MK138 for cooling (energize relay to cool)

hello to all here, this is my 1st post, i am no stranger to electronics.
i have the MK138 - the kit works as designed :slight_smile: the relay energizes when the temperature is low
(designed for heating mode)
i’d like to know what mods i can do to make it so that the relay will energize for cooling

the kit is designed for heating ( incubators / fish tank heaters etc. )

i am looking to use this kit for cooling ( modified TEC refrigerator for 12V / Solar power use )
so i wish to see the relay LED lit up when in cooling mode.
what must be done to make this happen ?

Hello there !

Just swap the resistor R4 and the NTC connected across SK 1

thanks, i found out also that if i swapped pins 12,13 on the IC1 then it does the opposite.
sure i had to cut the traces and use jumper wires, but it works…

using NC Contact instead the NO Contact or virso versa should work normally

true, but i wanted to see the led on when my fridge is cooling… :o)


Thanks dacflyer for posting the question. I was wondering that too…

I just assembled the MK138, had fun doing it, but it did not work as expected.
I thought it would turn on when temp went to 30 C or above, but this turned on the led / relay BELOW 20 c. I wondered what I did wrong.

Yes, as posted the NO tap on the relay made it work, and it turns on my 12v fan above 30c. To adjust the temp without hacking it up, I added a 15k ohm with the sensor. it puts the on / off 30c point about mid range on the pot.

if i remember right the circuit turns on when it gets too cold… i modified mine to turn on when it gets too hot…
in the schematic it tells you what resistor to change if you want to vary the temp range…

Thermostat kit MK138 uses a NTC (negative temperature coefficient) thermistor temperature sensor to detect temperature changes. The thermostat can respond to a set point temperature range from approximately 41°F to 86°F (5°C to 30°C).

the instructions also mention how you can change resistors for higher or lower temperature range.