diff --git a/Sensor-API.md b/Sensor-API.md
index 87cf6325..dbf6d6a0 100644
--- a/Sensor-API.md
+++ b/Sensor-API.md
@@ -86,4 +86,8 @@ AddLog(LOG_LEVEL_INFO);
 snprintf_P(log_data, sizeof(log_data), PSTR(D_LOG_I2C "MPR121(%c) " D_FOUND_AT " 0x%X"), pS->id[i], pS->i2c_addr[i]);
 AddLogSerial(LOG_LEVEL_INFO);
 ```
-#### void AddLogMissed(char *sensor, uint8_t misses)
\ No newline at end of file
+#### void AddLogMissed(char *sensor, uint8_t misses)
+
+### Useful pre-processor directives
+### `PSTR("string")`
+This pre-processor directive save RAM by storing strings in flash instead of RAM
\ No newline at end of file