The full name of this product is something like The GSE Controller Temperature & Min-Max with Hysteresis Speed!
Overly long name aside, this is a really well priced fan speed controller that is especially suitable for single fan extraction systems in grow tents. It has four dials on the front:
1. Temperature. This is your desired grow room temperature
2. Min Fan Speed. Sets your minimum fan speed down to 20% idle (this can actually be set to turn off as well, but obviously this is not recommended in most situations)
3. Max Fan Speed. Sets the maximum fan speed. Great for taking the edge of noisy fans, among other benefits.
4. Hysteresis. This feature allows you to add a temperature range that the controller will try to maintain between. For instance, if you set 24 degrees as the room temperature and a Hysteresis of 2 degrees, the controller will keep the room between 24-26 degrees. This feature combined with being able to control the min and the max fan speed make this a very versatile controller with a feature set normally only seen in controllers that cost around twice as much.
It's not using voltage stepping or frequency control technology so the fans will hum a bit, but as long as you don’t mind that this product represents excellent value for money - recommended.
Yep - it's kinda weird but it does highlight the main points about how the controller works, if you stick with it: