# What size resisters for signals??????



## Mark Stearns (Jan 4, 2008)

Hey all, I am in the process of installing an overhead layout at my sons school and will be using 2 Yardmaster's by RRconcepts to controll my blocks, station stop and searchlight signals so Thomas and Percy don't plow into one another on the layout. My question is this, I am using 2 search light signals from South Bend Signal Co. and had originaly planned on running power from the Bridgeworks 5 amp transformer to power the 2 Yardmasters. When I ordered the signals Jim sent out the correct resisters to use w/ the 30 somthing volts that the bridgeworks puts out on its accessory side. I have since decided to power the Yardmasters off of track power as shown in the RRconcepts hook up diagram for ease of hook up and not having to run 50 feet of cable to the yardmaster on the far side of the room. So I am trying to figure out what size resister would work with the lower voltage of track power as apposed to the 2.2K ohm resisters that were sent with the Signals. Thank you to anyone who can help.
Mark


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## Greg Elmassian (Jan 3, 2008)

you first need to know if the lights you are using are LEDs (which run on current) or incandescent lamps (which run on voltage) (this is really an oversimplification, but a good starting point)... 

Regards, Greg


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## Mark Stearns (Jan 4, 2008)

They are multi color LED's


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## Mark Stearns (Jan 4, 2008)

correction, they are dual color led's


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## Greg Elmassian (Jan 3, 2008)

You can either learn ohms law, or use an LED calculator.

*http://led.linear1.org/1led.wiz* 


Regards, Greg


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## toddalin (Jan 4, 2008)

You can key off of the information that the manufacturer provided. It's a straight linear equation.

If the mfg says to use 2,200 ohms for ~32 volts, at 16 volts you would want to use 1,100 ohms and at 8 volts it would be 550 ohms, etc.

The thing is, if you power the lights the from the track, the voltage continually changes with the speed of the trains. You have two options, use a voltage regulator so that the LEDs always see the same voltage after a certain level (e.g., 5 volts) is attained at the track, or as most of us do, just use a resistor big enough to take care of the higher voltages you may be running, and sacrifice some of the LED brightness at lower voltages. A ~1,000 ohm 1/2 watt resistor should suffice for your typical needs and give adequate brightness at lower speeds.


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## Greg Elmassian (Jan 3, 2008)

Actually, technically not a linear equation, it's algebraic, a = bx + c ( where a would be the source voltage, bx would be the product of the current and the resistance of the resistor, and c would be the voltage drop of the LED) 

...... remember to get the best calculation, you want to subtract the voltage drop of the LED from the calculation for the current... that's why I recommended the calculator OR learning what really happens. 

From a practical point, in many installations, where the source voltage is much higher than the voltage drop of the LED itself, it really makes very little difference, but it does make a lot of difference when you are have the source voltage close to the LED voltage drop. 

Regards, Greg


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## toddalin (Jan 4, 2008)

Posted By Greg Elmassian on 30 Apr 2010 03:11 PM 
Actually, technically not a linear equation, it's algebraic, a = bx + c ( where a would be the source voltage, bx would be the product of the current and the resistance of the resistor, and c would be the voltage drop of the LED) 

...... remember to get the best calculation, you want to subtract the voltage drop of the LED from the calculation for the current... that's why I recommended the calculator OR learning what really happens. 

From a practical point, in many installations, where the source voltage is much higher than the voltage drop of the LED itself, it really makes very little difference, but it does make a lot of difference when you are have the source voltage close to the LED voltage drop. 

Regards, Greg 

I look at the milliamps that are pushed through the LED and that is a straight linear equation. Based on the suggestion from the mfg for 2,200 ohms and I was assuming the rated output to be at 32 volts.

(32 volts x 32 volts)/2,200 ohms = 0.47 watts / 32 volts = 14.5 milliamps

(16 volts x 16 volts)/1,100 ohms = 0.23 watts / 16 volts = 14.5 milliamps

(8 volts x 8 volts)/550 ohms = 0.12 watts / 8 volts = 14.5 milliamps


BTW, about 20 milliamps is good for most LEDs and will ensure a nice long life. No sense in making things overly complicated here.


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## Greg Elmassian (Jan 3, 2008)

Yes, BUT, the voltage is not as you have it, but the voltage drop of the LED subtracted from that... that is the source of your error, I believe. 

