I made this simple alarm to have in the kitchen. It alarms of these events:
-Freezer temperature rises too high (broken)
-Kitchen ceiling temperature rises too high (stove left on, or fire)
-Water on the floor (dishwasher broken or water pipe bursted)
-Lights turned on (nice to know, or food burglar visiting the fridge)
-Loud enough sound (fire alarm, or burglar making noise)
-Mains power loss and return of the power
In addition it has green-yellow-red traffic light LEDs that indicates the electricity price, that it retrieves from internet in real time. When electricity is expensive, the production struggles. Then it is a good idea not to use the stove.
The alarm includes Ni-MH batteries as backup if mains power fails. It is based on the ESP8266 WiFi microcontoller. Alarms are sent over WiFi to my raspberry-pi based hub, that can send the alarms to my cellphone. The temperature and other data are also sent to the cloud to be viewed with a web browser to see past events and trends.
I decided to make the 2-sided circuit board using old fashion trough-hole technology. I make my circuit boards myself using photosensitive mask, UV-exposure and Ferrichloride etching. The photo mask I make by Laser printer on A4 overhead transparencies. Unfortunately my laser printer (Dell) does not make perfect black result so the large copper areas have a bit of "swiss cheese" holes in them. But it does not affect the performance.
