Which sensor schematic to build

Discussions on buying or building timers, solenoid start gates, light trees, weigh scales, and other race related electronics.
Post Reply
Wing Nut
Apprentice
Apprentice
Posts: 3
Joined: Tue Apr 18, 2006 6:50 pm
Location: Reedley, Ca.

Which sensor schematic to build

Post by Wing Nut » Wed Apr 19, 2006 3:55 pm

It's time to upgrade our church's Awana Grand Prix track to an optical finish line, at this point I like the looks of the GPRM software and would like to build a parallel port finish line.

So far I've found the 4 lane finish line detector based on pack 471 at GrandPrix Race Central
http://grandprix-race-central.com/downl ... system.gif , the modified version of that schematic without the 2nd quad comparator, found at http://www.wrightzoo.com/GPtrack/schematic.gif , and a third schematic found at http://pinewood.newtownbsa.org/document ... atic01.pdf that looks simplest of all without any comparator and using a 6 volt power supply (I already have a 6 volt wall wart laying around) instead of building a 5 volt version.

My question is if the complexity of the comparators is necessary, or will the simple version work just as well?
Next, can the optional led's found in the 2 more complex schematics which show the signals going into the parallel port be added to the simplified version?
Also will the 6 volt power supply work for all of these layouts instead of building a 5 volt supply?

Our next race isn't until next spring, but I want to get everything built and run a lot of tests before then.


For the Fun of it !!

NealOnWheels
Apprentice
Apprentice
Posts: 24
Joined: Thu Mar 30, 2006 12:38 pm
Location: Rolling Prairie, Indiana

Re: Which sensor schematic to build

Post by NealOnWheels » Thu Apr 20, 2006 11:58 am

The problem with parallel ports is there is no guarentee that all pins will have consistant electrical properties. They can possibly have varying input voltage thresholds and varying loading properties. The comparators provide a well defined input voltage threshold and provide a sharper transition on the port pins.

Without the comparators the parallel port can introduce additional lane bias as some lanes will react faster than others.

Keep in mind regardless of the use of comparators or not the sensors will also have some variation as well.

Also the first schematic has some comparators using the R9/R10 reference voltage and some using the R11/R12 reference voltage. They all should use the same reference voltage. The tolerances of the resistors can make a significant difference in the two reference voltages.



Wing Nut
Apprentice
Apprentice
Posts: 3
Joined: Tue Apr 18, 2006 6:50 pm
Location: Reedley, Ca.

Re: Which sensor schematic to build

Post by Wing Nut » Mon Apr 24, 2006 10:09 am

Sounds like the comparators may be the way to go. Both schematics using comparators use 2 sets of resistors for the reference voltage going to the comparators, would there be any problem with using 1 set to provide reference voltage to all 5 inputs on both the comparators ( 4 finish lines and the start circuit )? I wonder what the reasoning was to split their reference voltage up to two sets of resistors? :?

I did test voltage on the 6 volt wall wart that I have, and with no load it showed about 10-11 volts, looks like unregulated and might be to high for use with electronics. :shock: However I also checked a 5 volt power supply that is used for a dlink access point that I have, and it showed right at 5 volts with no load. Looks like that might be the way to go and still not have to build a 5 volt power supply. :D

I've got a bunch of parts ordered from glitchbuster.com ( they are a lot cheaper than the Shack, about $10 for all the leds, phototransistors, resistors, and comparators ) and am looking forward to warming up the soldering iron. 8)

A few more thoughts. :idea: What about mounting all the electronics above the track? Putting the IR leds right next to each sensor and aiming at a mirror mounted flush on each lane. That way everything is mounted and housed together, no wires running from the top to the bottom accept the start circuit. Just mount the finish bridge onto the finish section of track. It would make building a lot simpler. Also, would it be better to mount the led in front of the sensor, or side by side? Seems like one in front of the other would be more accurate on a car with a narrow front end. I guess that the main issue would be accurate alignment from the led to the mirror and back to the sensor. I suppose that the mirror could get dirty from lube off the cars, but with the sensor mounted below the track, it could also have the same problem. Another thought, reverse this and have all the electronics mounted below the track and just have a mirror bridge over the top? Has anyone tried the mirror idea and been successful? What are your thoughts? :?:


For the Fun of it !!

