The Thermo Plug works, basically. Here’s the configuration with an NTC sensor and a buzzer:

Sample output, reading out the ADC periodically:

As the temperature goes up, the resistance and the ADC readings both go down. The actual conversion to the corresponding temperature will require a calibration and either a lookup table or a polynomial fit, as described in a previous post.

Here is the board layout:

The buzzer can be replaced by a relay plus clamping diode. Since it’s driven via an NPN transistor, there will be enough current to drive several types of relays. The buzzer / relay are hooked up to PWR, not +3V.

Another configuration for this board is with the AD597 thermocouple sensor chip. Unfortunately, I forgot to add a wire between pins 1 and 3, so it has to be added by hand – as you can see here:

Another view, with the attached K-type thermocouple:

The thermocouple responds to temperature change much faster than an NTC – and it’s already calibrated (second number is °C):

When I touched the sensor with my hands, it took less than half a second to detect a 5° increase!

Here is the sketch I used for the above demo:

Ok, time to tweak the Thermo Plug and fix that missing wire in the next pcb run.