diff --git a/docs/gps.md b/docs/gps.md
new file mode 100644
index 0000000..e69de29
diff --git a/exercises/12_FiveTalk/platformio.ini b/exercises/12_FiveTalk/platformio.ini
index 0eff093..84b0b70 100644
--- a/exercises/12_FiveTalk/platformio.ini
+++ b/exercises/12_FiveTalk/platformio.ini
@@ -28,6 +28,7 @@ build_flags =
   -D GPS_WAKEUP_PIN=7
   -D GPS_1PPS_PIN=6
   -D GPS_L76K
+  -D NODE_SLOT_COUNT=7
   -D LORA_CS=10
   -D LORA_MOSI=11
   -D LORA_SCK=12
@@ -94,4 +95,6 @@ build_flags =
   -D NODE_LABEL=\"Guy\"
   -D NODE_SHORT=\"G\"
   -D NODE_SLOT_INDEX=6
+  -D GPS_UBLOX
+
 
diff --git a/exercises/12_FiveTalk/src/main.cpp b/exercises/12_FiveTalk/src/main.cpp
index 8c612cd..b292aab 100644
--- a/exercises/12_FiveTalk/src/main.cpp
+++ b/exercises/12_FiveTalk/src/main.cpp
@@ -27,6 +27,10 @@
 #define NODE_SHORT "?"
 #endif
 
+#ifndef NODE_SLOT_COUNT
+#define NODE_SLOT_COUNT 7
+#endif
+
 #ifndef NODE_SLOT_INDEX
 #define NODE_SLOT_INDEX 0
 #endif
@@ -63,8 +67,8 @@
 #define FW_BUILD_UTC "unknown"
 #endif
 
-#if (NODE_SLOT_INDEX < 0) || (NODE_SLOT_INDEX > 6)
-#error "NODE_SLOT_INDEX must be 0..6"
+#if (NODE_SLOT_INDEX < 0) || (NODE_SLOT_INDEX >= NODE_SLOT_COUNT)
+#error "NODE_SLOT_INDEX must be 0..NODE_SLOT_COUNT-1"
 #endif
 
 static const uint32_t kSerialDelayMs = 1000;
@@ -156,6 +160,20 @@ static uint8_t bestSatelliteCount()
   return (g_gps.satsUsed > g_gps.satsInView) ? g_gps.satsUsed : g_gps.satsInView;
 }
 
+static uint32_t computeFrameSeconds(uint32_t requiredSeconds)
+{
+  uint32_t frame = ((requiredSeconds + 4U) / 5U) * 5U; // round up to 5s
+  while (frame <= 60U && (60U % frame) != 0U)
+  {
+    frame += 5U;
+  }
+  if (frame == 0U || frame > 60U)
+  {
+    frame = 60U; // fallback
+  }
+  return frame;
+}
+
 static void logf(const char *fmt, ...)
 {
   char msg[256];
@@ -1030,7 +1048,7 @@ static bool initRadio()
     return false;
   }
 
-  logf("Radio ready for %s (%s), slot=%d sec=%d", NODE_LABEL, NODE_SHORT, NODE_SLOT_INDEX, NODE_SLOT_INDEX * 2);
+  logf("Radio ready for %s (%s), slot=%d/%d (2s each)", NODE_LABEL, NODE_SHORT, NODE_SLOT_INDEX, NODE_SLOT_COUNT);
   return true;
 }
 
@@ -1051,8 +1069,15 @@ static void runTxScheduler()
   if (!getCurrentUtc(now, epoch))
     return;
 
-  int slotSecond = NODE_SLOT_INDEX * (int)kSlotSeconds;
-  int secInFrame = now.second % 10;
+  uint32_t requiredTxSeconds = (uint32_t)NODE_SLOT_COUNT * kSlotSeconds;
+  uint32_t frameSeconds = computeFrameSeconds(requiredTxSeconds);
+  uint32_t slotSecond = (uint32_t)NODE_SLOT_INDEX * kSlotSeconds;
+  if (slotSecond >= frameSeconds)
+    return;
+
+  uint32_t secInFrame = (uint32_t)now.second % frameSeconds;
+  if (secInFrame >= requiredTxSeconds)
+    return; // idle guard interval
   if (secInFrame != slotSecond)
     return;