A circuit for controlling current flow in an instrumentation loop includes a light sensor receiving impinging light and delivering a signal varying with the intensity of the impinging light. The signal is amplified and produces a reference signal which is a current signal of about four milliamperes in the absence of incoming light and about twenty milliamperes in maximum light. A voltage controlled current driver presents an impedance to current flow through the loop which varies with the reference signal and with the intensity of the incoming light. The sensor includes a mounting plate, a photodetector mounted on the mounting plate, a filter mounted over the photodetector, and a diffuser mounted on the mounting plate forming a dome over the filter and photodetector. The diffuser disperses incoming light before the incoming light impinges the photodetector. The filter blocks infrared light thereby blue enchancing impinging light to approximate the light sensitivity response of a human eye. The sensor circuit uses a photovoltaic cell and an amplifier to obtain an output that is linearly proportional to the light level.

In a thermal-type airflow meter that measures an intake airflow rate in an internal combustion engine of an automobile or the like, there has been a problem that pulsation errors caused by external fluctuations under the environment including a large pulsation and reverse flow in the intake valve are difficult to be reduced. In order to solve this problem, the invention provides a flowmeter comprising a detection element that outputs a non-linear signal corresponding to a flow rate, and a regulation means that regulates an output signal of the detection element for controlling. With this construction, the regulation means executes output regulation processing that regulates irregularities of the detection element in the output signal, and inequality linearization processing that regulates an average value of the output signal after the output regulation processing by means of regulating parameters.

A loop-powered intelligent transmitter for process control variables is provided which can operate at an average current in excess of the minimum value of current that can flow in the loop. In order to keep the average current drawn down to the actual loop current, execution of a control program by a microprocessor incorporated in the transmitter is slowed down to the extent necessary to reduce current demand of the transmitter to that available. Slowing down is preferably accomplished by sensing when a current deficit occurs and placing the microprocessor in sleep mode to reduce current consumption until the deficit is made up. Entry into sleep mode may be deferred during time critical operations of the microprocessor.

A measuring transmitter assembly using the same lines for power input and for output signals and using a single power supply, has a first power input and signal output loop coupled to the power supply, a second power input and signal output loop coupled to the power supply, a first sensor for providing a first input signal in response to the value of a first parameter and a second sensor providing a second input signal in response to the value of a second parameter, and a controller coupled to the first and second sensors. The controller is also coupled to the first and second power input and signal output loops and provides a first output signal on the first power input and signal output loop in response to the first input signal and a second output signal on the second power input and signal output loop. The second output signal may be in response to only the second output signal or it may be in response to the first and second input signals. The assembly also has a digital isolator for electrically isolating the first power input and signal output loop from the second power input and signal output loop.