monitoring.I thinking in two ways to use the photo diodes:1. to connect the signals together and use one signal wire for every seed box.(on one seeder machine can be 4,6 or more seed boxes. Every seed boxes must have one pair of these sensors.)So I would use only 4 or 6 I/O pins on my uC.2. to use a smaller uC for every sensor and connect all the photo diodes to separated I/O pins on the uC.Then all the small uC's to connect to the main uC where the data are collected, computedand show the result on the screen or whatever...

author: Marco
author_rank: Super Contributor
timestamp: April 29, 2018, 09:02:41 pm
content: Connect their anodes (cathode should be biased by about 5-10V) and put the combined current into a resistor to convert it to a voltage (at these speeds you probably don't need a transimpedance amplifier). Connect a RC differentiator to the resistor and use a comparator to threshold the result.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 29, 2018, 09:12:07 pm
content: Hi Marco!Thanks for the quick reply.Can you please attach a schematic how you mean the circuit?Just to be sure does I understand your writing because my low English.Thank you.I'm thinking to try this kind of connection too.What you think about this?

author: Marco
author_rank: Super Contributor
timestamp: April 29, 2018, 09:44:52 pm
content: Sure, but no guarantees it worksFirst dimension R1 and the LED current to get a nice signal swing when a seed goes through (a couple 100 mV or more would be nice, dunno if you can do that without an amplifier). Then dimension R2/R3/C, make R2//R3 larger than R1 if possible, then C sets the time constant, needs to return to equilibrium fast enough for the next seed. I put the diodes below the detection resistor, because that way the pulse goes up when the seed goes through, which is more intuitive.

author: C
author_rank: Super Contributor
timestamp: April 29, 2018, 10:55:26 pm
content: When testing you need to test all states.No seed & one seed is a test.but you also could have two seeds at same time.or a hand full of seeds at a time.A hand full could be normal fast planting or an error indication.From what little you have said, I would look for a microcontroller with CAN bus for each seed counter.C

author: BrianHG
author_rank: Super Contributor
timestamp: April 30, 2018, 12:36:47 am
content: Marco has the correct circuit for this application.Though you need to properly tune the resistors and capacitor.The other choice is to get rid of the op-amp, feed the output pu of the capacitor into an MCU ADC input and software filter-isolate the signal into seed counting, but, this requires a lot of programming and software DSP filtering know how.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2018, 09:03:26 am
content: For detecting any interruption one could connect the photo-diodes in series. So the relevant output would be from the diode that gets the least light.Having the diodes in parallel would result in the sum of intensity. If the analog part is sensitive enough, one might still be able to detect a single interrupted light path.The current from photodiodes is usually rather low, so it might be a good idea to have some amplification close to the detectors.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 30, 2018, 09:58:06 am
content: Does it menas if the photo-diodes are in paralel and feed the op-amp that could result in a particle detector? I mean that could detect also the dust in the air where the seed travel. And that could be a false triggering.To sensitive...But of course it could be trimmed with the potmeter on the op-amp.I'm not sure how precise could that be in a practical meaning because of the rea lly small current of the diodes. I assume it Is somewhere in mA or even in uA region.Maybe a stupid question but no matter.Why you are focused to use an op-amp and not a Schmitt trigger?I mean if one of the diodes is triggered that should indicate that something was traveled through the light.That diode on-off time must be somehow declared in the uC or by hardware to avoid a seed bouncing effect. That bouncing effect if it is to long should trigger an error.Maybe to much seed is traveling or some other sort of problem exist.This signal shoukd be a to long high/low output, let's say maybe 50ms or 100ms...When I talked with people's who have this kind of machine, they say if they have to change to smaller seeds or bigger seeds than they have to change te sensor parts also. Becuase of the resolution I assume.From this info I know does one type of sensor is for one type of seeds. ..Sent from my GT-I8260 using Tapatalk

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2018, 01:25:43 pm
content: The current through Photodiodes would likely be in the low µA range, depending on the LEDs. So the signal tends to be rather small. With the diodes parallel I would expect more like a small drop in intensity (e.g. 20%) - this might get tricky to detect with a schmidt-trigger. The size of seeds is a factor too - coco-nuts is easy, something like papaver get difficult.It might need the extra amplification with something like the OP to also sense weak signal. I would expect a kind of adjustment option. Schmidt trigger like functions are often included with µC inputs. An analog reading to the µC is an option too. So the µC could learn the open intensity.

