according to the principles of the invention , the flame detecting system as a whole is composed essentially as described in fig1 namely : a detector head a , to be placed near the combustion chamber d in close optical relation with the flame f to be monitored and controlled , and equipped with an ionization tube t sensitive to ultraviolet radiation , polarization circuits , discharge control , and transmission of the pulse signals ; a line b connected to the detecting head , by means of two or more wires , to supply power to the head and to transmit the signals over a distance ; an electronic control station c placed at a proper distance , connected to line b , and having apparatus which performs the following functions : a . an electric power supply to supply power to the head at proper voltage and power levels ; b . means for receiving and converting the received pulse signals to develop a quantitative indication thereof , on local or remote display instruments i ; c . means for receiving the pulse signals and , on the basis of their intensity and sensitivity levels predetermined by calibration , causing the flame relay to be energized or de - energized , for the purpose of actuating the safety devices of the burner in case of absence of flame . this latter function ( c ) is by far the most critical for the purposes of the safety of the entire system , and , therefore , a special objective of the present invention , which consists of a new and improved type of control station , which combines with excellent performance the advantage of being fully fail - safe . this characteristic is completely independent of whether or not there are devices and / or circuits employed for the periodic checking of the tube operating characteristics , which , therefore , can be done at times and in ways most suitable for the particular case . for a complex electronic apparatus consisting of a series of stages to be fail - safe , it is sufficient that the following two conditions are met simultaneously : a . each individual state must be fail - safe , in the above sense ; b . the series must be conceived so that non - occurrence of the characteristic signal in any one stage prevents the output of the characteristic signals in all subsequent stages . to obtain the degree of safety required in flame detector stations , an apparatus has been developed which meets these requirements and which in its preferred realization operates according to the block diagram of fig2 . independently of the methods of transmission of the pulse signals to the control station , each stage , represented by a &# 34 ; block box ,&# 34 ; is made to operate dynamically , so that each component continuously and periodically runs through all points of its operating characteristic . for this purpose , where necessary , periodic test signals of a sufficiently high frequency ( for example , 1 khz ) are introduced , and the output relay can be energized only if the entire apparatus correctly responds to the test signals . in the diagram of fig2 the oscillator stage o furnishes the test signals , which may be rectangular wave signals of a given frequency ( for example 1 khz ) or other signals of properly defined characteristics . to prescind from the symbols used in the diagram , the nature of function of each stage is as follows : stage 1 ( preferably a transformer ) receives the signals from the line and transfers only the pulse signals , while adapting the impedance of the line to the input impedance of the control station . stage 2 ( preferably a rectifier and pulse shaper ) gives to the signals a specific form and level . stage 3 ( preferably a capacitor ) utilizes the alternation of levels to transfer energy to the following stages . stage 4 ( preferably an adjustable potentiometer ) calibrates the sensitivity of the system , suitably proportioning the transfer of energy . stage 5 ( preferably a capacitor ) stores the energy received from the signals , giving at the output a voltage level corresponding to the integral mean . stage 6 ( preferably a controlled switch ), by a special test signal , samples the voltage at the input . stage 7 ( preferably a trigger circit ), generates a digital output signal whenever in a sampled signal it exceeds a certain threshold level chosen on the basis of the required maximum sensitivity . stage 8 ( preferably a capacitor ) utilizes this alternation of levels to restore the level of energy stored by it and from which it furnishes a continuous signal . stage 9 ( preferably a controlled switch ), by means of the test signal , samples the input signal . stage 11 ( preferably a transformer ) lastly transfers to the final flame relay k the signals resulting from the chain in normal operation , constituting a barrier for those signals which may derive from breakdown or non - function of the preceding stages . other purposes , characteristics and advantages of the invention will become evident from the following description , with reference to the diagram shown in fig3 which shows a preferred realization of a complete control station for flame detection constructed according to the invention . to the terminals a and b is applied the feed voltage of the apparatus , of 24 v d . c . the voltage is stabilized by the transistor q1 to a value of + 15 v . the transformer t1 , with the transistor q2 and q3 , operates as an inverter self - oscillating at a frequency of about 1 khz , which furnishes the negative voltage of - 15 v , the periodical test signals of rectangular wave fluctuating between + 1 . 5 v and - 1 . 