tempmodule: fix reference voltage.

1 files changed, 6 insertions, 3 deletions

diff --git a/src/tempmodule/main.ino b/src/tempmodule/main.ino
index 9288deb..d0db8ea 100644
--- a/src/tempmodule/main.ino
+++ b/src/tempmodule/main.ino
@@ -194,18 +194,21 @@ static void discover_client(void)
  * Rt:	Thermistor resistance
  * Ra:	Voltage divider resistor
  * Vt:	Tap voltage
- * Vcc:	Input Voltage (3.3V)
+ * Vcc:	Input Voltage (3.31V)
  *
  * Rt = Ra * Vcc / Vt - Ra
  *
  * Using 64-bit arithmetic as 32-bit would overflow.
- * On WeMOS D1 Mini the A0 reads 3.3V as 1023, that is our Vcc.
+ * On WeMOS D1 Mini the A0 reads 3.2V as 1023; our Vcc is 3.31V
+ * The correct value of the reference volume obtained by measuring
+ * resistances with a multimeter, taking the ADC reading and applying
+ * Ohm's law.
  * We shift the range by one to avoid division by zero.
  */
 static int get_resistance(int64_t vt, int64_t ra)
 {
 	++vt;
-	return ra * 1024 / vt - ra;
+	return ra * 1080 / vt - ra;
 }
 
 static int get_temperature(int res)