In SX Microcontrollers, SX/B Compiler and SX-Key Tool, vchaney wrote:

Thank you all for your quick and informative replies.

Peter - I will try the capacitor across the motor leads right away. One of the articles you referenced mentioned shielding the motor. I didn't mention that I am successfully using another DC motor on the same SX and with the same method. That motor is shielded, but the train motor is not shielded, in fact most of the train motor is quite open. I wonder if it would help to have the capacitor right at the train track, and not on the circuit board which is 3 feet away. The train gets its power through the wheels in contact with the track. I wonder if this less-than-ideal connection is contributing to the noise.

The power suppies have a common ground.

The relay is to change the direction of the motor, so I can't put a diode directly across the motor. Since the +12V to the motor is also connected to the +12V to the ULN, then the ULN's diode should also be across the motor. I don't see what harm there would be in putting another diode near the relay, so I'll try that, too.

JonnyMac - I assume the com pin is the +12V? If yes, then I am OK there. My ULN just has 7 in and out pins, ground and positive power.

Thanks again for the help. I'll know about the extra diode and capacitor in a few hours.

Vic Chaney

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