Improved examples; last one needs work.

docs/reference/radio/send-number.md

@@ -28,6 +28,50 @@ input.onButtonPressed(Button.A, () => {
 })
 ```
 
+### Light level sender
+
+This example broadcasts the level of the light around it:
+
+```blocks
+radio.setGroup(99)
+basic.forever(() => {
+    let level = input.lightLevel()
+    radio.sendNumber(level)
+})
+```
+
+This example shows the light level from the sender (above):
+
+```blocks
+radio.setGroup(99)
+basic.forever(() => {
+    let level = radio.receiveNumber()
+    basic.showNumber(level)
+})
+```
+
+### Mailbot
+
+This example takes the signal from the light level sender (above)
+and shows a text string if the light level becomes much brighter.
+You can put the light level sender in your mailbox and it will
+tell you when someone opens the box. You can try this with a normal
+box too, like a present.
+
+```blocks
+radio.setGroup(99)
+let max = 0
+basic.forever(() => {
+    let level = radio.receiveNumber()
+    if (level > max) {
+        let max = level
+    }
+    if (max > 10) {
+        basic.showString("ALERT")
+    }
+})
+```
+
 ### See also
 
 [receive number](/reference/radio/receive-number), [on data received](/reference/radio/on-data-received)