Power-on indicator: flash the green LED while the screen is dark

When the display doesn't stay lit — any backlight mode where the screen goes
dark, including "None" — the green LED now flashes for 100 ms every 3 s, so a
dark screen doesn't leave you wondering whether the radio is still switched on.

The flash stands down whenever the LEDs are already in use: while receiving
(DMR slot RX or an analog/digital signal), while transmitting, and while muted
(which flashes the red LED instead). On standing down it restores the green LED
to the current RX state, matching the existing mute-indicator handling. Writes
go through LedWrite(), so the CPS "all LEDs disabled" option still applies.

Docs (README, MANUAL, CHANGELOG) updated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JCK7s3WnwRHZNtQLdyyWBu

CHANGELOG.md

@@ -7,7 +7,11 @@ single running list until the first versioned release.
 
 ## Unreleased
 
-_Nothing yet._
+- **Power-on indicator** — when the display doesn't stay lit (any backlight mode
+  where the screen goes dark, including **None**), the green LED flashes for
+  **100 ms every 3 s** so it's obvious the radio is still switched on. The flash
+  pauses whenever the LEDs are already in use: while receiving, while
+  transmitting, and while muted (which flashes the red LED instead).
 
 ## 0.12.0
 

MANUAL.md

@@ -99,6 +99,14 @@ fixed. Every element (S-meter, frequencies, menus) follows the selected theme.
 ### Mute indicator — 🟢 [FreeTRX]
 While muted, the **red LED flashes** (0.5 s every 2 s). See *Mute* below.
 
+### Power-on indicator — 🟢 [FreeTRX]
+Whenever the backlight is off — any backlight mode where the screen goes dark,
+including **None** — the **green LED flashes** for **100 ms every 3 s**, so a dark
+screen doesn't leave you wondering whether the radio is still on. The flash pauses
+while the LEDs are needed for something else: while receiving, while transmitting,
+and while muted (which flashes the red LED instead). It also follows the CPS
+"disable all LEDs" option.
+
 ---
 
 ## Audio

MDUV380_firmware/application/source/applicationMain.c

@@ -1541,6 +1541,37 @@ void applicationMainTask(void)
 			}
 		}
 
+		// Power-on indicator: when the display doesn't stay lit, flash the GREEN LED for
+		// 100ms every 3s, so it's still obvious the radio is switched on while the screen
+		// is dark. Only while nothing else owns the LEDs: a LED lit by RX/TX already shows
+		// the radio is alive, and muting flashes the RED one.
+		{
+			static bool heartbeatFlashOn = false;
+
+			bool rxGreen = (slotState == DMR_STATE_RX_1) || (slotState == DMR_STATE_RX_2) ||
+					currentRadioDevice->analogSignalReceived || currentRadioDevice->digitalSignalReceived;
+			bool ledsAreFree = (rxGreen == false) && ((trxTransmissionEnabled || trxIsTransmitting) == false) &&
+					(audioAmpIsMuted() == false) && (lastVolume != -99);
+
+			if ((displayIsBacklightLit() == false) && ledsAreFree)
+			{
+				bool flashOn = ((ticksGetMillis() % 3000) < 100);
+
+				if (flashOn != heartbeatFlashOn)
+				{
+					heartbeatFlashOn = flashOn;
+					LedWrite(LED_GREEN, (flashOn ? 1 : 0));
+				}
+			}
+			else if (heartbeatFlashOn)
+			{
+				// The backlight came on, or RX/TX/mute took the LEDs over: end our flash and
+				// restore the green LED to whatever the current RX state requires.
+				heartbeatFlashOn = false;
+				LedWrite(LED_GREEN, (rxGreen ? 1 : 0));
+			}
+		}
+
 		while(ticksGetMillis() < startTime + 1)				// ensure this Task runs at 1ms intervals. Regardless of clock speed.
 		{
 			vTaskDelay(0U);

README.md

@@ -57,6 +57,9 @@ FreeTRX keeps OpenGD77's core (DMR/FM, hotspot, CPS compatibility) and adds:
 - **Battery bars display** — a third battery status option alongside % and
   voltage: a battery-shaped icon with up to four 25 % segments (Display Options →
   Battery).
+- **Power-on indicator** — whenever the backlight is off, the **green LED flashes**
+  for 100 ms every 3 s, so a dark screen never leaves you guessing whether the
+  radio is still on. It pauses while receiving, transmitting or muted.
 - **DMR reliability fix** — resolves a hard-fault (freeze needing a battery pull)
   on the DMR path that affected recent self-built OpenGD77 revisions.