Assume 3 volts drop on the led for example... 

(29 x 29 ) / 2200 = .382 watt / 29 = 0.013 amps (of course don't know why you went to watts to get back to amps... 

29 volts / 2200 ohms = 0.013 amps ... 


So, if you let me complete the simplification: (I repeat your calculations, but using the actual voltage applied to the resistor) 

32 volts source and 2200 ohms: 29 volts to the resistor / 2200 = 0.013 amps 

16 volts source and 1100 ohms: 13 volts to the resistor / 1100 = 0.012 amps 

8 volts source and 550 ohms: 5 volts to the resistor / 550 = 0.009 amps 

Therefore, if you ignore and do not use the voltage drop of the LED, it is NOT a linear equation... the current varies in the real case 

Regards, Greg


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## krs (Feb 29, 2008)

Posted By Mark Stearns on 29 Apr 2010 09:11 PM 
I have since decided to power the Yardmasters off of track power as shown in the RRconcepts hook up diagram.......
Mark


Mark -

Can you post a link or the circuit as to which diagram you mean?
All the Yardmaster hook-ups that I see on the RR-Concepts page require some fixed voltage, typically from the power pack accessory terminal.

I don't see ant options where you can power the yardmaster using track power.

Your question re value of dropping resistor for the LEDs in the searchlight signal depends on the voltage the Yardmaster provides at the output that feeds the signal.


Unfortunately, I can find absolutely no specs for the Yardmaster on the RR-Concepts page - something as basic as the input voltage range allowed for the Yardmaster would be helpful.


- Knut


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## Greg Elmassian (Jan 3, 2008)

Page 13 of the online yardmaster manual says "6 to 20 volts AC or DC". 

You should not try powering from the track if it will go under 6 or over 20, but you REALLY should give it a fixed voltage. 

It says that the signal outputs are the same voltage as the input (that will only be true if these outputs are relay driven), but you can assume the input voltage worst case. 

Yes, part of the manual shows the Yardmaster driven from the track, BUT page 15 says you can use track power UNLESS you want to drive turnouts or 12v signals, reinforcing my second sentence. 

No offense, but you really should RTFM, as we say in engineering. Take the acronym in good humor please. 

Regards, Greg 



p.s. found the site and the manual in about 6 seconds... link right on the product page... Google is your buddy...


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## krs (Feb 29, 2008)

Greg - 

The problem I had is that I have flash turned off in my browsers, so all I get is a stylized "F" not the button that says "manual" 
I can just click on that stylized "F" if I want to watch the flash video (which is what most people use flash for) - never occurred to me that someone would use flash for a link button on a web page. 
Still don't understand why people would use flash for that - the Flash application has to be the worst application that's relatively main stream. 

But back to the topic at hand - sort of a bit puzzling that 20 volts AC or DC is specified as the maximum voltage for the Yardmaster but per the OP the unit was originally going to be hooked up to about 30 volts of the bridgewerks supply.


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## Greg Elmassian (Jan 3, 2008)

Well, I thought the Bridgewerks was supposed to be a nominal 24 volts. Still too much voltage per the manual.


Yep, overuse of flash is common... using flash just for a button... lame programming.

Regards, Greg


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## krs (Feb 29, 2008)

Posted By Greg Elmassian on 30 Apr 2010 05:14 PM 
It says that the signal outputs are the same voltage as the input (that will only be true if these outputs are relay driven), but you can assume the input voltage worst case. 


Just had a chance to flip through the manual.
Looks good on the surface, but I think it neds some work to make it clearer and more consistent.
As to the input and output voltages - yes they are stated to be the same, but the input is shown as 12 VAC and the corresponding output as 12 VDC (Page 16 and some other pages in the manual as well)
I haven't found any page where it states the DC output voltage with a DC input voltage, ie track power.

It's probably buried somewhere in the text, same as the input voltage range.