author: Marco
author_rank: Super Contributor
timestamp: April 30, 2018, 04:50:47 pm
content: At 10 mA a LED would put out about 0.2 mW/sr ... so if the distance between the LED and photodiode is 5 cm, that gives 0.2/25 = 0.008 mW/cm2, gives 0.008*50 = 0.4 uA.It's not much, but the capacitance is only 11 pF. So even with a Meg resistance you're looking at a time constant of 44 uS.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2018, 05:29:11 pm
content: Phototransistors could be considered for higher signal.

author: C
author_rank: Super Contributor
timestamp: April 30, 2018, 06:26:47 pm
content: Think of all the different changes switching seed can make.You listed dust and for some seed types a bag of seed will contain dust.Seed size and with this smaller is often planted closer together increasing seed detection rate for same planting speed.Some let light shine through the seed.So in addition to proper detection, the speed must be there for planting rate.C

author: Marco
author_rank: Super Contributor
timestamp: April 30, 2018, 07:03:47 pm
content: Need to know the size of the seeds and their speed to say anything useful really. You can get a reasonable signal swing with a 44us time constant with just a resistor, if it needs to be faster use amplification and a smaller resistor or a transimpedance amplifier.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 30, 2018, 07:42:06 pm
content: The seed dimension, weight, size are very unpredictable, also because of this unknown there is no way to calculate an accurate math I think.But, here is what I setup days ago and how I played with this stuff and the result on my scope:My setup is on my breadboard.I used 1 photo sensitive diode and 1 IR LED diode all they 5mm diameter.They are 6 cm far from each other.The actually distance on a seeder machine is below 4cm.So, my setup is a bit over distanced.The photo sensitive diode is connected to the 1A on a 74HC14 Schmitt trigger.And the CH1 of my scope also connected to the 1A of the IC.The CH2 is connected to the 1Y of the IC.When I drop a corn seed to interrupt the light I measure the result with my scope:As you can see, the CH1 is the analog part of the signal and the CH2 is the output of the Schmitt trigger on 1Y.The seed is traveling around 5.8mS until it left the sensing region.I tried hundreds of time with a couple of hundred of different corn seeds and the time was between 3ms up to 7-8ms.Depending on the seed weight and dimension I assume.This signal looks me fine to use and is very stable until the test on my bench.I don't had the ability to try it in real life on seeder machine for now.Why can we not use this signal in some way together with each other rather than using the uA etc.region what are really nasty to setup and control that small amount of current.Maybe that could change his behavior on a more hot day, or more colder day, or more moisture or whatever.And this is a signal when I press a button on my DVB remote control.The same distance of 6cm from the photo sensitive diode is my DVB remote control.Sorry for this kind of interrupting but this signal looks really good for me.What you are think about this?Don't pay attention to the ripples on CH1, that comes from my breadboard....

author: Marco
author_rank: Super Contributor
timestamp: April 30, 2018, 08:13:20 pm
content: If the seed only blocks part of the sensor it might not drop the signal low enough to trigger. Dirt and dust will affect the reliability in a way a differential detection is less vulnerable to.Most importantly though if you use the photodiode in photovoltaic mode and you just connect it direct to a logic input, you are relying on the dark+input leakage current to drop the signal when the seed blocks it. Especially the leakage current is not well characterized, designing a circuit purely experimentally is risky. Unless you have a really good idea how the inverter input operates, at least put a resistor in parallel which dominates the pull down, so you know what it does at any temperature and with any in-spec inverter from principle.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 30, 2018, 08:43:15 pm
content: Thanks for this valuable info.Tomorrow I will make the circuit from post no #9 what you are shared and I will play a bit with it,to see how that is working.I have LM324 IC in my home stock, I hope that would be good enough.I use the TSAL6200 IR LED and the forward current is 100mA in the datasheet.Should I let the IR LED full sine or not?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2018, 09:07:32 pm
content: The typical 100 mA specs for the IR LEDs is rather optimistic. They already get quite warm if not cooled well via the board or wires. So I would use 20-50 mA.Using a number of IR lEDs increases the chance that a seed will only block a part of the light to one detector.For pulse of a few ms, the LM324 might be just fast enough.The logical circuit would be a series resistor and 2-4 detectors in series.The circuit in #9 should give a similar result if there is a pull up behind the diodes, but slightly less sensitive.

