I’d like some advice on how to wire the spur power for my layout. I’m using a DCS system with TIU and AIU. My goal is to use the AIU to control the power for the spur.

I’ve read Barry’s book where he talks about two methods to do this - 1. Use the AIU as the switch or 2. Use the AIU to control a relay. Since my track is powered by 14 gauge wire and it’s too big to fit into the AIU terminals, I think the way to go is the relay route (option 2).

Can you recommend a relay that’s powered by the 14v output on my Z4000 and will complete the 21v circuit from my star bus to the spur?

Thank you

Original Post

Here’s what I’ve used.  Easy instructions and pretty inexpensive.  You can get them in blocks of 2,4,8 and sometimes even 16.  I use them to activate yard tracks as well as whisker tracks on the turntable.  You can get them in 12v or 5v.  I used 5v because that’s what was available more quickly at the time.  You’ll also need a matching voltage wall wart to power them.  There are several threads on their use on this forum and so you can do a search, usually with advice from GRJ or Stan2004.  Feel free to email me if you have any questions.


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Forgot to mention that you can also get a Voltage converter rather than the wall wart which will drop your voltage to 12 or 5 for these relays and so you can power it the way you’d like.  I still prefer the wall wart.

Thanks Bob!

Do you think the signal relay can handle 14v as that’s the voltage I have for my accessories? The other option I have on the Z4000 is 10v.

I’m really not sure as I’m not an expert on the electronics of the relay nor its  tolerances.  If you do choose to go this way, I’d try the 10v option first and that may work if the relay is located relatively close to the Z4000 to minimize voltage drop due to wire size and distance.  Again, it may not be enough to both activate and hold the relay closed.  If you use the 14v terminals, I’m thinking that you will shorten the life of the relay, possibility to a very short life (that is a quick burn out).  Again, these are just my observations and frankly, they are guesses on my part.   I use the wall wart to focus the power of the Z4000 on running the trains.  I plug the wall wart into the same power strip that I use to power the transformers so all turns on and off with the main supply.  It’s a cleaner application for my wiring setup.

Maybe someone more versed in the electronics can weigh in with a more definitive view?

The AIU will switch AC or DC voltage with no problem. Your "14v output on my Z4000" is AC. I whole heartedly agree with Bob's recommendation of the relay board. I use a 6 Channel board on my layout.

Relays have two things to consider. The COIL is what closes the contacts. The COILS on the relay boards Bob is recommending are DC. You cannot use your Z4000 to power them (unless you add a bridge rectifier). The easiest thing is to use a wall wart as previously recommended to power the coil. Pick a voltage that matches the relay coil. The current draw on the relay coil is small (100ma or less). IF you can find a relay with a 14 VAC coil (good luck), go for it. You can use your Z4000 to power the circuit.

The CONTACTS are what actually switch the load. I would suggest selecting a relay with a contact rating of at least 10A. 

Last edited by Gilly@N&W


Thanks for chiming in on this.  I forgot about the DC versus AC aspect, which should have been obvious to me.  Always good to have other sets of eyes on the questions surfaced here. 



Thank you the information. I should have known better.. That said, this is what I’m thinking:

1. Use the relay Bob posted

2. I purchased a 5vdc wall wart

3. The + from the wall wart goes to the + on the coil. The - is split with one lead going to the - on the coil. The other - goes to the IN on the AIU ACC port. The AIC 1 output goes to the control on the coil.

4. My 21vac track power gets connected to the COMM and the NO on the relay.

5. Activating thr AIU sends the signal to the relay and closes the switch which powers the isolated spur.

Is this logic correct?


Bob and Gilly have you headed in the right direction with a separate DC supply to operate the relays . Bob also has the right idea with the relay selection. Those are rated for 10A and will work well for this application. I am using two 8 bank relay sets as I am powering 9 roundhouse bays and 7 whisker tracks around my turntable. My setup differs a bit as I also incorporated some additional 555 relays to activate the watch dog signal for DCS. Here a pic of my setup.



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Pretty good except step 3. Either the + goes directly to the coil and the - gets switched by the AIU or the other way around. You would not have both + and - from the wall wart connected directly to the coil. If you connect both wires directly to the coil, the coil will energize and stay energized as long as power is applied.

As for which wire to switch, if the relay is listed as NPN, switch the - . If the relay is listed as PNP, switch the + . It is possible that it may not care.

Last edited by Gilly@N&W

I want to build on Tom’s advice on your #3.  These are relays that allow you to choose to trigger on either high or low/no voltage.  Accordingly, there are jumpers on the relay that must be set.  My experience is that they are initially set to high when I received them.  As I understand it, this choice allows people using Arduino boards to use both high and low triggers for their programming logic when using multi-relay boards.   To provide power to the relay, you first bring both + and - to the screw terminals on the relay board and that will light it’s power-on LED.  Subsequently, to use the high option for your circuit #1, the + supply from the wall wart goes the AIU input and then from the AIU output to the IN #1 screw terminal of the relay.  The jumper is set to the high position.  To use the low option, switch the jumper to low and run the - through the AIU instead of the +.

Here’s a 4 unit relay and I’ve wired the first relay for demo purposes.  Jumpers are in high position.  Red wire is + and white is -.  Green is hooked to the IN #1.

Same relay now hooked to wall wart.  Note the LED is now on indicating wall wart is providing power.

The next photo shows the green wire touching the positive terminal on the wall wart and it has triggered the relay as shown by the red LED now lit.  For this demo I’ve omitted the AIU, but as I noted above, the green wire would go from the wall wart to the AIU and then from the AIU to the relay in #1.


In the next photo I moved the jumper to low and hooked up the green wire to the - supply on the wall wart and that likewise triggered the relay.  

Another photo with a closer view showing the jumper having been changed.  I hope you find this helpful.  I took you at your word at being a noobie.  I’ve learned a lot from this forum and continue to do so as there are so many good people willing to share.


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You're awesome! I’m a big visual learner and you're pics cracked the code for me.

One last question before the Amazon truck arrives on Saturday-

Is it ok for the relay to stay energized for a long duration? Once I activate the relay and power the spur, I’d like to keep it powered for a while.

Thanks again for taking the time to send those great pics.

You’re welcome!  I’m visual as well and you can’t beat the old saying that a picture is worth a thousand words!  I don’t think keeping it powered should be a problem.  But note that once you remove power from the wall wart, the relay turns off and is restored to the normally open position.  Assuming you plug it into the same power strip that you use to power your trains, and assuming that you power down after your operating session, you’re really not keeping it on all that long.  Alternately, simply unplug the wall wart.  If you want the spur powered at your next operating session, just close it again via the AIU.  Good luck and stay healthy!



Above photo hijacked from this OGR thread might apply to the picture-is-a-thousand-words school of thought.

Some earlier brought up the DCS watchdog "problem".  The linked thread (you kind of have to wade thru a lot of posts) discusses it in exhausting detail.  Basically, if you turn on a spur do you really want/need a DCS engine to powerup in the shutdown (silent) state.  If so, then you have options as discussed in the linked thread.

The other point I made in the above linked thread is to "draw out" a wiring diagram specifically showing where you have thick wire and thin wire (so to speak).  The thick wire carries the high-current track power (e.g., AWG 14-16) while the thin wire carries the low-current relay control wiring which can be thinner gauge (e.g., AWG 24).  The idea is to minimize the use of thick wire as this wiring also carries the DCS command signal.  By spending a few minutes up front to plan where to place the AIU, relay modules, etc., you can save on wiring cost (thick wiring being more expensive than thin) and better maintain DCS command signal integrity.


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Last edited by stan2004

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