Once the UDS reaches the pre-set temperature the blower turns off. The blower turns back on when temperature drops by a few degrees, then turns off once the temp goes up again just like a thermostat although much more sensitive. This cylic on/off switching of the blower serves to regulate the temperature inside the drum to within +/-3 deg-F for hours on end, as long as there’s enough charcoal in the fire basket. A temperature probe goes through the side of the drum and serves to sample the actual temperature inside drum.
An alarm sounds off once the temperature drops by about 20 deg-F from the set point and this prompts me to add more charcoal. The set point for the alarm automatically follows the temp setting on the knob; there’s no knob for the alarm set point. Now I can sleep without worrying about my cooking. 
The analog meter serves as a rough (un-calibrated) indication of the UDS temperature. I put it there cuz I had one lying around. Not really happy with it cuz I get more resolution from my digital thermometer.
Analog electronics reside inside the small black box. The temperature can be set anywhere from 180 deg-F to 300 deg-F by turning a knob. LED indicators for power and blower on. Alarm enable/disable switch. The black box is magnetically-mounted to the side of the drum for quick & easy mounting & de-mounting (no screws). Power is supplied by either a wallwart or a 12v battery and the unit is reverse-polarity protected, less than 2 watts power consumption.
after I installed this controller I never have thick dark smoke coming out of my UDS anymore and the temperature is REALLY constant now. No more guesswork and tweaking of the air vent holes which take a long time to figure out and varies with ambient temperature.
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Post time: Jun-12-2017