- Knut


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## Mark Stearns (Jan 4, 2008)

Well I went with a 460 ohm on one side and a 1k on the common side to keep the red from overpowering the green when the light flashes yellow per Jim at South Bend Signals. As far as powering the lights off the track the speed will remain constant due to the overhead nature of the railroad and the starts and stops are handled through blocks by the Yard Master. I am using a setup as described on page 4 of the Yard Master manual. Curtis has been great at helping me figure out what to do to get this layout to do what I want. The Yard Masters have been very easy to install and I plan to use more of his products in the future. I have no personal gain from RRConcepts other than the products seem to do exactly what I want and Curtis is fast to respond with answers to my many questions I have asked him. I am a very satisfied customer of RR Concepts and would recomend them w/ no quams. Thanks to all for your responses and I am glad to be back on MLS after a long time off.


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## toddalin (Jan 4, 2008)

Posted By Greg Elmassian on 30 Apr 2010 03:50 PM 
Yes, BUT, the voltage is not as you have it, but the voltage drop of the LED subtracted from that... that is the source of your error, I believe. 

Assume 3 volts drop on the led for example... 

(29 x 29 ) / 2200 = .382 watt / 29 = 0.013 amps (of course don't know why you went to watts to get back to amps... 

29 volts / 2200 ohms = 0.013 amps ... 


So, if you let me complete the simplification: (I repeat your calculations, but using the actual voltage applied to the resistor) 

32 volts source and 2200 ohms: 29 volts to the resistor / 2200 = 0.013 amps 

16 volts source and 1100 ohms: 13 volts to the resistor / 1100 = 0.012 amps 

Therefore, if you ignore and do not use the voltage drop of the LED, it is NOT a linear equation... the current varies in the real case 

Regards, Greg 

You can either learn ohms law, or use an LED calculator.

*http://led.linear1.org/1led.wiz* 


Regards, Greg 






The voltage drop on the LEDs varies with the LEDs, so when using single LEDs, by not considering the drop (which we may not actually know), we provide a _conservative estimate_ of the resistor value that includes a built in margin for error. In this case, yes, it is an easy linear relationship.

I show watts so that the wattage value for the resistor (1/2 watt) is evident.

Ultimately, we both know that any 1/2 watt resistor between 750 - 1,200 ohms will work fine with good life and decent brightness for typical single LEDs driven off of typical track power, and this is really all the original poster wanted to know. You could simply have stated this up front.


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## Greg Elmassian (Jan 3, 2008)

Yep, conservative is good, and I believe I mentioned/addressed the ignoring of the LED as a simplification/practicality measure in my 3rd post, 3rd paragraph. 

Ahh... yes, always good to remember wattage, many people go to radio shack and just buy 1/4 watt resistors. I've had a number of cases where I had to go to 1 watt (I have a higher track voltage). 

But, when you say "and this is really all the original poster wanted to know. You could simply have stated this up front.", well, you also quoted my post with the simplest answer: "use an LED calculator..." 

I gave my best advice... either learn it or use a tool. Why do I say this than just tell someone "use any resistor between 750-1200 ohms"? 

Because you are really giving the wrong answer, because in my "X" years in the hobby, and my "3x" years as an engineer, people will take that "answer" to mean it's good for any application. 

So I get tons of emails, where someone used a 750 ohm resistor and 24 volts and blew things up, because they were using something that could not take over 20ma. 

(I won't mention the cases where their power supply put out 30 volts under light loads, but it has happened) 

So, ok, maybe I'm too down in the details... I'll admit I want to give answers that are as close to "bulletproof" every time. 

Maybe what I say is "overkill" 90% of the time. But, those 10% where someone's electrics were saved email me and thank me. It's worth it to me. 

Really not trying to get on your case Todd (although linear versus algebraic was a fun exercise).... 

Regards, Greg


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## krs (Feb 29, 2008)

Posted By toddalin on 01 May 2010 12:08 PM 
Ultimately, we both know that any 1/2 watt resistor between 750 - 1,200 ohms will work fine with good life and decent brightness for typical single LEDs driven off of typical track power, and this is really all the original poster wanted to know.


I don't see how you can come to that conclusion.
The voltage drop across a "typical" single LED is pretty consistent within a fairly small voltage range depending on the colour of the LED, but track voltage can vary from a few of volts to 24 or even a bit higher and over that range a 750 ohm resistor on the one hand would allow too much current for a typical LED at 24 volts and a 1200 ohm resistor on the other extreme would barely light the LED if the track power is only 3 or 4 volts when you're running slow.


