A device of a cart includes a control, a speed regulating potentiometer, a microprocessor, a digital-to-analogue converter, a motor controlling unit, and a switch; the control is angularly displaceable and near to handlebars of the cart; the potentiometer is connected to the control so as to output a voltage, which depends on displacement of the control; the microprocessor can produce digital signals according to the output voltage of the potentiometer; the converter is used for converting the digital signals into analogue voltage; the motor controlling unit is used for making a motor run at a variable-speed mode, at which mode the speed of the motor will depend on output voltage of the converter; immediately after the rider presses the switch, the microprocessor will ignore output voltage of the potentiometer, and start and continue giving a same digital signal to the converter so that the motor starts running at constant speed.