Heatermeter output control

I'm building a bread proofing box and would like to use Heatermeter 4.2/ Arduino (RPi A+). I'm using the 12vdc output (for blower) as one of the outputs with a 1k resistor to an IR510 MOSFET to control 24vdc external power supply via relay. The relay controls a 110vac outlet connected to a small (2x2) frig. I inverted the output so when the temp is high, the output is on and turns on the frig. I planned on using the 5v output for the servo to control the other relay via a TIP120. This relay would control a 15w 110vac bulb to produce heat if the temp drops below the SP. The grounds are tied and am switching the low side (N Channel). The 12v MOSFET portion works fine however, I did not realize that the 5v (servo) power was constant (assumed it was switched). So the TIP120 is not going to work. I did notice that the signal output (servo) shifts between 142 and 155mv (OFF/ON respectively). My apologies I don't have a schematic to upload but its pretty straight forward. Is there an option I can use with that low a voltage or is there another output I can use? Or possibly control the constant 5v servo power via script? I'm not an electrical genius and fairly new to Heatermeter but spent quite a few years controlling industrial cooling logic. Any help would be greatly appreciated.

The IRF510 MOSFET is completely inappropriate, it is not a logic level device and needs 10V to
switch properly. You want a logic-level MOSFET with a low in-resistance (the IRF510 is ancient
technology, modern devices are 100x better, literally).

Please draw your schematic and post it, it is not at all obvious from your description, few here
have the patience to decode long descriptions of a circuit when you can draw it, and post the photo.