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Boosting IR sensitivity

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Boosting IR sensitivity
Posted by BigCityFreight on Sunday, February 2, 2020 3:40 PM

Reposting here:

I have a grade crossing signal board that I have six IR detectors wired to (signals protect three tracks, so three IR emitters/detectors on each side of the crossing, one per track.) All the IR pairs are the type that are in a little black plastic brick, side by side, with a small plastic divider between emitter and detector. 

I have the IR bricks between the rails, but the sensitivity is disappointing. If I put a small piece of white tape on rolling stock bottom, they work fine (I know black absorbs more IR energy, so the white tape reflects better). Before going the tape route for hundreds of pieces of rolling stock, is there a way to boost the sensitivity? This board does not have a separate adjustor for it, so it would have to be something else.

Thanks - I'm learning electronics as I go, so all help appreciated.

Todd

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Posted by gmpullman on Sunday, February 2, 2020 3:49 PM

Hi, Todd

I use IR detector/emitter pairs from the kits included with LogicRail Signal Simulators and the Grade Crossing Pro boards. They mount one (HO) tie apart and are angled toward each other at about 10°. 

These seem to function pretty well althouth there is a sensitivity pot on each board. Maybe the problem you're having is related to the IR bits being housed in the same cube. 

Perhaps try to find separate emitter/detectors and mount them a little distance apart, aimed toward a central point about where your car floors bisect the beam?

Good Luck, Ed

*I was typing more but when I hit "save changes" I got kicked out then found out the thread had been locked Bang Head

Toward the end of this document there are some pointers regarding the use of IR detectors. 

http://www.logicrailtech.com/od-1-ir_inst.pdf

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Posted by speedybee on Sunday, February 2, 2020 4:06 PM

Hi Todd,

I know the detectors you're talking about because I tried them too and had the same problem.

I switched to using a single LTR301 phototransistor instead of the emitter-detector pair. I found it works much better. The LTR301 is much smaller (it's about 1.5mm thick) so it can slide underneath the track with its little detector spot between ties. It's small enough that you don't even need to plan for its placement; I use foam roadbed and can just slide one in anywhere.

I don't need any IR emitters with it. I pull its collector up to 5V via a 100k ish resistor. When there is no train on it, ambient light activates it, and the collector is pulled down to ground. When there is a train sitting on top of it, ambient light is blocked enough that the collector remains pulled high. I find it much easier than the detector-emitter pair, and no taping/painting of rolling stock is required.

Datasheet: https://www.sparkfun.com/datasheets/Components/LTR-301.pdf

It's called a infrared phototransistor but I've found it seems to work fine with whatever ambient light is bouncing around my room. I don't even use incandescent lights.

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Posted by rrinker on Sunday, February 2, 2020 4:24 PM

 There's not much you can do but try different sensors with different characteristics. Adjusting the position of the ones you have might help. 

                                                  --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by gregc on Sunday, February 2, 2020 7:44 PM

BigCityFreight
I have the IR bricks between the rails,  ... is there a way to boost the sensitivity?

are they angled such that their lines intersect at the height of the bottom of the car?

have you maxed the current to the emitter and maxed the resistance on the detector?

low level TTL is fairly low (~0.8V), arduino atmel 328P is ~0.3V.   you could try a 555 which is 1/3 VCC,  ~1.7V at 5V.

greg - Philadelphia & Reading / Reading

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Posted by RR_Mel on Sunday, February 2, 2020 8:33 PM

I’ve been dinking around with IR detection for the last couple of years and I have found out that not all of the sensors are as sensitive as others.  I would try a different pair of emitters and sensors.  In particular 3mm sensors generally are much more sensitive than 5mm.
 
A matched pair of 3mm are good for about 12” reflecting off a non black surface and 3” off a flat black surface.
 
I have had very good luck with the Arduino FC-51 obstacle detection modules.  I angle the beam at 12° flush with the ties which puts the reflective surface ½” above the rails.
 
I made an alignment jig for drilling the holes between the ties on Atlas code 83 track.
 
 
 
I have about 40 of the FC-51 modules in and working flawlessly off black car and locomotive bottoms.
 
 
 
 
Mel
 
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by BigCityFreight on Sunday, February 2, 2020 9:55 PM

Thanks -- I'll try these out. Does it matter which side the resistor is on? 

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Posted by gregc on Monday, February 3, 2020 3:35 AM

BigCityFreight
Does it matter which side the resistor is on? 

no

what are you connecting the detector to?

greg - Philadelphia & Reading / Reading

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Posted by BigCityFreight on Monday, February 3, 2020 6:57 AM
How do you max out the current and the resistance? There is no adjustment for sensitivity on the board, so not sure I can change the current. How do I max out the resistance? (Sorry, I'm just learning electronics.)
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Posted by gregc on Monday, February 3, 2020 10:11 AM

BigCityFreight
All the IR pairs are the type that are in a little black plastic brick, side by side, with a small plastic divider between emitter and detector. 

i assumed you have a emitter/detector pair similar to (b) in the lefthand image below.   there are 2 wires on either side, one pair for the emitter and the other for the detector which are presumably connected to some circuit similar to the one below.

