A measurement circuit includes a switch unit with a number of keys selectively pressed to output different resistance regulating signals. A resistance setting circuit receives the resistance regulating signals and connects different resistances to a control circuit. The control circuit obtains a voltage according to the chosen resistance by the resistance setting circuit and compares the voltage with a preset voltage. If the voltage is greater than the preset voltage, the control circuit outputs a high level signal to a control pin of a pulse width modulation (PWM) controller, to control a voltage unit to output a voltage. If the voltage is less than the preset voltage, the control circuit outputs a low level signal to the control pin of the PWM controller, to control the voltage unit to not output a voltage. A display unit displays the chosen resistance.