Patent ID: 7325892

Claim:
An ink level detecting unit including an excitation pulse generating part which applies an excitation pulse to a vibration element provided for an ink tank which houses ink, a sensor which detects a frequency of a counter electromotive force waveform from the vibration element based on residual vibration by resonance with a medium in the ink tank, and a judgment part which judges the existence of ink on the basis of the frequency detected by the sensor, wherein said sensor includes at least one filter which causes only a waveform in a predetermined frequency band, which is based on the presence or absence of ink, to pass, and a frequency detecting circuit which binarizes the counter electromotive force waveform from the vibration element, counts the number of pulses of the binarized counter electromotive force waveform, counts time from a predetermined number-th pulse to a predetermined number of pulse, and detects the frequency of the counter electromotive force waveform on the basis of this time; said filter comprises a band pass filter for ink presence, and a band pass filter for ink absence, which cause only the waveforms in the predetermined frequency bands that have been previously assumed according to the ink presence and ink absence respectively to pass; and said frequency detecting circuit detects the frequency of the counter electromotive force waveform that has passed through said band pass filter for ink presence or band pass filter for ink absence; and an insensitive band is provided between the pass bands of both band pass filter for ink presence and band pass filter for ink absence.