R1 can be made small to maximize the current thru the emiter for a know supply voltage.   R2 is made large so that even when a limited detection occurs it creates a large change in voltage across the resistor.

i the circuit below, i'm not sure why they have a transistor.   The transistor requires ~0.7V to start conducting and R4 raises that voltage as soon as it does.

   

greg - Philadelphia & Reading / Reading

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Posted by rrinker on Monday, February 3, 2020 1:55 PM

gregc

  the circuit below, i'm not sure why they have a transistor.   The transistor requires ~0.7V to start conducting and R4 raises that voltage as soon as it does.

   

 

 If the voltages called out on the circuit are to be beleived, the .86 volts at the R2/R3 junction is just slightly above the maximum 'low' voltage for the 74HCT14  inverter. So it would appear to be acting as just a level converter. A different sensor, with a different IR phototransitor, could probably make that unecessary.

                                        --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by BigCityFreight on Monday, February 3, 2020 2:19 PM

gregc

 

I have the Vishay tcrt5000. Datasheet here: https://www.vishay.com/docs/83760/tcrt5000.pdf
 
What kind of resistors would I need to maximize the sensitivity? I'm struggling to understand much of this. 
BigCityFreight
All the IR pairs are the type that are in a little black plastic brick, side by side, with a small plastic divider between emitter and detector. 

 

i assumed you have a emitter/detector pair similar to (b) in the lefthand image below.   there are 2 wires on either side, one pair for the emitter and the other for the detector which are presumably connected to some circuit similar to the one below.

R1 can be made small to maximize the current thru the emiter for a know supply voltage.   R2 is made large so that even when a limited detection occurs it creates a large change in voltage across the resistor.

i the circuit below, i'm not sure why they have a transistor.   The transistor requires ~0.7V to start conducting and R4 raises that voltage as soon as it does.

   

 

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Posted by gregc on Monday, February 3, 2020 3:57 PM

rrinker
So it would appear to be acting as just a level converter.

i don't understand how the voltages in the diagram are possible since there's typically a ~0.7V drop across the base-emiter.   and why a 5k resistor in series with the base?      it's just not clear to me.

fig 5.4.2 on that page shows a slotted opto being tested which doesn't require much sensitivity.   I think reflected sensitivity may require a different type of circuit.

 

BigCityFreight
What kind of resistors would I need to maximize the sensitivity? I'm struggling to understand much of this. 

according to your datasheet, the Forward current (emitter) is 60ma.   under Basic Characteristics, the Forward voltage (drop) is 1.5V.   With 5V supply, a  59 Ohm ((5 - 1.5) / 0.06) resistor results in 60ma.  (something more to be on safe side.

but it's still not clear how you're using the opto, what are you connecting it to?

and is the detector subject to ambient incandescent lamps/sunlight in the room

greg - Philadelphia & Reading / Reading

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Posted by BigCityFreight on Monday, February 3, 2020 5:59 PM
BigCityFreight
What kind of resistors would I need to maximize the sensitivity? I'm struggling to understand much of this. 

according to your datasheet, the Forward current (emitter) is 60ma.   under Basic Characteristics, the Forward voltage (drop) is 1.5V.   With 5V supply, a  59 Ohm ((5 - 1.5) / 0.06) resistor results in 60ma.  (something more to be on safe side.

but it's still not clear how you're using the opto, what are you connecting it to?

It's connected to a cheap ebay board that controls a grade crossing signal. 

and is the detector subject to ambient incandescent lamps/sunlight in the room

 No - only flourescents. 
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Posted by gregc on Tuesday, February 4, 2020 3:54 AM

BigCityFreight
t's connected to a cheap ebay board that controls a grade crossing signal. 

can you post a link from ebay?

greg - Philadelphia & Reading / Reading

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Posted by gregc on Tuesday, February 4, 2020 8:20 AM

i was expecting to see a part no.  that I could look up to see a schematic for what the opto is connected to.   Based on your questions, it seems unlikely that you'd modify the circuit.

so at this point, the only thing i can suggest is the alignment of the detectors which also doesn't seem likley without modifying the housing.

there is of course that pot on the board the opto connects to.  is it for adjusting the sensitivity?

greg - Philadelphia & Reading / Reading

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Posted by RR_Mel on Tuesday, February 4, 2020 8:25 AM

The “IR Block” does not appear to be angled, my experience is the sensitivity is best when the beam is angled at 12°.  The focal point (reflected) at ½” above the rails works the best for black bottomed rolling stock and locomotives.
 
My detectors will detect my hand at 3” above the two angled sensors.  
 
 
 
 
Mel
 
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by BigCityFreight on Tuesday, February 4, 2020 2:23 PM

The pot is labeled as "speed" and I think somewhere on ebay description it says it's for the flash rate, though the flashing board (yellow one) has its own adjustment pot. I will try the one on the green board and see what happens. 

The sensors come out of the brick pretty easily and I already have an oversized hole cut, so I will try angling them. I also have the optical sensors on order that another poster suggested to swap in as a backup plan.

If all else fails, I have plenty of white tape to put on the bottom of rolling stock.

I appreciate everyone's efforts here. I'm learning a lot.

Thanks!

Todd

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