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Last post 10-02-2009 2:10 PM by JSperan. 7 replies.
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09-30-2009 9:01 AM
Offline seaside
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Joined on 07-28-2008
Posts 27

Simple Train Detection for Staging Tracks

I am having trouble getting the circuit described in Robert Frey's October 2009 issue of Model Railroader to work.  Does anyone know if there is an error in the circuit or have any trouble-shooting thoughts?  I have hooked everything up according to the diagram on a breadboard.
09-30-2009 4:22 PM In reply to
Offline Mark R.
Top 500 Contributor
Joined on 04-25-2004
Posts 906

Re: Simple Train Detection for Staging Tracks

I kind of wondered about that !

I looked at the wiring diagram and SOMEthing just didn't look right to me - but I couldn't put my finger on it ! (?) Hoefully MR will print a correction if necessary.

Mark.
09-30-2009 5:04 PM In reply to
Offline cacole
Top 10 Contributor
Joined on 07-23-2003
Sierra Vista, Arizona
Posts 9,315

Re: Simple Train Detection for Staging Tracks

 Several resistor values were not right.  I recalculated all of the values according to Ohms law and the specifications of the reflective opto sensor and transistors and found several errors.

The two resistors marked as 470k are WAY off.

09-30-2009 7:21 PM In reply to
Offline rrinker
Top 25 Contributor
Joined on 02-14-2002
Reading, PA
Posts 8,237

Re: Simple Train Detection for Staging Tracks

 Might have flopped them around. The IR LEDs in the sensors are 1.7V, two in series powered by 12 volts with 20ma would be a 430 ohm resistor, 470 being close enough. Also looking at the picture of the circuit, the middle resistor, which may or may not be the middle resistor int he schematic, is 470 ohms - yellow, violet, brown. Can't see the colors on the two standing up.

                                        --Randy

 
09-30-2009 8:58 PM In reply to
Offline seaside
Not Ranked
Joined on 07-28-2008
Posts 27

Re: Simple Train Detection for Staging Tracks

Randy, you are correct!

I replaced the 470k ohm current limiting resistor for the IR-LED pair with a 470 ohm resistor and the circuit worked.

How do I get a message to MR so they can print a correction before others run into the same problem?

 

John
09-30-2009 9:06 PM In reply to
Offline rrinker
Top 25 Contributor
Joined on 02-14-2002
Reading, PA
Posts 8,237

Re: Simple Train Detection for Staging Tracks

I suspect someone had already seen this. However there is an option to contact the editors somewhere back up at the top of the site, I used it to submit a correction to a DCC comparison article and was later contacted by Andy Sperandeo because they wanted to print my comment. My only claim to fame ever - I had a letter to the editor printed in MR!

 It's Contact Us under the Our Magazine menu, I think.

                       --Randy
10-02-2009 10:55 AM In reply to
Offline Robert Frey
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Joined on 08-08-2006
Greendale, WI
Posts 28

Re: Simple Train Detection for Staging Tracks

In the October 2009 issue of Model Railroader, the article “Simple train detection” in the Fig.2 Wiring diagram on page 65, the 470 and the 470K values got interchanged.  The value between the two 1K resistors should be 470 ohms. (The 470 ohm resistor controls the 20 ma. current that flows through the “E” of the Infrared sensors.)  The value to the left and right for each transistor is 470K.  (By making the size of the 470K resistors across each transistor smaller, then the “S” sensor can resist higher amounts of ambient lighting (such as incandescent overhead lights), but then the sensing distance to the bottom of a HO car becomes less.  
 Model Railroader has been notified.  They said there will be a correction printed in the Railway Post Office section of the December 2009 issue pointing out the reversal of the 470 and the 470K resistors.  I was glad that Randy was able to get the circuit to work.  Sorry for the artwork error on the two IR sensors with a 4 wire cable.  That was a very clever way of solving the artwork problem by looking at the actual picture of the middle resistor as 470 ohms – yellow, violet, brown.  
 In the following picture, I have attached a circuit for a single IR Detection circuit.  The 470 allows about 20 ma. to flow through “E”.   When 0.2 ma. of detection flows through “S” into the 470K resistor, then about 1 volt is produced across the b to e of the NPN transistor.  When the transistor turns on, about 15 ma. then flows through the 680 resistor and also through the LED.  This makes for about a total of 35 ma, through “E”   This increase of the current in “E” makes for a very quick turn-on point.  It can sense the tip of your finger at a distance greater than 1/2 inch in a room with overhead fluorescent lights.   The current rating of “E” is 50 ma. max.
 Robert Frey

 
10-02-2009 2:10 PM In reply to
Offline JSperan
Not Ranked
Joined on 05-23-2009
Posts 207

Re: Simple Train Detection for Staging Tracks

 Robert,

Thanks for taking the time to clear this up and for providing more great information! 

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