Greg definitely provided the correct answers including the one which stated that track voltage really varies too much, a ten to one ratio, to run an LED directly from it.

The OP uses a fixed track voltage as I understand it, so one can calculate the proper resistor value for an LED but not if one uses the traditional, variable DC voltage to run the trains.


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## NavyTech (Aug 2, 2008)

Well Greg you certainly know how to put someone in there place. I would never second guess you or say you were wrong again either.


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## Greg Elmassian (Jan 3, 2008)

Not the intent at all. 

But, I am really serious about the emails and phone calls I get when someone has "locked on" to a seemingly "universal" solution and damaged their loco's lights or wiring. 

There's a lot of enthusiastic people out there hoping for that "universal solution", but there always seems to be an exception or "gotcha". 

My philosophy is to put a little more effort and caution into it, yeah, maybe it takes more work to measure the current in an existing circuit, yeah, maybe you have to go buy a $10 voltmeter, but in the long run, it pays off. 

Many people in this hobby have limited funds for their "pride and joy" and have to do modifications that may be "tricky", maybe somewhat "beyond" their capabilities. 

I used to tell people "well, if you don't understand it, you should get someone else to do it", but that does not wash when you have been scrimping to buy that $500 loco, and there is not another $100-$150 to get someone to mod it for you. 

So, my advice often gets lengthy and detailed, but again, trying to be "bulletproof" so no one damages anything. 

It's not a competition, but a forum. I've given extensive explanations on why I do things a certain way. Todd is certainly within his right to tell me that I'm overboard. 

Readers are free to take or ignore any advice given here. I know you think I do this to put people down. You are dead wrong. 

Greg


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## White Deer RR (May 15, 2009)

I appreciated the link to the LED calculator, just the thing I needed. I'm just messing around using alkaline batteries to light a few structures and perhaps the track cleaning car as a first step, not track power, but I'd take your word over the kid at Radio Shack, as nice as he was. Technically I see the nice kid nailed the correct resistors for the green but not the red LED's. Information is good, even if I don't understand all of it. ;-) Best!


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## toddalin (Jan 4, 2008)

Posted By krs on 01 May 2010 01:07 PM 
Posted By toddalin on 01 May 2010 12:08 PM 
Ultimately, we both know that any 1/2 watt resistor between 750 - 1,200 ohms will work fine with good life and decent brightness for typical single LEDs driven off of typical track power, and this is really all the original poster wanted to know.


I don't see how you can come to that conclusion.
The voltage drop across a "typical" single LED is pretty consistent within a fairly small voltage range depending on the colour of the LED, but track voltage can vary from a few of volts to 24 or even a bit higher and over that range a 750 ohm resistor on the one hand would allow too much current for a typical LED at 24 volts and a 1200 ohm resistor on the other extreme would barely light the LED if the track power is only 3 or 4 volts when you're running slow.


Greg definitely provided the correct answers including the one which stated that track voltage really varies too much, a ten to one ratio, to run an LED directly from it.

The OP uses a fixed track voltage as I understand it, so one can calculate the proper resistor value for an LED but not if one uses the traditional, variable DC voltage to run the trains.




From OP:
When I ordered the signals Jim sent out the correct resisters to use w/ the 30 somthing volts that the bridgeworks puts out on its accessory side. I have since decided to power the Yardmasters off of track power as shown in the RRconcepts hook up diagram for ease of hook up and not having to run 50 feet of cable to the yardmaster on the far side of the room. So I am trying to figure out what size resister would work with the lower voltage of track power as apposed to the 2.2K ohm resisters that were sent with the Signals. Thank you to anyone who can help.
Mark


Nothing about this being a "fixed" voltage.

Obviously anyone who tries to power an LED off variable track power is subject to compromise. But few run their trains as high as 24 volts at least for any extended period. I run mine at typical speeds and usually keep the voltage between 12-17 volts to the track (and can go as high as ~22.5 volts).

Sure the LEDs will be dim when the trains are toddling around and will be bright then under full steam. But other than using a regulator of some sort, there is not much choice in the matter. Even at 24 volts a 750 ohm resistor still keep the current under 30 mA, and most LEDs can easily take that current, and more. Use too big a resistor and the low speed lighting suffers. Remember, life is full of compromises.


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## krs (Feb 29, 2008)

Nothing about this being a "fixed" voltage. 

