Have you ever left the house and had that little voice in your head say “Did I remember to turn off the stove?" Or maybe you're concerned about a loved-one living independently. With LoFi, you can eliminate needless worry.

Here is an example application where LoFi wirelessly monitors a stove using an adjustable heat detector. The steel casing is magnetically mounted so that it is out of the way, but does not require permanent installation.

You can see this in action towards the end of the Semifinals video.

TECHNICAL DETAILS

Contactless temperature sensors provide an opportunity to instrument major appliances without opening or altering them. It is possible to connect a wireless transmitter to the LEDs on a modern oven display to determine if the stove is active. However, a less invasive technique is to simply measure the temperature externally using infrared.

The Melexis infrared sensor (MLX90614) sees -70C to +380C. It is as simple as aiming the sensor at the stove top and measuring the temperature. LoFi transmits changes in the analog value, and can trigger transmissions on either those changes or a digital trigger pin.

The Melexis thermal sensor can be read with either I2C or PWM (app note). LoFi is designed for analog inputs. To convert the Melexis PWM output to analog, attach a 10 kilohm resistor and 0.1 μF capacitor to the output pin.

For user friendliness, I wanted an adjustable “on" threshold and an indicator light. Because only one input pin is needed by LoFi, it would have been cheapest, smallest, and easiest to modify the code to have LoFi's ATtiny chip perform the comparison operation and light the LED with the spare pins.

For the sake of being a purist, I instead chose to create an interface board containing a comparator, so that LoFi would still be considered 'stock'. That is, this was an opportunity to document a solution for people that aren't comfortable with programming.

The TLV3702 was chosen because it works down to 2.5 volts. C2 and R2 convert the IR sensor PWM to analog. R3 is the adjustment threshold for determining when the stove is 'on'.

Below is the implementation of the circuit. Again, the comparator is only necessary if you don't want to modify the LoFi source code.

There are a wide variety of interesting things to monitor in a home. With LoFi, the price is low enough to make it affordable.