referring to fig1 there is shown in diagrammatic form a digital magnetic compass of the type with which the invention is especially useful . such a compass is shown for example in the aforesaid copending patent application . the compass includes a transparent disc 10 having a plurality of concentric tracks each having a different number of alternately transmissive and opaque segments 12 defining codes representative of compass headings . a compass magnet 14 is affixed to and rotatable with disc 10 . an array of photosensors 16 is disposed typically along a radius of disc 10 with a sensing cell associated with each coded track of the compass disc . an illumination source , such as a light emitting diode 18 , is disposed on the opposite surface of disc 10 from array 16 and is operative to illuminate the coded tracks , the light being transmitted by the transparent segments of the coded tracks being received by the sensor array 16 . for different angular positions of disc 10 with respect to array 16 , the array provides a plurality of output signals correspondingly coded to represent the angular disc position and therefore also representative of compass heading . the output signals are processed by suitable electronic circuitry to provide an output indication of compass heading sensed from disc 10 . the electrical energy required to illuminate source 18 is a major portion of the total energy required for compass operation since in preferred implementation the electronic circuitry of the digital compass is of micro - circuit form requiring extremely little electrical power . according to the invention , source 18 is energized only for so long as necessary for receipt of a proper output signal from array 16 . the novel photoelectric control circuit is shown in fig2 . a light emitting diode d1 has its anode coupled to a source of operating potential v cc by way of a resistor r1 and its cathode connected to the collector of a transistor q1 the emitter thereof being coupled to ground or other source of reference potential . the light emitting diode is arranged to transmit light through disc 10 , the transmitted light being received by phototransistors 22 and 24 . these phototransistors are associated with respective tracks of disc 10 such that at least one phototransistor always receives light from source 18 . the collectors of phototransistors 22 and 24 are coupled to voltage source v cc while the emitters thereof are respectively coupled via respective resistors r2 and r3 to ground . respective capacitors c1 and c2 are provided in shunt with respective resistors r2 and r3 . the emitter of phototransistor 22 is coupled to an input of a comparator 26 , while the emitter of phototransistor 24 is coupled to the input of a second digital comparator 28 . a reference signal is applied to both comparators 26 and 28 from a suitable source v ref . the output of the comparators is coupled via an or gate 30 to an input of a latch circuit 32 which also receives a control signal from a control source 34 . the output signal from latch circuit 32 is coupled via a resistor r4 to the base of switching transistor q1 . circuit operation will be described in conjunction with the waveform diagrams of fig3 a - 3e . a timing cycle is defined by a control signal shown in fig3 a provided by control source 34 . the provision of a control signal to latch circuit 32 causes an output signal to be provided to the light emitting diode . this output signal , shown in fig3 b , causes energization of the light emitting diode for the duration of this latch output signal , as shown by the waveform of fig3 c depicting the current flowing while light emitting diode is energized . the threshold voltage v ref is depicted as a dotted line in fig3 d , while the output voltage v pt from the phototransistors 22 and 24 is shown in fig3 d as a rising voltage which exceeds the threshold level and which thereafter decays to a zero level . the digital comparators 26 and 28 each provide an output signal upon exceedance of the threshold v ref by the phototransistor signals , and these output signals are applied as a reset signal via or gate 30 to latch circuit 32 . this reset signal is shown in fig3 e . resetting of the latch circuit causes discontinuance of the latch output signal thereby de - energizing the light emitting diode by switching off transistor q1 . during each control cycle defined by the duration of the control signal provided by source 34 , the light emitting diode is energized until detection of a photosensor signal at least equal to the predetermined threshold level . upon such detection , the light emitting diode is de - energized for the remainder of the operating cycle . thus , the illumination source is energized only during a portion of each operating cycle and for only so long as is necessary to assure detection of a valid photosensor signal equal to or greater than the threshold level . power is thereby conserved by minimizing the time duration for which the light emitting diode is energized . in the embodiment of fig2 two phototransistors are shown illuminated by a common light emitting diode . it will be appreciated that the invention is applicable to various light coupling arrangements . for example , a single light emitting diode or other illumination source can be associated with a corresponding photosensor , or a plurality of photosensors can receive light from a single illumination source . in some instances , it may be desirable to energize the light emitting diode throughout an operating cycle but to vary the illumination in order to maintain a substantially uniform electrical output signal level . the embodiment of fig2 can be modified to accomplish such gain control , by replacing the latch circuit 32 with a variable gain circuit such as a transistor amplifier which provides an output signal of amplitude variable in response to the magnitude of the phototransistor output signal . the invention is not to be limited by what has been particularly shown and described except as indicated in the appended claims .