5 v at 1 khz and also furnishes via the diode 17 , the capacitor 16 and the protection resistor 13 , to the terminals c and d , a continuous voltage of 700 v , which is coupled to the detector head over a line to provide the ionization tube the necessary voltage of the proper polarity . using the wire coupled to terminal d as a common wire , the pulse signals generated by the discharge of the tube due to the ultraviolet rays reach the control station over a third wire ending at the terminal e . the pulse transformer t2 which corresponds to stage 1 in fig2 transfers the signals separating the continuous voltage levels , and brings about the best impedance adaptation for all alternate components which constitute the signal . the diode bridge 18 , the capacitor 22 and the amplifier formed by q4 constitute the stage which full wave rectifies and shapes the input to give to the pulses a specific form ( stage 2 in fig2 ), as on the collector of q4 the voltage switches from 15 to 0 v and then goes up again . the capacitor 26 with the diodes 27 and 28 ( stage 3 in fig2 ) carries out a fail - safe coupling and stores energy while q4 is turned off ( charging through 27 ), and then transferring a negative voltage to the next stage when q4 becomes saturated . the adjustable potentiometers 30 - 31 ( stage 4 in fig2 ), properly limiting the current flow , permit calibrating the sensitivity from the outside of the control station to obtain the best discrimination between near - by flames . the sensitivity of the control station may be changed by closing an external contact 19 across the terminals h and i . this causes the switching of contact s1 by coupling terminal 71 to 0 v ( terminal 72 being coupled to - 15 v ). a visually observable light signal is generated by means of the luminescent ( i . e . light emitting ) diode l . the capacitor 32 , in the presence of flame signals ( stage 5 in fig2 ) establishes a voltage at terminal 32a which is negative relative to ground bus 0 v , the mean value of which , as the frequency of the pulses at the input increases , has an approximately logarithmic response owing to the zener diode 33 and the resistor 34 . an ideal response curve is thereby obtained in that it permits maintaining the discrimination properties of the apparatus at an optimum level within an extremely wide range of radiation intensities , as they occur in practical cases , avoiding the phenomena of uncertainty of measurement or saturation . the voltage present on the capacitor 32 is applied to the following stages intermittently , by effect of the transistor q5 ( stage 6 in fig2 ) piloted ( i . e . alternately operated between conduction and non - conduction ) by the test signal coupled thereto by transformer t1 . the next stage is a schmitt type threshold discriminator ( stage 7 in fig2 ), comprising transistors q6 and q7 , which , in turn , detects the presence of the flame with fail - safe reliability as it must continuously switch by effect of the flame signal . in the preferred realization , the threshold of the schmitt discriminator is put at 2 v . that value is not determining for the purposes of safety , but only to specify the sensitivity of the trigger circuit . capacitor 40 , in the same manner as capacitor 26 , serves to make a fail - safe connection . in particular the energy accumulated by capacitor 40 in the switching cycles is sufficient to maintain in operation the downstream circuits and hence to keep energized the flame relay k for a pre - established time , after the non - occurrence ( i . e . termination ) of the flame signals . resistors 43 and 44 serve to adjustably determine the de - energization delay time of relay k , to a greater or smaller elapsed interval according to the ohmic values of the resistors and the position of switch g . ( the capacitor 40 in combination with the diodes 41 and 42 with either resistor 43 or 44 corresponds to stage 8 in fig2 ). normally the de - energization time is 1 second . the transistor q8 ( stage 9 in fig2 ) acts in a manner analogous to transistor q5 , to make the transfer of the energy accumulated in capacitor 40 to the following stages intermittent in nature . the transistors q9 and q10 and the resistors 46 , 47 , 48 are the components of the power amplifier stage ( stage 10 in fig2 ) which , like the others , must operate dynamically . the two input terminals of this stage are formed by the terminals 46a and 46b of resistor 46 , while the two output terminals are represented by the collector 73 of q10 and by the + 15 v line coupled to terminal 74 . for this stage , the cyclic dynamic operation is achieved , for example , as follows : in this stage , the test signal coincides with the useful input signal . this signal is periodically variable , as the preceding stage ( stage 9 in fig2 ) furnishes a periodically sampled useful signal . in every period of its cyclic dynamic operation , the stage ( 10 in fig2 ) can assume two extreme operating points , corresponding to the simultaneous non - conduction or saturation of the two transistors q9 , q10 . with the cyclic dynamic operation of the stage , which occurs between said two extreme points , also each component of the stage ( q9 , q10 , 46 , 47 , 48 ) periodically and continuously runs through all points of its operating characteristic . lastly the transformer t3 with the diodes 49 and 50 ( stage 11 in fig2 ) provides for the energization of relay k , in an absolutely fail - safe manner . a separate circuit , formed essentially by the operational amplifier 70 , serves to provide a quantitative indication of the flame intensity on a measuring instrument m1 included in the apparatus , and , if further desired , on a remote instrument m2 , assuming as zero the minimum value which is sufficient to energize the flame relay , regardless of the calibration sensitivity of the apparatus . the input signal at the amplifier 70 is obtained from the collector of q5 by means of the resistors 57 , 58 and 59 and is filtered by the capacitor 56 . it follows that no energization of relay k can result from any failure of the amplifier a . while a preferred form of realization of the invention has been illustrated and described , various modifications thereof will be evident to the specialists , and it is therefore understood that the invention is not limited to the form of realization set forth or to the details thereof , and that it includes the departures therefrom in the spirit and scope of the invention derivable from the description , drawings and claims .