Patent ID: 8514092

Claim:
An alarm circuit for a fan, the alarm circuit comprising: a direct current converting circuit comprising an input connected to a speed pin of the fan, wherein the direct current converting circuit converts pulse signals from the speed pin of the fan to a direct current voltage when the fan operates; a first electronic switch comprising a first terminal connected to an output of the direct current converting circuit, a second terminal grounded, and a third terminal connected to a first power supply, wherein the first electronic switch is turned on when the first terminal of the first electronic switch receives the direct current voltage from the direct current converting circuit; a second electronic switch comprising a first terminal connected to the third terminal of the first electronic switch, a second terminal grounded, and a third terminal connected to a second power supply, wherein the second electronic switch is turned off when the first electronic switch is turned on; a clock chip comprising a power pin connected to the second terminal of the second electronic switch, a reset pin connected to the second terminal of the second electronic switch, a trigger terminal connected to the second terminal of the second electronic switch through a first resistor and a second resistor in series and grounded through a first capacitor, a threshold pin connected to the second terminal of the second electronic switch through the first and second resistors in series and grounded through the first capacitor, and a discharge pin connected to a node between the first and second resistors; and a light emitting diode comprising an anode connected to an output pin of the clock chip, and a cathode grounded.