The minimum voltage to drive this circuit is determined by the "pickup" voltage of the relay. The enable pin may be switched on and off very quickly for PWM (pulse width modulation) speed control.īoth control pins are connected to the microcontroller via 220 Ohm resistors to limit current. The "Base" of the second TIP120 is the "Enable Pin" - turning it on causes the motor to actually run. This is used to turn the motor on and off. The "Base" of the first TIP120 is the "Direction Pin" - turning it on and off switches the direction of the motor.Ī second TIP120 switches power to common on the relay. Since the microcontroller can't quite produce enough current to drive the relay - a transistor (TIP120) is used to switch it on and off. This in effect reverses the wiring whenever the relay is turned on or off. The motor is connected to both normally closed and normally open (in reverse) sides of the relay. This circuit uses a DPDT (Double Pole Double Throw) relay to switch which direction the motor is turning.
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