Plant watering system (#419)

* added youtube movie

* plant watering system

* added project entry

docs/projects/plant-watering/code.md

@@ -0,0 +1,59 @@
+# Code
+
+Let's add the code so that _when the soil moisture level is low, the servo waters the plant._
+
+From the **soil moisture** project, we know that the moisture is low when the ``reading`` is roughly less than ``500``.
+We can use this information to add an ``if reading < 500`` in the code.
+
+```block
+let reading = 0
+if (reading < 500) { }
+```
+
+The servo is connected to pin ``P2`` so we can use ``pins servo write`` block to change the angle of the servo.
+We need it to change the angle to ``0``, wait until the water has fallen off, and move it back to ``80``.
+
+```block
+let reading = 0
+if (reading < 500) {
+    basic.showIcon(IconNames.Umbrella)
+    pins.servoWritePin(AnalogPin.P2, 0);
+    basic.pause(3000)
+    pins.servoWritePin(AnalogPin.P2, 80)
+    basic.pause(3000)
+    pins.analogWritePin(AnalogPin.P2, 0)
+}
+```
+
+We insert the code above in the **forever** loop of the [soil moisture code](/projects/soil-moisture/connect).
+
+```blocks
+radio.setTransmitSerialNumber(true)
+radio.setGroup(4)
+led.setBrightness(64)
+let reading = 0
+basic.forever(() => {
+    pins.analogWritePin(AnalogPin.P1, 1023)
+    reading = pins.analogReadPin(AnalogPin.P0)
+    radio.sendNumber(reading / 4);
+    pins.analogWritePin(AnalogPin.P1, 0)
+    led.plotBarGraph(
+        reading,
+        1023
+    )
+    if (input.buttonIsPressed(Button.A)) {
+        basic.showNumber(reading)
+    }
+    if (reading < 500) {
+        basic.showIcon(IconNames.Umbrella)
+        pins.servoWritePin(AnalogPin.P2, 0);
+        basic.pause(3000)
+        pins.servoWritePin(AnalogPin.P2, 80)
+        basic.pause(3000)
+        pins.analogWritePin(AnalogPin.P2, 0)
+    }
+    basic.pause(5000);
+})
+```
+
+https://youtu.be/7eC_VjH1eP0