An inductive loop has its frequency of oscillation controlled by means of a counter, a reference frequency source, a comparator, and an attenuator. The frequency of the loop is compared by the comparator to the frequency of the reference source to produce an error signal which controls the attenuator. The amplitude of the output signal of the attenuator is used to adjust the loop frequency to a desired value.

A vehicle detector installation includes a loop oscillator the loop of which is laid in the roadway and which is locked in operation to a voltage controlled oscillator (VCO). Vehicle detection is effected by a phase detector monitoring the phase difference between the oscillators. The VCO is incorporated in a phase-lock loop (PLL) that is capable of locking to a multiple of a reference frequency oscillator over a range of multiples. To achieve the locking of the loop oscillator to the VCO, means are provided for disabling the normal operation of the PLL and sweeping the VCO over its range of frequency until the phase detector indicates that the loop oscillator and VCO frequencies are equal. This indication activates the PLL to its normal operation to pull the VCO and therewith the loop oscillator to an adjacent multiple of the reference frequency. The PLL is maintained by a repeated charge/discharge cycle of the VCO capacitor that is dependent on the phase of the reference oscillator and the VCO.

A microprocessor controlled loop detection system to be connected to a number of inductive loops which are buried in a road bed and which is used to detect the presence of motor vehicles above the loops to control the operation of traffic lights at an intersection. A common oscillator is connected to each loop on a time-shared basis under the control of a microprocessor, and the frequency of the oscillator in each instance is dependent upon whether or not there is a vehicle above the loop. The microprocessor provides a null time as the system is switched from one loop to another to prevent spurious responses in the system due to transient signals. The microprocessor acts to count the number of cycles of the output signal of the oscillator which occur during a predetermined time interval as each loop is connected to the oscillator by the microprocessor to determine the oscillator frequency for each loop and thereby detect the presence of a vehicle in the particular loop. A synchronizing circuit is provided connecting the oscillator to the microprocessor for controlling the start and finish of each such counting interval by the microprocessor.

A sensor, especially for detecting misfeeds in a stream of sheets of paper, comprises a pair of sensing plates sensing changes in the capacitance between the plates, a first oscillator providing an output signal at a frequency representative of the capacitance, a second oscillator set to a reference frequency which is the same as or close to that frequency generated by the first oscillator when no object is between the sensing plates, gating means to produce an output representative of the frequency difference between the oscillator outputs, a low-pass filter, and a microprocessor which responds to changes in the difference frequency.

The temperature of an operator circuit is measured. A relationship between the temperature of the operator circuit and characteristics of the operator circuit is determined. The relationship is applied to the measured temperature of the operator circuit to create an adjustment action. The detection threshold is adjusted by the adjustment action.