author: Marco
author_rank: Super Contributor
timestamp: April 30, 2018, 10:11:47 pm
content: Quote from: Chriss on April 30, 2018, 08:43:15 pmIn that case buffer the output of R1 first before you do the CR differentiation, without a buffer the size of the resistors you would need for R2&R3 would become problematic.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 30, 2018, 11:30:33 pm
content: I found some LM358 also, would that be a better choice?

author: Chriss
author_rank: Frequent Contributor
timestamp: May 02, 2018, 06:53:13 pm
content: Hi!I played with the schematic frompost #9https://www.eevblog.com/forum/projects/how-to-use-multiple-photo-diodes-as-on/?action=dlattach;attach=420283I found it very helpful and excellent.The most impressive is the possibillity to fine tune the sensitivity.But, I have a question.What does the capacitor in series mean in this schematic?I not really understand the writing:"...then C sets the time constant, needs to return to equilibrium fast enough for the next seed."Can somebody help me to understand why is this C put in the circuit?Thank you.

author: german77
author_rank: Regular Contributor
timestamp: May 02, 2018, 07:06:08 pm
content: The capacitor is acting as a high pass filter. It's needed to eliminate DC and slow changes. The smaller the capacity the faster the signal needs to change to pass the filter.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 02, 2018, 07:06:31 pm
content: The series capacitor is there to measurechangesin the signal instead of the signal level itself - i.e., a derivative of the signal. This makes sense, since the conditions change, any dust / mist etc. changes the amount of light (DC level), LEDs age, alignment might get a bit off... But the "seed dropping" event is always a quickie, so it's best to filter out the DC light level and only detect the fast changes.You can, of course, just measure the signal level, including DC, and do all of this in software, with quick enough sample rate (a few kHz at least could be required, I think).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2018, 07:19:41 pm
content: Quote from: Chriss on April 30, 2018, 11:30:33 pmThe LM358 is the dual version to the LM324 quad OP. So no real difference in the OPs.A good choice of OP would be something like TLC272 or MCP600x.Just for a first test the LM358/LM324 should be OK - not high performance, but very cheap and usually bread board compatible.The capacitor in series makes the OP only react to changes in the intensity not to a constant (low frequency) background. This can be a good method to separate small changes from a unknown background, e.g. due to dust collected on the LEDs, or temperature changes.The circuit shown use the OP as a comparator - many µCs already contain an comparator (usually better performance than the LM358) and would thus not even need that OP.With small seeds passing by a line of detectors, I would expect to see a drop in the intensity to 1 or maybe 2 detectors with the other detectors essentially unchanged. Having the diodes in parallel and thus measuring the sum is possible, though is looses some sensitivity. Still the parallel circuit is likely the best simple one.The series connection of detectors would measure the minimum current and would be more sensitive when the starting current is the same, but would be a problem it not - so my original idea is not that good (it might work with some parallel resistors if the differences are small). So is the similar diode minimum circuit suggest by someone else, as one would not notice a small drop in intensity at the detector with the highest background.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 02, 2018, 07:41:18 pm