Mark mentioned the "fixed" voltage in his later post - at least that's what I assume he means when he says "the speed will remain constant." 


Well I went with a 460 ohm on one side and a 1k on the common side to keep the red from overpowering the green when the light flashes yellow per Jim at South Bend Signals. As far as powering the lights off *the track the speed will remain constant* due to the overhead nature of the railroad and the starts and stops are handled through blocks by the Yard Master. I am using a setup as described on page 4 of the Yard Master manual. 

However, what would bother me using the set up on page 4 of the manual for a layout that runs many hours a day is that either full running voltage or zero volts is applied to the loco. That's not very healthy for the gears starting and stopping the loco like that many times a day. 
Anyway, that's what I understood the yardmaster to provide in this circuit - basically a latching relay as the output that applies the track voltage set by the throttle or cuts it off totally.


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## toddalin (Jan 4, 2008)

Posted By krs on 01 May 2010 02:52 PM 
Nothing about this being a "fixed" voltage. 

Mark mentioned the "fixed" voltage in his later post - at least that's what I assume he means when he says "the speed will remain constant." 


Well I went with a 460 ohm on one side and a 1k on the common side to keep the red from overpowering the green when the light flashes yellow per Jim at South Bend Signals. As far as powering the lights off *the track the speed will remain constant* due to the overhead nature of the railroad and the starts and stops are handled through blocks by the Yard Master. I am using a setup as described on page 4 of the Yard Master manual. 

However, what would bother me using the set up on page 4 of the manual for a layout that runs many hours a day is that either full running voltage or zero volts is applied to the loco. That's not very healthy for the gears starting and stopping the loco like that many times a day. 
Anyway, that's what I understood the yardmaster to provide in this circuit - basically a latching relay as the output that applies the track voltage set by the throttle or cuts it off totally. 





No. My take is that he will use an adjustable power supply, but because the trains are overhead, he'll just set them to a speed he likes and let them run for time on end rather than use them to do switching, drops, pick-ups, etc. (Who in their right mind would just put a "set voltage" to the track for track-powered engines with no way to throttle that voltage other than turn it off?) I do the same thing, as do most of us who do an open house where trains are just running around for hours on end for all to enjoy. That speed could be...


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## krs (Feb 29, 2008)

If you look at the first post, Mark is setting up blocks using the Yardmaster so the two trains he is running on the same loop don't run into each other. 
The Yardmaster will detect where the trains are using the track sensor and when one gets too close to the other, it will shut off power to a short section of track so the second train stops for a short while. 
Al least that's what I understood from his posting and looking at the page 4 diagram he is using. 
There should really be a deceleration and acceleration section built into this set up - I don't think the Yardmaster provides that but I could be wrong - maybe that's built in - only requires a few components to do that.


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## toddalin (Jan 4, 2008)

Posted By krs on 01 May 2010 04:27 PM 
If you look at the first post, Mark is setting up blocks using the Yardmaster so the two trains he is running on the same loop don't run into each other. 
The Yardmaster will detect where the trains are using the track sensor and when one gets too close to the other, it will shut off power to a short section of track so the second train stops for a short while. 
Al least that's what I understood from his posting and looking at the page 4 diagram he is using. 
There should really be a deceleration and acceleration section built into this set up - I don't think the Yardmaster provides that but I could be wrong - maybe that's built in - only requires a few components to do that. 


Blocks don't change the need for variable speed, nor does the Yardmaster require that trains be limited to one set speed.

Agreed accelleration/decelleration would be nice and easier on the equipment, but are not imperative. LGBs ran for years on end in store windows starting and stopping day in and day out without any reduction in speed.

On my railroad, I use inrush suppressors and diodes or resistors to reduce the speeds where trains start and stop continually for hours on end making things a little easier on the equipment, but there is no real-time accelleration/decelleration per se and the trains just run a little slower through these sections.


