The BasicDCC Project

BasicDCC Booster

I needed a DIY DCC Booster to go along with my DCC controller project. While looking around the net for further information, I stumbled upon various Stepper Motor Driver modules that would appear to handle the switching and current requirements for a decent 3amp DCC booster. I picked up a few of the different driver modules from the internet and looked at the NMRA specs to see how to interface them to a standard DCC controller. The NMRA specs state that the input of all boosters must be optocoupled when connected to an approved controller. This ensures that the controller won't suffer any damage in the event of a serious booster failure. I wanted my booster to be compatible with the various controllers available on the market so I decided to go with an optocoupled input. I settled on a straight forward voltage divider circuit with reverse voltage protection that feeds either a PC817 or 6N137 optocoupler. This provides ample protection to the controller while meeting the NMRA spec. I also needed some way of handling the SIGNAL DETECTION, FAIL SAFE and OUTPUT LOGIC functions. I settled on the ATTiny microcontroller to take care of this. This tiny 8pin chip has plenty of code space for the task and enough output drive to feed the various driver modules directly. I laid out the design and named it the BasicDCC Isolator. I built it up and tried it with the driver modules that I ordered. Some smoked at less than 1amp and some handled heavier loads quite well. I ended up settling on a module based on the L298N that fit my 3amp target nicely. It is actually rated at 6amps but I like things to run cooler, so I treat it with a 3amp rating. This leaves plenty of head room. It works well with most 12-16vdc power adapters with a rating of 700ma -3amp. (That old laptop power adapter works well if the voltage is within tolerance). I found that an adapter rated at 700ma runs 4 Atlas N Scale locos quite well. I had 9 locos running with a 3amp source and the L289N was just luke warm after an hour of continuous running.

The Microcontroller



The ATTiny25/45/85 processors will also work in the circuit

The ATTiny25/45/85 processors will also work in the circuit

The Opto-Couplers

The board has been laid out to use either the PC817 or 6N137 optocoupler. The difference between the two optocouplers boils down to the type of output provided by the device. The 6N137 has a digital TTL type output that is designed for compatibility with TTL devices. The output is fed by a comparitor that sharpens the output and brings it into TTL spec. The PC817 has an fast open collector type output. The output is fed directly from the detector and has slower rise and fall times that require sharpening. Luckily, the input pins of the ATTiny processor do a great job of sharpening the signal and can accept the output of the PC817 directly. Both have similar inputs that feed the internal LED directly so the same input circuitry can be used for either device.

The PC Board

I planned to experiment with the optocouplers and microcontroller so I used machined pin IC sockets for both of these devices. I find that placing the microcontroller into a separate socket and using it as a carrier while programming, allows for easy in and out. This avoids bending the pins on the microprocessor and reduces issues due to socket wear.

The Driver Module

The drive circuit is built around an L298N PCB module that easily handles the 3amp drive with pretty decent built in short-circuit protection. I ran with my breadboarded prototype for almost a year and it stood up well to the many short circuits encountered along the way.

* Isolator Wiring Chart *
Isolator AOUT2 AOUT1 BOUT2 BOUT1 ENA-A ENA-B	Module IN1 IN2 IN3 IN4 ENA ENB