NealOnWheels
Apprentice
Apprentice
Posts: 24
Joined: Thu Mar 30, 2006 12:38 pm
Location: Rolling Prairie, Indiana

Re: Which sensor schematic to build

Post by NealOnWheels » Mon Apr 24, 2006 12:04 pm

I see know reason not to use a common resistor pair for all comparators. It would also be good practice to add a small capacitor (.1uF) accross R10.

Your 6V supply may not be that bad as soon as it is loaded by your leds. There are resistors in series with the 6V supply so that should limit the current to the 5V parallel port. Not a practice I like to use especially when the characteristics of an unregulated power supply is unkown. Your regulated 5V power supply is a better choice.

I have often thought about using mirrors. I would be reluctant to do that as it may be possible a car could have a finish that could reflect the light. If it did it is possible it would never register. And as you said it may be difficult to align.



User avatar
gpraceman
Site Admin
Site Admin
Posts: 4686
Joined: Fri Jun 20, 2003 12:46 am
Location: Highlands Ranch, CO
Contact:

Re: Which sensor schematic to build

Post by gpraceman » Mon Apr 24, 2006 12:07 pm

Wing Nut wrote:Sounds like the comparators may be the way to go. Both schematics using comparators use 2 sets of resistors for the reference voltage going to the comparators, would there be any problem with using 1 set to provide reference voltage to all 5 inputs on both the comparators ( 4 finish lines and the start circuit )? I wonder what the reasoning was to split their reference voltage up to two sets of resistors? :?
I am not sure why they are split. The original Pack 471 plans had them this way, so I built the 4 lane version that way as well. Maybe some electronics guru out there can answer that. I have never had a issue with my system reporting an incorrect finish order and I have put it through a lot of testing with my GPRM software.
Wing Nut wrote:A few more thoughts. :idea: What about mounting all the electronics above the track?
Personally, I would not have the sensors above the track. I have heard of refective light issues (off of the track or the cars) that have caused timing problems. You can better shield the sensors from reflected light by recessing them down into the track. Most all of the commercial timers available have the sensors below the track.


Randy Lisano
Romans 5:8

Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

User avatar
gpraceman
Site Admin
Site Admin
Posts: 4686
Joined: Fri Jun 20, 2003 12:46 am
Location: Highlands Ranch, CO
Contact:

Re: Which sensor schematic to build

Post by gpraceman » Mon Apr 24, 2006 12:34 pm

For housing the electronics, I like the solution below. You can just slide it over the end of the finish line section and then secure it in place with a couple of fasteners (this one uses alignment pins instead).

Image

Image

Image


Randy Lisano
Romans 5:8

Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

Wing Nut
Apprentice
Apprentice
Posts: 3
Joined: Tue Apr 18, 2006 6:50 pm
Location: Reedley, Ca.

Re: Which sensor schematic to build

Post by Wing Nut » Tue Apr 25, 2006 9:45 am

That's a great looking finish line! :D Gives me some ideas, however our track lays flat on the floor, and the bottom of the bridge where the sensors are mounted would make it have a hump there. Although it might not be a bad idea to raise the whole track off the floor a few inches because when the kids sit up front to see their cars coming down, they block the view for the rest of the audience. The only problem is when a car comes off the track it could get damaged worse if the track is to high off the floor. :cry:

Back to the sensors. Top emitters/bottom sensors sounds like the right way to go. If the sensors are that sensitive to see reflections from the cars, I can see the advantage of top mounted emitters and bottom mounted sensors. My kid's cars have ended up pretty shiny themselves. 8)

One more picky finding. :roll: On the 4 lane schematic with 2 comparators, it shows 3 lanes and the start gate on one comparator, and the 4th lane on the 2nd comparator. I think that I'll put all 4 lanes on one comparator, and the start gate on it's own. Just seems more logical. It almost looks like that is what was intended on the schematic, because the start gate ends up going to pin 13 and lane 4 goes to pin 15 which seems backwards.