content: Hmmmm...I was thinking in this direction does the C could be some sort of high pass filter but I not reallyrecognized the meaning in the circuit, but now I got the point and makes me sense.Ok.Thank's guys for this help.Any idea what value of cap should I start?Or how to calc this cap? I don't have a clue for this.I will switch to TLC272 later, for now I haw in stock the LM358 and LM324 but now I set up my LM358 on my breadboard..Somebody before wrote the input voltage to the comparator IC should be around 100mV or above.Would it be a problem if I would go higher? maybe around 1v?Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2018, 07:52:58 pm
content: A larger signal to the comparator is not a problem, as long as the voltage is inside the supply range. So no problem with too large a signal here. Even if due to the capacitor it might go a little outside the supply range the current is very limited (if there is no extra amplifier) and due to this limit not a problem.One might get problems if the signal get way below about 10 mV. Than some amplification before the comparator would be good.The RC combination is a high pass filter. The lowest important frequency present in the signal would be at about 1/time to pass the detector. With a speed in the m/s range and mm size seed this would be ms. Large seeds would give longer but higher signal and thus would not mind loosing some to the filter. One could argue that instead of size of seed the size of the detector would be relevant. The RC time constant should be about in the range of the signal. Not to loose too much signal strength the resistors behind the capacitors should be larger than the ones at the detector. So a reasonable combination would be something like 1 MOhm and about 1 nF-10 nF to get a time constant in the 1-10 ms range.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 02, 2018, 09:11:27 pm
content: I can smell this sensor calibration will be a long story with lot's off coffe.The average size of the corn seed is 7 x 6 x 10 in mm.The setup which started to recognize the seeds ( 7pcs from 10pcs was recognized ) is:R1 - 150kR2&R3 - 180kC - 220nFVR1- 270 Ohm ( tuned into a detection region )It is still not sensitive enough.To be continued...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2018, 04:20:37 pm
content: If looks a little like R2/R3 are rather small. I would expect better performance with R2/R3 > 3 * R1. For the AC signal R1,R2 and R3 are in parallel. For the DC signal only R1 is active and thus can not be very large.With a puls of only a few ms long, depending on the cable capacitance, one might reach the speed limit due to the relatively high resistance.With the rather large capacitor the signal seen by the comparator will be rather unsymmetric with a relatively large positive excursion and a small, but longer negative one. So there are two possible setting for the trimmer to work - the higher voltage point is likely the better choice.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 03, 2018, 04:27:59 pm
content: Can you pls help me to figure out the R1 resistor?I'm stuck on the datasheet of this specific photo diode what I use, I cant figure out the data from it.Here is the datasheet I use:https://media.osram.info/media/resource/hires/osram-dam-2495942/SFH%20203%20FA.pdfI cant figure out the correct equations to calculate the resistor value.What parameters are used for this kind of calculation from this datasheet?Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2018, 05:31:18 pm
content: If one wants to calculate the current from the photodiode, one needs the intensity from the light sources and the sensitivity of the detectors. As this is only one number one can alternatively measure the signal strength and if needed adjust the resistor. One should aim for something like 1-2 V (DC) over R1.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 03, 2018, 06:27:50 pm
content: I did it first with a trimmer and setup the output voltage on output for 2.5v with my scope.After that I changed the trimmer with a fixed resistor.So, I did it correct.I really was overheated when I started to figure out the correct equation from the datasheet.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 03, 2018, 07:46:33 pm
content: QuoteJust ask if I understand this correct...

