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Ross Crawford / projects / dcc

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DCC for LEGO trains

Introduction

DCC is a digital protocol that allows multiple trains (and other devices) to run on a single track, controlled by a single controller through the rails.

It consists of a controller, which generates digital signals telling decoders what to do, and provides the power, and a decoder in each train / device that decodes the signals, and thus drives the train or operates the device as required.

Implementation

Well, since Mark Riley came up with DCC controller firmware for the RCX, I finally decided to bite the bullet and install a DCC decoder in one of my train motors to try it out. So I looked around for what was available in Australia, and came across the TCS M1. These are available fairly cheaply by mail order from Gary Spencer at MRC in Sydney.

The M1 spec says it handles 1A continuous and 2A peak current, so it seemed plenty for a LEGO motor. And it’s nice & small, fits easily inside the LEGO motor. It uses standard NMRA DCC wiring, so instructions by others can be followed to the letter. I left the metal contacts and diode on the motor.

As I only intend to use it with LDCC i considered not using a resistor in series with the accessory connector. However on reflection I decided to use a 68 ohm 1/4W resistor, in case I ever decide to run it on a 12V layout. Both accessory wires are connected to the accessory connector.

Here’s the obligatory pic

The M1 has a nice feature called “dither”, to make slow running much smoother. It basically applies an extra voltage at a given frequency when running slow, the voltage & frequency are set by CV values in CV57 & 56. I found the default voltage (CV57=15) was a bit low for my locos - values of 18-22 performed better. And the default frequency (CV56=6) was way too slow, I had to increase it to 1. This is probably because the LEGO motor has no flywheel, whereas most model locos for which this decoder is designed do. The decoder is very quiet at slow speeds so I didn’t bother connecting a capacitor circuit.

Anyway, after picking the best setting for a loco with no train, the dither continues to help up to around 25% of full throttle, so it works well hauling trains too.

The only slight downside is that it doesn’t work with the analog train controller on settings less than 3 - I guess it needs that much to get the decoder working. But I think I can live with that restriction.

Conclusion

Well, after some playing, I’m very happy with this decoder, and will probably be installing more. It works well with the my locos, with the 4 cylinder engines running off the accessory connector.

Many thanks to Mark Riley for your LDCC software, without which I probably would never have bothered trying this!

Site last modified November 4, 2004 Home

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