# Controlling 12v model railway electric points

Hi all!

I'm seeking some advice with regards to the following:

I have a model railway setup built with 60+ year old hornby dublo 3 rail components, which includes 6 sets of electric points. Each set of points requires 12v DC current, and has three pins : ground, left and right. To switch points requires a momentary pulse (25ms) on the appropriate left or right pin.

I have put together a circuit (based on the DC motor control example from arduino docs) and sketch (using Jcbutton togglebutton library) that includes a transistor, diode, resistors, led and push button so that a press of the button successfully flicks the switch in a single direction and sets the led on or off depending on the current state. As a proof of concept that works fine.

The issue I have is that I need to repeat this entire configuration twice for each set of points, which requires 12 of everything. Each set takes up a minimum of 6 full columns on a breadboard, and that won't fit.

I am wondering if there's a simpler way to do this so that I could conserve space and components. Eg. Could I construct something to use the same transistor and diode to send voltage to either left or right on a given switch dependent on whether the button state is on or off?

I can manage the arduino sketch, but the electrical configuration is beyond my abilities/knowledge. I just can't help feeling this should be possible.

Cheers,
Stu

This probably involves two solenoids, and the first thing is to measure the resistance so that the current required is known - then the switching devices and free-wheel diodes can be selected.

Can the "ground" pin on the points actually be 12V and the other two pins momentarily shorted to ground to operate? If so then thats much more convenient.

Further thought - I feel like I need a simple switch to choose between left or right before firing the 12v pulse... But the only way I know to do that is by adding more transistors and that's even worse.

MarkT:
This probably involves two solenoids, and the first thing is to measure the resistance so that the current required is known - then the switching devices and free-wheel diodes can be selected.

Can the "ground" pin on the points actually be 12V and the other two pins momentarily shorted to ground to operate? If so then thats much more convenient.

As far as I can tell the solenoids don't care about polarity so, yes, they could be reversed but it's still 12v and needs a transistor to allow the arduino to control the pulses (I think?). I tried using the 5v out from the arduino and it's just not enough.

Moving forward I plan to use an arduino mega and a computer power supply to drive everything, including trains, points, lighting, signals etc. Probably controlled by Bluetooth and Android app. Eventually I might get pcb made so I don't have a stack of breadboards with loose connections, but right now I just want to build a poc.