This timer is a cyclic timer with a fixed total ON + OFF period of 1 minute 30 minutes or 1 hour. Three onboard jumpers can be used to limit the total ON+OFF period to 1 minute 30 minutes or 1 hour. You can then set the ON period of the cycle using the variable resistor. For example if you set the variable resistor to 1/3 of its range the ON period will be about 20 minutes and the OFF period will be about 40 minutes (assuming the jumper is set to the 1hour mode).