TIMERS, COUNTERS AND RATE MEASUREMENT 



power (Figure 41.9); thus a chain of 4 circuits makes a counter on the 

 scale of 16. Most of us are accustomed to count on a scale of ten. Ten is 

 not given by raising two to any integral power and for this reason this 

 circuit is not very convenient as it stands. It may be made to count in 

 decades by a simple modification, as follows: 



The presentation of counters of this type is achieved by connecting small 

 neon indicator lamps between the right-hand triode anode of each Eccles- 

 Jordan circuit and earth. When the count is zero all the right-hand triodes 

 are conducting and all the neons are extinguished because the anode 

 potentials are below the necessary maintaining voltage. We call this 

 'circuits off'. In a scale of 16 the lamp associated with Eccles- Jordan 1 is 

 labelled 1, Eccles-Jordan 2 is labelled 2, