Hi LMR! If you haven't seen my robot MAEP already, it has two power supplies, one for the motors and one for the Arduino. I decided to add an LED indicator to show when power is being supplied to the motors (which are servos). The LED is just connected in series with a 1k resistor to the power lines from the motor power supply, a 4.8V NiCad battery. As you expect, the LED is off when motor power is off and on when the motor power is on. But when just the Arduino is on and not the motors, the LED is lit dimly, and changes brightness as I wave my hand over the sensors.