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How to Build an Auto-Reverse HO Trolley Circuit

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I though it would be really cool to have a trolley go through the center of town in my HO scale train layout. I wanted the trolley to head through the center of town, and when it reaches either end of the line it would pause and reverse, but I didn't know how to accomplish this.

The problem

How am I going to control the movement of the trolley so that it stops in the center of town for a period of time, then continues to the end of the line, pauses and goes back in the opposite direction? Also, how can I determine the position of the trolley on the track?

At first I thought about employing a circuit that uses a PIC Microcontroller to perform the logic needed, but then I settled for a simple transistor circuit to do the job.


Determining the position of the trolley on the track

I came up with the idea of using reed switches and magnets to determine the location of the trolley on the track. The reed switch was attached to the rail at either end of the trolley line and one in the center. A magnet at the bottom of the trolley caused the reed switch to close when it was directly over the reed switch.


Here is a picture of the magnet on the bottom of the trolley that activates the reed switch. I removed the magnets from Playskool Alphabet letters, then glued them to the base of the trolley.

Trolley auto-reverse circuit design


The circuit consists of two PNP transistors that make up a bistable circuit. The bistable circuit changes state when the reed switches attached to either end of the track are activated with the magnet on the trolley closing the circuit. One transistor of the bistable circuit is attached to a driver transistor. This activates a relay that changes the direction of the trolley by changing polarity of the voltage on the track.

There is also another circuit consisting of a 555 timer integrated circuit that pauses the trolley each time it changes direction and when it is in the center of town. The 555 timer integrated circuit connects to a second relay that disrupts power to the track and causes the trolley to stop. The 555 timer integrated circuit is activated when the bistable circuit changes state during an auto-reverse, or when the reed switch in the center portion of the trolley track is activated.

The auto-reverse trolley circuit is powered by the AC terminals on the model railroad power supply. A bridge rectifier circuit and capacitor rectify and filter the AC to a relatively steady DC current. Then a 7805 regulator regulates the voltage to a steady 5 volts. The DC terminals on the model railroad power supply provide power to the trolley through the track, and the polarity of the track voltage is switched by the relay of the auto-reverse circuit.

Here is a schematic of my trolley auto-reverse circuit. I used LEDs (Light Emitting Diodes) in this circuit to indicate the state of the bistable circuit. Caution: Build this circuit at your own risk! I am not responsible if you burn out your model train power pack, motor in your trolley, or damage to you or your track. You should have a basic knowledge of electronics and electricity before building the auto-reverse HO trolley circuit.


Bread boarding the auto-reverse HO trolley circuit

I built the power supply for the auto-reverse HO trolley circuit on a separate solderable perfboard. No need to breadboard this part of the circuit as the design was straight forward. I bread boarded the rest of the auto-reverse HO trolley circuit so that I could make changes to the circuit on the fly if needed. I always recommend bread boarding an untested circuit design first before committing it to solderable perfboard.


Testing the circuit

Next I setup up a test track next to my workbench so that I could test the auto-reverse HO trolley circuit. This made it easy for me to make changes on the fly without affecting my HO scale layout.


Preparing the trolley track to be added to my HO scale layout

Testing is complete, my auto-reverse HO trolley circuit works like a charm. Now it is time to add it to my HO scale layout. The first thing I did was add the reed switches to both ends and to the center of the section of trolley track. I recommend that you install the reed switches to the track before installing it on your layout so you don't get hot glue and solder on your layout.

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I then soldered the track sections together so they wouldn't work loose during operation.

Be careful that you do not get solder on the top or inside section of the rail as solder in these areas could derail your trolley.


I used several 2x4 pieces of wood to elevate to the trolley track to make it easier to assemble.


I routed the wires attached to the reed switches along the side of the rail. I used very thin black wire picked up at the hobby store to attach to the reed switches. I could use very thin wire for the reed switches because these wires will carry a very small electric current, less than 1 milliamp.

Mounting the trolley track to my HO layout

I routed the trolley track right through the center of my layout with all of the wires exiting on the right hand side of the track.


I drilled holes through the plastic ballast section of the railroad tracks so that I could route the wiring through these holes leaving the ballast flush with the table. I need to keep my HO layout easy to disassemble in case I need to move it.


Mounting the trolley auto-reverse electronics in the control console

The model train power supply, auto-reverse circuit and power supply perf boards are mounted in the control console of my HO layout. Make sure you mount the model train power supply so that you can easily adjust the speed of the trolley.

Running the trolley

The trolley is now ready for operation! It will pause in the center of town then move to the end of the track and pause then return in the opposite direction, pause in the center of town again then travel to the end of track in the other direction pause again and reverse.

Video of auto-reverse trolley in action!

Here is a video of my trolley in action on my HO layout.

What if I want the trolley to make more than one stop?

Simply add more reed switches connected in parallel to reed switch SW2. Place the added reed switches at the desired location on trolley track.


With some basic electronic knowledge and wiring skills, you too can have a working trolley system in your HO layout.


