the invention is an electronic device which measures the time elapsed between two electrical input signals , produces an electrical output signal after a period of time proportional to the elapsed time between the two input signals and the operating frequency of a variable oscillator . if a given time interval represents the time of arrival of an object at two locations separated by a known distance d 1 then an output signal occurs a time interval after the object has travelled an additional distance d 2 , which time interval is determined solely by a specific relationship between distance d 1 and the variable oscillator frequency . distance d 2 is independent of the object &# 39 ; s speed provided only that its average speed remains constant over distances d 1 and d 2 . the basic time and distance relationships upon which the invention &# 39 ; s concept of operation is based are illustrated diagrammatically in fig1 . in this figure , sensors 1 and 2 represent devices which produce an electrical output signal when the object of study is in close proximity . they must be located so that the object , moving at speed u , passes them before passing the recording device , and are separated by an arbitrary , but known , distance d . the horizontal lines labelled &# 34 ; start signal &# 34 ; and &# 34 ; stop signal &# 34 ; symbolize the electrical outputs of the object sensors as a function of time . the time interval between sensors &# 39 ; signals is δt 1 . the horizontal line labelled &# 34 ; count up &# 34 ; and the vertical lines on it represents the electrical signal that would result if an electronic oscillator operating at frequency f 1 was turned on at the start of time interval δt 1 and then turned off at its end . similarly , the line labelled &# 34 ; count down &# 34 ; represents the electrical signal that would result if a second oscillator operating at frequency f 2 were turned on only so long as necessary for exactly the same number of cycles of its signal to occur as occurred on the &# 34 ; count up &# 34 ; line during the time δt 1 . this interval is δt 2 . the &# 34 ; output &# 34 ; line represents an electrical signal capable of triggering the operation of the device with which the moving object is to be recorded . it is located an arbitrary , but known , distance d 2 from sensor 2 . the time delay computer utilizes these relationships as follows : frequency f 1 pulses are generated by a fixed - frequency oscillator and counted during interval δt 1 . at the end of this interval , which is signified by the arrival of sensor 2 &# 39 ; s output signal , the fixed - frequency oscillator is disconnected from the devices which counted the pulses . the counting devices are then immediately connected to a variable frequency oscillator in such a way that each of its pulses subtracts one count from the total which was reached when the fixed oscillator was connected . the frequency of the variable oscillator may be the same , higher , or lower , than the fixed oscillator frequency . the delayed output signal pulse is generated when the counting devices reach zero . if the object speed u is constant during its passage from sensor 1 to the recording device , then the following mathematical relationships hold : ## equ1 ## but , also note : ## equ2 ## therefore : ## equ3 ## or : ## equ4 ## equation ( 1 ) shows that for any set of values f 1 , f 2 , and d 1 , the distance from the second sensor to the point at which the object is located when the output signal occurs is a constant that is independent of the object &# 39 ; s speed , provided only that the speed has not changed between the first sensor and the camera . equation ( 2 ) gives the frequency at which the variable oscillator should be set so that the output signal will occur when the object is at the same distance d 2 for any given d 1 and f 1 . all of the quantities in equations ( 1 ) and ( 2 ) may be measured with any desired precision in advance of the object &# 39 ; s passage , thus determining the object &# 39 ; s location when the recorder is operated with essentially the same precision . fig2 in this attachment is a block diagram showing the manner in which the invention &# 39 ; s operating principle is implemented . ff1 is a digital electronic flip - flop which is triggered by the sensor 1 output signal . the flip - flop &# 39 ; s output causes the fixed - frequency oscillator to be connected to the count - up input of a multiple - decade counter which was initially set at zero . when sensor 2 &# 39 ; s signal triggers ff2 at the end of period δt 1 , ff1 &# 39 ; s output is returned to the inactive state which disconnects the fixed oscillator . ff2 &# 39 ; s output simultaneously causes the variable frequency oscillator to be connected to the decade counter &# 39 ; s count - down input . when the counters reach zero , a signal representing an attempt to borrow is produced by its least - significant - digit part , which produces the delayed output signal . the borrow signal also clears ff2 and the device is back in its initial condition . this is the essence of the invention . fig3 is a schematic diagram of a time delay computer which was fabricated according to the principles