Anschluss von Gardena 9V Impulsventilen
Since the activation and deactivation of the electrovalve requires opposite voltage, the drivers in the motor shield helps to provide positive and negative voltage. However, since the rates and currents are different, simply attach the output of the DC motors of the motor shield is not sufficient. A strategy to vary the voltage and current of the output (9v 210ma for opening, -1.6v 50mA for closing) is required.
Using this circuit, assuming the + terminal is at top, when a positive 9v voltage is applied, the top branch acts as a short circuit and the current flows freely through the diode and activates the valves, while when a negative 9v voltage is applied, the top branch acts as an open circuit and the current must pass through R1, the voltage drops to -1.6v and the current is limited according to the specification. Actually another resistor should be applied in series with the diode, but since the valve opens even without using it, I decided not to include it.