I like the led's that show the signal going back to the parallel port on the computer, but how hard would it be to light the fastest lane's led, latch it on, and disable the other 3 lanes from lighting, so the audience could see the winning lane. Then reset the circuit when the start gate is reset back to its ready position? I guess you could get carried away with this idea and it could lead all the way to putting led numbers above each lane with the finish positions 1-4. Oh well, being obsessed with the best makes it more fun anyway. :lol:


For the Fun of it !!

User avatar
gpraceman
Site Admin
Site Admin
Posts: 4686
Joined: Fri Jun 20, 2003 12:46 am
Location: Highlands Ranch, CO
Contact:

Re: Which sensor schematic to build

Post by gpraceman » Tue Apr 25, 2006 1:40 pm

Wing Nut wrote:One more picky finding. :roll: On the 4 lane schematic with 2 comparators, it shows 3 lanes and the start gate on one comparator, and the 4th lane on the 2nd comparator. I think that I'll put all 4 lanes on one comparator, and the start gate on it's own. Just seems more logical. It almost looks like that is what was intended on the schematic, because the start gate ends up going to pin 13 and lane 4 goes to pin 15 which seems backwards.
Thase are 4 lane plans that are meant to be easily adapted to 3 lanes, without really having to rearrange the circuit. For 3 lanes you would not need the 2nd comparator and all that is connected to it.
Wing Nut wrote:I like the led's that show the signal going back to the parallel port on the computer, but how hard would it be to light the fastest lane's led, latch it on, and disable the other 3 lanes from lighting, so the audience could see the winning lane. Then reset the circuit when the start gate is reset back to its ready position?
The circuit was really meant to be a simple sensor system with a computer doing the timing. You could, of course, make such modifications, but it would be better to go all the way and make a timing system that doesn't need a computer (but can connect to one), so you can use it if your computer konks out.


Randy Lisano
Romans 5:8

Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

John Shreffler
Merchant
Merchant
Posts: 154
Joined: Wed Feb 04, 2004 9:32 pm
Location: Vienna, VA
Contact:

Re: Which sensor schematic to build

Post by John Shreffler » Thu May 11, 2006 7:23 pm

Looking for small variations in voltages using comparators is definitely NOT the way to go. Too many adjustments. And prone to triggering by flash cameras.

Just put enough power into the emitters, and enough sensitivity into the sensors so that the sensors are saturated. When the car breaks the beam, the photodiode comes out of saturation, rising all the way to the supply voltage.

This approach is rather hard to pull off using common solid state components if the separation is greater than 8 inches, which is why a lot of folks use an incandescent light as the infrared source.


John Shreffler
Maker of The Judge

User avatar
gpraceman
Site Admin
Site Admin
Posts: 4686
Joined: Fri Jun 20, 2003 12:46 am
Location: Highlands Ranch, CO
Contact:

Re: Which sensor schematic to build

Post by gpraceman » Thu May 11, 2006 7:45 pm

John Shreffler wrote:Looking for small variations in voltages using comparators is definitely NOT the way to go. Too many adjustments. And prone to triggering by flash cameras.
I never had a problem with my homebuilt system regarding flashes and there was no adjustment needed for sensitivity. While developing GPRM, I put it through a lot of testing and had no problems even while trying to force a tie situation.


Randy Lisano
Romans 5:8

Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

John Shreffler
Merchant
Merchant
Posts: 154
Joined: Wed Feb 04, 2004 9:32 pm
Location: Vienna, VA
Contact:

Re: Which sensor schematic to build

Post by John Shreffler » Thu May 11, 2006 7:53 pm

You might be missing the point. A comparator is designed to look at rather small differences in two inputs, and report via the output which goes rail to rail. It is certainly possible to use this approach to detect shadows. However, an IR diode by itself will do the exact same job.


John Shreffler
Maker of The Judge

Post Reply