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– 55 – A Meter Indicator For Use With Electrical Counters Using Scale-Of-2 Circuits.

Abstract.—A meter method of indicating the count of a series of coupled scale-of-2 circuits is described. The method is particularly suitable when a scale-of-16 unit has been converted to act as a decade counter.

High-speed electrical counters or chronographs are, at present, being used in increasing numbers and in an increasing variety of applications. When an instrument is first devised and is performing a new function, a certain amount of inconvenience in use is tolerated, but when it becomes an everyday tool it is desirable that the operation be as simple as possible. During the war period electrical counters or chronographs underwent considerable delevopment, one of the main uses being the measurement of short time intervals in determining the muzzle velocity of shells. High-speed counters have also been used for many years in radioactivity measurements, each disintegration in a sample producing a count.

The fundamental unit of a high-speed electrical counter is a scale-of-2 circuit (Fig. 1), which consists of two electronic valves coupled together so that the system has two stable states. In one stable state, the first valve, A, is conducting, and the second, B, non-conducting. If a suitable impulse is fed to the input, the conducting states are reversed, B becomes conducting and A non-conducting. Another impulse to the input returns the system to the original state, and by taking an output signal from one valve of the pair

– 56 – one output pulse can be obtained for two input pulses. Hence the name, Scale-of-2. By using a number of scale-of-2 circuits in series, it is possible to build up scales-of-4, 8, 16, etc.

The normal method of indicating the count is to have one neon lamp connected to each scale-of-2 circuit (across one of the plate load resistors). If the lamp is glowing, the count is one, if the lamp is extinguished, the count is zero. Hence for a scale-of-16, there would be four lamps (Fig. 2). At the start of a counting period all the lamps would be extinguished. When one input pulse is received, the first lamp glows. With a second input pulse, the first lamp goes out, but the second lamp goes on. The counting value for the second lamp is two, the value for the third lamp four, and for the fourth lamp eight. So if, after a counting period, the first and fourth lamps were glowing, the number of pulses received would have been 1 + 8, i.e., 9. In a large sealing unit using 10 scale-of-2 circuits and giving, in effect, a scale-of-1024, there is a considerable possibility of error in recording the count. The trouble is that the final state of the instrument is not simply interpreted in terms of our decimal system of counting.

There have been two developments to simplify the operation and use of high-speed electrical counters. The first improvement was the introduction of the decimal or decade counter. One type, described by Potter*, uses a very ingenious method of resetting a normal scale-of-16 circuit to its original state after ten pulses have been received. Potter used four neon lamps as indicators for each decade, thus making a small amount of arithmetic necessary when finding the number of units, tens and hundreds in a count. A second type has been described by Lewist.† This method uses a scale-of-2 circuit followed by a ring-of-5. For indicators, Lewis suggests neon lamps or magic-eye tubes.

[Footnote] * J. T. Potter. A Four-tube Counter Decade. Electronics: 110–113. June, 1944.

[Footnote] † W. B. Lewis. Electrical Counting. P. 91. Cambridge University Press, 1942.

– 57 – The second improvement has been the introduction of a meter to indicate the count, the meter pointer moving over the scale in steps, and the reading giving the state of the counter directly. Quite recently, a decade counter using the scale-of-2 and ring-of-5 system with a meter indicator, has been described by West.* This particular counting system had been described earlier (during the war period) in a report which had a restricted circulation.

The remainder of this paper describes a meter indicator system which can be used with electrical counters consisting of a series of scale-of-2 circuits. The method is particularly convenient when used with a decade counter of the Potter type, i.e., a scale-of-16 converted to act as a scale-of-10.

Fig. 3 shows the principle. Each pair of valves forms a scale-of-2 circuit. In the initial state before a counting period, valves 1, 3, 5 and 7 are conducting, and valves 2, 4, 6 and 8 non-conducting. The current through the plate load resistor of a conducting valve is about 2.5 ma., and the current through the other plate load resistor about 0.5 ma. (due to the resistors of the coupling circuit). When a suitable pulse is received and the scale-of-2 circuit changes its stable state, the change in current through each plate load resistor is about 2 ma. In the initial state of the counter, the 0.5 ma. currents produce a total voltage drop across R1, R2, R3 and R4 of about 37.5 mV. A similar potential drop is arranged across R5, so, at the start, there is zero potential difference between points × and Y.

When the first pulse is received and valve 2 becomes conducting, the extra 2 ma. through R1 produces a potential change of 0.01 V. at X. With a 2nd pulse, 2 becomes non-conducting, but 4 becomes conducting. The 2 ma. through R1 and R2 produces a potential change of 0.02 V. at X. A 3rd pulse makes valve 2 conducting besides valve 4, so giving 4 ma. through R1 and 2 ma. through R2 to provide the change. This produces 0.03 V between × and Y. With a 4th pulse we get 0.04 V. at X, since valves 2 and 4 become non-conducting but 6 conducting. A 5th pulse gives 0.05 V., etc. Hence each pulse received causes the potential of × to change by equal increments. In the case of the circuit shown, the 15th pulse causes valves 2, 4, 6 and 8 to be conducting, and a potential at × of 0.15 V. The 16th pulse makes these valves non-conducting, causes one output pulse from the unit, returns the potential difference between × and Y to zero, and brings the system to the original state as at the beginning of the counting period. The sole effect of Potter's modification of the circuit is to make valves 2, 4, 6 and 8 non-conducting on the receipt of the 10th pulse instead of needing to wait for

[Footnote] * S. S. West. An Electronic Decimal Counter Chronometer. Electronic Engineering. 19:3–6, Jan., 1947. 19:58–61, Feb., 1947.

– 58 – the 16th. A direct-reading instrument is made by connecting a suitable meter between points × and Y. A 0–500 microammeter with resistance about 150–200 ohms would be satisfactory. Of course, the introduction of the meter will alter the distribution of currents and potentials slightly from those described above, but this does not interfere with the meter's action of indicating the count.

Note: The plate-to-grid coupling condensers marked 10μμF. should be 100μμF.

The undesired couplings between different scale-of-2 circuits due to the common resistors R1, R2 and R3 are very small, and no trouble has been experienced due to them. The instrument can be returned to its initial state, i.e., the meter reading returned to zero, at any time, by opening the reset key for a moment. This makes valves 2, 4, 6 and 8 non-conducting.

High-speed electrical counters using scale-of-2 circuits have been used for a number of years with neon lamps. These indicators need to be interpreted by the operator, and there is a considerable possibility of human error in the case of large scaling circuits. The meter indicator described in this paper simplifies the interpretation.

Discussion.

Mr. R. L. Taylor asked what was the maximum scale of counting that could be covered by the use of a meter indicator.

The speaker replied that this depended on the size and ease of reading of the meter scale. With the ordinary type of panel meter available he did not advocate the use of this method above, say, a scale-of-16 counter. A larger meter would enable higher scales to be distinguished. He advocated the method particularly for decimal counters.