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2018, 06:05:51 am
content: Quote from: Chriss on May 02, 2018, 09:11:27 pmThe idea was that the current through R1 would essentially work as a voltage source for the C-R2//R3 high pass filter, that only really works when R2//R3 is substantially larger than R1 ... which might cause problems with bias current for the comparator if they got too big. That's why I said you might want to put a buffer after R1, you seem to have a free opamp so why not?

author: Chriss
author_rank: Frequent Contributor
timestamp: May 04, 2018, 06:35:16 am
content: I got the idea and I'm familiar wit it.The circuit is now working well but there is to tune something.Here is my setup now:R1 - 60KC - 100nFR2&R3 - 180KDistance between transmitter and receiver are 5cm.The result of counted seeds with different weight and dimensions 8-10 counted from 12 seeds.The error can be due to a hardware problem in the path of the seed until it reach the sensors.And the sensor are are in openair now, so maybe ambient light could also make a small error...But I can smell it is put in a good direction thanks to you guys!What you mena under "buffer"?Do you mean I should put a cap beside the R1?Sorry I use some time translator to better understand the writings...Thanks for so much support.Sent from my GT-I8260 using Tapatalk

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2018, 06:44:44 am
content: Buffer, as in an opamp with the signal connected to the positive input and output to the negative input. A x1 or unity gain amplifier, otherwise known as a buffer.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 04, 2018, 06:46:46 am
content: ThanksI got it.Will try.Sent from my GT-I8260 using Tapatalk

author: Chriss
author_rank: Frequent Contributor
timestamp: May 04, 2018, 10:14:43 pm
content: Just a thinking, how would affect the circuit if I would swap the photodiode to photo transistors?Is that a good or bad idea?Sent from my GT-I8260 using Tapatalk

author: Marco
author_rank: Super Contributor
timestamp: May 05, 2018, 12:40:56 am
content: The circuit is complex enough to do it's own amplification ... which will be predictable rather than the extremely temperature dependent behavior of the phototransistor.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 05, 2018, 10:39:12 am
content: QuoteYou mean the photo transistor circuit is complex enough and can do his own amp?QuoteYou mean the temperature swinging can impact the behavior of the photo transistor,and it will be hard to predict how the counting system would work?I know the photo transistor have a lover response time than a photo diode, but I think also a photo transistor would beeasier to control the output signal than on a photo diodes.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 05, 2018, 12:46:52 pm
content: Phototransistors are a little slower than photo-diodes. However when used in analog mode and not in saturation (like often done with opto-couplers) they are not that slow. There is some, but not that much (unless you look at very low intensity) temperature dependence. However this would not really matter in a counting system that starts with a differentiation - thus would suppress slow changes in dark current or amplification. The larger temperature effect is likely with the LEDs.I would consider photo-transistors OK with this application, especially if the alternative includes a long cable between photodiode and amplifier. The amplifier could be just with R1 and a follower or alternatively a trans-impedance amplifier and thus no "R1".A disadvantage with photo-transistors is more that they tend to be often rather small area and thus more gaps to not detecting tiny seeds. So the spacing might need to be closer with some sensors. A small angle of acceptance might also help - and that is often easier to get with photo-transistors. So the details on the optics (LEDs and detectors) can make quite some difference.Adjust the sensor is probably helps to use a scope to look at the pulses both at R1 and after the capacitor.

author: Marco
author_rank: Super Contributor
timestamp: May 05, 2018, 12:51:32 pm
content: Quote from: Chriss on May 05, 2018, 10:39:12 amA phototransistor is in essence a transistor with a photodiode driving the base. It's okay if you just want a simple binary detection, but as soon as you do any significant processing on the signal and you want linearity and stable performance over temperature you're better off doing your own amplification.A photodiode is pretty consistent over temperature, a resistor is very consistent over temperature, an opamp amplifier is very consistent over temperature ... the current multiplication of a transistor is all over the place with temperature. Output for a phototransistor I looked at doubles for around 75 degrees temperature difference, or to put it in ppm to compare it to a resistor, 13333 ppm/°C. It's really really bad.

author: Chriss
author_rank: Frequent Contributor
timestamp: May 06, 2018, 06:30:44 am
content: QuoteThe length of the cable from the photo sensor to the controller can reach 3-4m in the best scenario.I assume it is to long.QuoteThis could be the answer for one of my questions I set to me on the beginning of this project.I saw several seed sensors and some of them was made with 3-4 5mm maybe photodiodes maybe photo transistors (I never figured out) and some of them was made by 6 or eve up to 10pcs 3mm phtoto sensitive elements.And now I think they are actually photo transistors.Check the picture where are 6 tiny photo reacting elements.The temperature is not so high in the region where the sensor is fitted until working condition.Around 20oC.The seed counting timing is also not high, maybe one seed/30sec.The dimension of the seed what comes in counting mode is around 5mm x 5mm, smaller seedsactually not counted just the seed flow is monitored.The error what are present in a very expensive seeder ( I mean not a DIY seeder electronic like this what I try to make ) is between 5% - 10%.Actually, I think the precision of this device is not so accurate and is not so important but the "is seed flowing" monitoring is the most important think.I got this infos when a talked yesterday with a couple of peoples who own this type of device and have some practical experience with it.

author: Marco
author_rank: Super Contributor
timestamp: May 06, 2018, 08:31:52 am
content: Quote from: Chriss on May 06, 2018, 06:30:44 amEven if you absolutely positively don't want to pull a 3 wire cable to put the electronics next to the photodiodes, you can pull 2 wires and do 4-20ma signalling. It makes no sense to transport these signals unprocessed, the amount of electronics required is tiny.