Coloradokid on December 11, 2019:

I'm in the same boat as steveg808. I too would like a readable copy of the circuit and parts list. My old eyes just can't clear up the fuzziness even with a magnifier. Thanks. on December 08, 2019:

Hello! does anyone have a good copy of this circuit and parts list!? just what im looking for! thanks in advance

Jim S on February 07, 2019:

I'm interested in putting a trolley on my layout. I like the circuit you designed. Could you send me a jpg or pdf copy of the circuit?


k9ikx on July 30, 2017:

I too could use a clear copy of the schematic. Thanks.

graydon on January 01, 2016:

i would like to build this circuit for my layout. could you send me a clear schematic. thanks.

graydon on December 10, 2015:

i really like your circuit. could you send me a schematic . my e-mail is thanks.

jbdkz (author) from Cleveland on November 05, 2013:

@nearboston: Please provide your e-mail address and I will send you a schematic in jpg format.

nearboston on November 05, 2013:

This looks great and I'd really like to build one, but I can't read the schematic

jbdkz (author) from Cleveland on December 15, 2012:

@anonymous: This was one of my first attempts at an electronic project in 15 years and served as a great refresher course in basic electronics. I am more comfortable with PICs now and may redesign this circuit using one.

anonymous on December 14, 2012:

Your circuit would be OK if it was 1975. Nobody would use all those discrete components today. Your first suggestion to use a PIC would be much easier and use the same ideas. Of course you would need a PIC programmer and a PC. I made a similar circuit for the train club I belong to using a PIC.

jbdkz (author) from Cleveland on April 01, 2012:

@Write4U LM: What I recommend is use the trolley tracks for power and to mount the reed sensor switches, then string black thread overhead between electrical towers as a fake overhead electrical system.

Write4U LM on April 01, 2012:

This is a great option for model trolleys. Do you know if this can be made to work with live overhead?

anonymous on March 17, 2012:

Can I have a copy of your original image of the parts list/schematic and bread board. Please email to

anonymous on July 15, 2011:

@jbdkz: could you email me the original parts list, too? sales at mic-net dot netThanks!Jesse

jbdkz (author) from Cleveland on April 15, 2011:

@tssfacts: It is fun! The trolley car is the crown jewel of my layout!

tssfacts on April 15, 2011:

This looks like a lot of fun.

jbdkz (author) from Cleveland on April 09, 2011:

@anonymous: I didn't document the breadboard layout,.......but I will see if I can find any close-up pictures of the breadboard layout and send them to you.

anonymous on March 27, 2011:

Any chance of more details of how you made a breadboard circuit from this schematic?I'm about halfway through assembling a breadboard circuit for a small bench top end to end HO layout Ive built and think your reversing trolley circuit would work well on it but I'm finding converting the schematic to breadboard a challenge...

anonymous on March 24, 2011:

@jbdkz: If not too much trouble can you send me a copy was well. Love what you've done here. Would like to do something similar. Thanks!

anonymous on February 14, 2011:

@jbdkz: I would appreciate it if I could get a copy of the parts list. My email is ki4hok at gmail dot com.ThanksTed

anonymous on January 15, 2011:

Do you sell this? Are you able to put more stops in?

jbdkz (author) from Cleveland on January 02, 2011:

@anonymous: i can send the original image of the parts list/schematic if you provide me an e-mail address to send it to.

anonymous on January 02, 2011:

Hi. I'm really blown away by your auto reversing trolley circuit. I would love to build one, but can't read the list of component parts below the schematic. Could you possibly email a legible parts list so I can get started on this great project. This is for the spouse, so you understand my need here. :)

jbdkz (author) from Cleveland on December 08, 2010:

@anonymous: Possibly, you would need a more substantial transformer to provide the fixed AC and variable DC needed. You also need to make sure the relay contacts are rated high enough to accomidate the numbers of amps the loco motor pulls.

anonymous on December 03, 2010:

I have a "G" scale LG Christmas train set I'd like to use the same way as your Trolley. Is your circuit limited to HO or do you think it would work on mine/

anonymous on November 30, 2010:

Could I get a copy of the circuit diagram as well? A trolley would look great running in my front window. Thanks in advance,

anonymous on November 29, 2010:

@jbdkz: jbdkz, I would also like a larger readable version of your schematic. Your circuit would come in very handy on my "future" train layout. Some auto-reverse circuits I've seen use optical (photo cell) sensors, versus your magnetic reed switches. Do you have an alternate circuit that uses optical sensors? Thanks for your very useful model train circuit. My e-mail address is Don

jbdkz (author) from Cleveland on November 14, 2010:

@shopjock: Please provide me you e-mail address and I will send a larger version of the schematic.

shopjock on November 08, 2010:

jbdkz1I just acquired a HO scale trolley for a diarama i'm working to contruct. I found your circuit to be something I would like to add to provide some animation. Would it be possible to obtain a legible copy of your circuit diagram? Thanksshopjock

AuthorNormaBudden on March 19, 2010:

The initials of your username resemble bdkz, except for the additional letter in front. Interesting, indeed!Congrats on the purple star! I would love for you to submit your purple star lenses at

marsha32 on March 12, 2010:

model trains are an awesome hobby and so much fun....congrats on your purple star!

anonymous on March 03, 2010:

very cool -- do you by any chance have a parts list?

bdkz on December 28, 2009:

Excellent lens!

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