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## Mark Stearns (Jan 4, 2008)

This post has brought up a point I didn't think about in regards to slowing the trains to a stop. The Yardmaster doesn't do this though for longevity of the locks it would be great, is there a relatively easy way to do this sing the setup I am using? I k now Todd said he slows the entire track section and that might be a way to do it but seeing that the yardmaster is basicly a switch would it be possible to put something inline with the wire that goes to the track from the yardmaster to slow the trains to a stop when the sensor is triggered? I am finishing up the install on Wednesday and will post pictures later in the week. Thanks to you all for your responses and help. Mark


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## toddalin (Jan 4, 2008)

Posted By Mark Stearns on 02 May 2010 09:31 AM 
This post has brought up a point I didn't think about in regards to slowing the trains to a stop. The Yardmaster doesn't do this though for longevity of the locks it would be great, is there a relatively easy way to do this sing the setup I am using? I k now Todd said he slows the entire track section and that might be a way to do it but seeing that the yardmaster is basicly a switch would it be possible to put something inline with the wire that goes to the track from the yardmaster to slow the trains to a stop when the sensor is triggered? I am finishing up the install on Wednesday and will post pictures later in the week. Thanks to you all for your responses and help. Mark 






If you put one these in series with the wire leading to that section of track where trains stop, it will both slow the trains through that section (they will not decellerate to a stop though), and reduce the inrush of power when they start up. In the past, they had a 2.5 ohm, 6.5 amp unit and I use several of them around the railroad at any point where trains would automatically stop/start.

5 ohm 7 amp inrush supressor


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## Mark Stearns (Jan 4, 2008)

So if I lengthen the block and put the sensor at the beginning of the block the train would just decelerate through that section and accelerate to normal speed after clearing the block unless the yardmaster clears the block before the train gets to the end and the train would just accelerate from whatever point it is at in the block. That would be cool as long as the other train has had enough time to clear the station stop. Now is there anything that could be done about the station stop itself?


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## toddalin (Jan 4, 2008)

Posted By Mark Stearns on 02 May 2010 07:16 PM 
So if I lengthen the block and put the sensor at the beginning of the block the train would just decelerate through that section and accelerate to normal speed after clearing the block unless the yardmaster clears the block before the train gets to the end and the train would just accelerate from whatever point it is at in the block. That would be cool as long as the other train has had enough time to clear the station stop. Now is there anything that could be done about the station stop itself? 


Without reviewing the Yardmaster, if the sensor to which you refer is to detect the train (e.g., reed switch), it seems to me that you would want to put the sensor toward the center so that when the train enters that block it slows down because of the 5 ohms of additional resistance imbedded in the inrush thermistor before it encounters its stop. Also, when it restarts, the combination of the inrush suppressor and 5 ohms will start the train at a reduced speed until it clears that block.

I have a similar area that uses a reed switch to stop the trains at the service facility. Initially, I had the reed switch too close to the preceeding block such that when long engines tripped the reed switch, their back axle had not yet entered the reduced speed/kill block. But the reed killed the block when the engine entered, so the long engines would just stop from their full speed.


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## Mark Stearns (Jan 4, 2008)

Okay so i would just add the thermister To the wire that connects the yardmaster to the isolated block of track which would then limit the voltage to that section and when the train triggers the Reed switch it would kill power to the block stopping the train. I could do that on both the station stop section as well as the block section if I'm looking at it right, is this correct Todd? That would be a neat feature and keep the beating on the locos to a minimum.


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## toddalin (Jan 4, 2008)

Posted By Mark Stearns on 02 May 2010 10:45 PM 
Okay so i would just add the thermister To the wire that connects the yardmaster to the isolated block of track which would then limit the voltage to that section and when the train triggers the Reed switch it would kill power to the block stopping the train. I could do that on both the station stop section as well as the block section if I'm looking at it right, is this correct Todd? That would be a neat feature and keep the beating on the locos to a minimum. 

Yes, sounds like you have it. Good luck and enjoy.


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## Mark Stearns (Jan 4, 2008)

Well the layout is up and runing, everything but the Thermisters have been put in place ( I am still waiting for them ) and it all woks great!! The signals are the perfect addition because the kids now watch the lights to know when the trains will start moving. The school is doing a "Grand Reviel" tomorrow and all the kids in the school will get to see it in action for the first time. Thanks to all who have helped out with answers and I look forward to implementing the Thermisters that Todd recomended when I get them and will let you know how it goes. Overall this project has been a huge success and will be leading into the installation of several other layouts around our area as time allows. I am thrilled to have done it and want to just say thanks to all my friends here on MLS for their help and support. 
Mark


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