outlined above . all parts are standard , commercially available , electronic components . the commercial part identification numbers and / or component values are listed in table 1 , time delay computer parts list . the computer functions in the following manner : transistors q1 and q2 serve as input buffers and inverters to properly condition non - ttl ( transistor - transistor logic ) input signals . they are operated as unbiased saturated switches in which the minimum &# 34 ; on &# 34 ; input voltage is set by base - circuit voltage dividing resistors r1 and r2 . these buffers may be bypassed if ttl - standard start and stop signals are available . integrated circuits ic1 and ic2 are one - shot multivibrators wired to produce a single 30 nanosecond output pulse each time their inputs go low . the start and stop input signals must be of such a nature that they transition only once during the time delay computer &# 39 ; s operating cycle . ic1 &# 39 ; s output pulse is generated when the device &# 39 ; s input is triggered by the sensor 1 signal . this event signifies the start of object speed measurement . the pulse is coupled to the preset input of ic3a , one - half of a dual flip - flop . ic3a &# 39 ; s q1 output is anded with the fixed - frequency oscillator ( f 1 ) in ic4a , one - fourth of a quad nand gate . q1 &# 39 ; s transition applies the oscillator signal to the count - up input of the 6 decade up - down counter chain formed by ic5 through ic10 . this chain may be cascaded to any required length if the relationship between f 1 , d 1 , and u is such that more than 10 6 f 1 cycles may occur during δt 1 . when the moving object causes an output from sensor 2 , ic2 &# 39 ; s one - shot pulse presets ic3b , the q2 output of which clears ic3a , thus disconnecting the fixed - frequency oscillator from the counter chain . ic3b &# 39 ; s q2 output is anded with the variable frequency oscillator in ic4b , so that this signal is applied to the counter chain count - down input with one clock pulse of the variable frequency oscillator &# 39 ; s disconnection . the borrow - out output of the counter chain &# 39 ; s most - significant digit component , ic10 , is connected both to ic3b &# 39 ; s clear input and to the clock input of ic11 , a one - shot multivibrator wired for a single 50 microsecond output pulse . when the counter chain reaches zero after counting down from the up - counted total at a rate determined by the variable frequency oscillator &# 39 ; s frequency , ic10 &# 39 ; s borrow - out pulse simultaneously disconnects the variable frequency oscillator from the chain and produces the system &# 39 ; s delayed output pulse through ic11 . the delay computer &# 39 ; s fixed - frequency oscillator signal was provided by a crystal controlled module ( m1 ) operating at 10 . 000 mhz . the variable frequency oscillator &# 39 ; s signal was provided by an external signal generator capable of generating ttl - compatible square wave signals at any frequency from 0 to 5 mhz . it is indicated on the schematic by the legend &# 34 ; external clock &# 34 ;. both fixed and variable frequency oscillators may be external to the basic time delay computer circuit . in the device described , it is possible to frictionally check total system operation by dry - cycling it and measuring the time required for the counter chain to count completely around ( 10 6 counts ). this is accomplished by triggering the delay computer stop input with no start input . switches s1 and s2 permit such manual triggering and switch s3 clears the counter chain and all flip - flops , as well as generating an output pulse . ic1 and ic2 pulse outputs were made externally accessable in the device as &# 34 ; start &# 34 ; and &# 34 ; stop channel &# 34 ; pulse outputs . these signals were used to trigger external digital timers which thus documented the time intervals involved . while the time delay computer was implemented with small and medium scale integrated circuits , the same concept of operation may be realized more compactly with commercially available lsi ( large scale integration ) up - down devices . the time delay computer was used in the course of a gasdynamic shock tube experiment to trigger a spark lamp so that schlieren photographs of mach 3 shock waves could be obtained at a series of closely spaced predetermined locations . the following table i indicates specific components that are suitable to use in the above described time delay computer . table 1______________________________________time delay computer parts list______________________________________q1 , q2 2n2222a silicon npn transistoric1 , ic2 sn74121 monostable multivibratoric3 sn7476 dual j - k flip - flopic4 sn7400 quad nand gateic5 - ic10 sn74912 bcd up - down counteric11 sn74123 dual monostable multivibratorr1 , r2 10 kω miniature trimmer resistorr3 1 kω , 10 % composition resistorc 0 . 1 μf , 60 v ceramic capacitorc1 1 . 0 μf , 60 v ceramic capacitorm1 k1091a 10 . 000 mhz crystal oscillator module ( motorola ) ______________________________________ while the invention has been described in its preferred embodiment , it it understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects .