Update cheat-sheet.md

Author: karlsoderby

content/hardware/02.hero/boards/uno-r4-minima/tutorials/cheat-sheet/cheat-sheet.md

@@ -188,6 +188,39 @@ In addition, analog pins A0-A5 can also be used as digital pins. Note that **A4/
 
 The reference voltage of all digital pins is 5 V.
 
+## LED
+
+The UNO R4 Minima has a total of four LEDs, three of which are programmable:
+- **ON** - power LED, cannot be programmed.
+- `LED_BUILTIN` - classic "built-in LED", attached to pin 13.
+- `RX_LED` - LED labelled "RX" on the board.
+- `TX_LED` - LED labelled "TX" on the board.
+
+To control these, define them as outputs and write desired state. The below example blinks each LED every second.
+
+```arduino
+void setup(){
+  //define pins as output
+  pinMode(LED_BUILTIN, OUTPUT);
+  pinMode(RX_LED, OUTPUT);
+  pinMode(TX_LED, OUTPUT);
+}
+
+void loop(){
+  //turn on all LEDs
+  digitalWrite(LED_BUILTIN, HIGH);
+  digitalWrite(LED_RX, HIGH);
+  digitalWrite(LED_TX, HIGH);
+  delay(1000);
+
+  //turn off all LEDs
+  digitalWrite(LED_BUILTIN, LOW);
+  digitalWrite(LED_RX, LOW);
+  digitalWrite(LED_TX, LOW);
+  delay(1000);
+}
+```
+
 ## DAC
 
 ![DAC Pin](assets/dacpin.png)