An image forming apparatus includes a processing unit, a sensor, and a processor. The processing unit has a developing device using a developer containing two components of a toner and a carrier. The sensor outputs a detection value corresponding to a toner concentration of the developer during operation of the processing unit. The processor performs a process of determining whether a service call to a service person is necessary based on the detection value of the sensor. The processor operates the processing unit at either a high process speed or a low process speed to acquire the detection value of the sensor, before determining whether the service call is necessary. The processor determines the process speed at which the processing unit is to be operated based on the detection value. The processor operates the processing unit at the determined process speed to determine whether the service call is necessary.

A developing device includes a housing for housing therein a developer containing toner and magnetic carrier, and a first sensor including an oscillation circuit and a coil board which is part of the oscillation circuit and on which a coil pattern is formed. A fitting portion formed by recessing an outer surface of the housing is provided in the housing so as to allow the coil board to be fitted at a position closer to the developer than the outer surface of the housing. The coil board is fitted into the fitting portion.

A developing device is equipped with a developing roller rotationally driven for developing an electrostatic latent image formed on a surface of an image supporting body with toner, a screw rotationally driven of which a ratio of number of rotations to number of rotations of the developing roller being rotationally driven is constant, and a processor. The processor is configured to detect toner density in developer by using a sensor installed facing the screw, during rotationally drive of the screw, and detect information indicating a rotational position of the developing roller, based on ripples which occur in the toner density detected.

An image forming apparatus determines a toner density erroneous detection state based on a patch image density in patch detection auto toner replenishing and, when the toner density erroneous detection state is determined, offsets a target toner density by the amount of erroneous detection so that the actual toner density becomes a desired toner density.

An image forming apparatus includes an image bearing member, a developing apparatus, and a controller. The developing apparatus includes a developer container, a developer bearing member, a conveyance member, and a detection unit. The detection unit comprises a detection surface arranged to face to the conveyance member and is configured to detect a toner density concerning the developer in the developer container and output a signal. The controller configured to adjust a toner density concerning the developer in the developer container based on a peak value of a waveform of an output signal of the detection unit.

Plural sensors are arranged respectively for plural toner colors, and oscillate to generate and output detection signals with frequencies corresponding to toner densities using oscillator circuits. An analog switch selects one detection signal among the detection signals. A filter decreases amplitude of the selected detection signal. A first signal line transmits the detection signal with the decreased amplitude. A comparator compares the transmitted detection signal with a predetermined threshold value and outputs as the detection signal a signal of which a level is set as a high level or a low level in accordance with the comparison result, and thereby increases amplitude of the detection signal. A receiver-side IC receives the detection signal with the increased amplitude at a predetermined port, determines a frequency of the received detection signal, and determines the toner density on the basis of the determined frequency.

A liquid supply device and an image forming apparatus in which the maintenance of a concentration detecting device is improved are obtained. The liquid supply device includes a flow passage through which liquid to be imparted to paper flows; a case including an inlet that is fixed to the flow passage and introduces a liquid from the flow passage, and an outlet that leads the liquid out to the flow passage; and a concentration detecting device that is openably and closably or attachably and detachably attached to the case and includes a detection unit, which detects the concentration of the liquid, at a position that faces the case.

Plural sensors are arranged respectively for plural toner colors, and oscillate to generate and output detection signals with frequencies corresponding to toner densities using oscillator circuits. An analog switch selects one detection signal among the detection signals. A filter decreases amplitude of the selected detection signal. A first signal line transmits the detection signal with the decreased amplitude. A comparator compares the transmitted detection signal with a predetermined threshold value and outputs as the detection signal a signal of which a level is set as a high level or a low level in accordance with the comparison result, and thereby increases amplitude of the detection signal. A receiver-side IC receives the detection signal with the increased amplitude at a predetermined port, determines a frequency of the received detection signal, and determines the toner density on the basis of the determined frequency.

A developing device includes a housing for housing therein a developer containing toner and magnetic carrier, and a first sensor including an oscillation circuit and a coil board which is part of the oscillation circuit and on which a coil pattern is formed. A fitting portion formed by recessing an outer surface of the housing is provided in the housing so as to allow the coil board to be fitted at a position closer to the developer than the outer surface of the housing. The coil board is fitted into the fitting portion.

A developing device is equipped with a developing roller rotationally driven for developing an electrostatic latent image formed on a surface of an image supporting body with toner, a screw rotationally driven of which a ratio of number of rotations to number of rotations of the developing roller being rotationally driven is constant, and a processor. The processor is configured to detect toner density in developer by using a sensor installed facing the screw, during rotationally drive of the screw, and detect information indicating a rotational position of the developing roller, based on ripples which occur in the toner density detected.