How to Build an Auto-Reverse HO Trolley Circuit
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.
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.
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.