diff --git a/door.py b/door.py
index 77d3b82..0ff6c77 100644
--- a/door.py
+++ b/door.py
@@ -6,6 +6,9 @@ from datetime import datetime
 
 # https://docs.python.org/3/library/datetime.html#datetime.datetime.now
 
+# DHT22 with an rpi
+# https://web.archive.org/web/20200819162120/http://www.home-automation-community.com/temperature-and-humidity-from-am2302-dht22-sensor-displayed-as-chart/
+
 # the door sensor
 door = Button(23)
 btn = Button(19)