So I just finished putting together my MK138 which will eventually control the fan cooling my home theater cabinet. Unfortunately I ran into some problems . . .
I plugged the MK138 into a 12V power supply and it switched on for a second, the LED lit up and I heard the relay switch on, but then the LED shutoff and the relay switched off. I tried adjusting the pot but the relay/LED would not trigger again.
EDIT: I proceeded to unplug the power then wait a few seconds and plug the power back in. Every time I did this the LED/relay would switch on for around a second then switch off and become unresponsive. If I quickly unplug the power and plug it back in then nothing happens (capacitors still charged?). If I turn the pot to the max temperature position the LED/relay will stay on longer before becoming unresponsive. At the lowest temperature setting for the pot the LED/relay will only flash on for an instant when the power is applied. 
I figure it’s probably a problem with something I did but I checked all my solder joints with my multimeter and everything appears to have good contact. I even confirmed that the LM324 was being supplied 12V whenever the MK138 was powered.
Here are my high res pics (sorry for the US imagehost):
Thanks for your help! You guys make great stuff!
As far as we can see, soldering and component placement looks OK.
Make sure you did not swap the 1N4148 and the zener diode.
Check 1N4148.
Check if you have 5V over ZD1 and at pin10 and pin8 of op-amp.
Check if voltage at pin13 changes with temperature
Check if voltage at pin12 changes when pot is turned.
[quote=“VEL417”]As far as we can see, soldering and component placement looks OK.
Make sure you did not swap the 1N4148 and the zener diode. Confirmed: (ZD1 has what appears to be “C55” written on it. For D1 and D2 I can just barely make out ‘1’, ‘N’ and ‘8’.)
Check 1N4148. Confirmed: (I had to desolder one of the legs of D1 to test it. Both D1 and D2 passed the diode test on my multimeter.)
Check if you have 5V over ZD1 and at pin10 and pin8 of op-amp. Confirmed
Check if voltage at pin13 changes with temperature Confirmed: (3.04V ambient [30C] to 3.18V when I pinch the thermistor with my fingers [33C]*)
Check if voltage at pin12 changes when pot is turned. Confirmed: (2.84V at max and 1.05V at min)[/quote]
A weird thing happened when I turned on the MK138 to test it as you suggested. When I plugged in the power the LED/relay lit up and stayed on for several minutes. I then turned the pot from the max position towards the min position and the LED/relay instantly switched off.
EDIT: I may have mixed up the LM324 pin numbers while experimenting with the “stuck on” phenomenon so I’m removing that data.
After several more experiments the “stuck on” phenomenon is no longer occurring. Given ambient temperature levels it leads me to believe there is a bad solder joint perhaps?
Thanks so much for your help! Any other ideas though? 
*Temperatures measured with multimeter thermocouple
I’ve never been so happy to be an idiot 
It works all along, I just had the behavior of the relay backwards in my head and it’s been too hot here for me to figure it out. 
Sorry, I’m a bit of a novice.
Thanks so much for your help!
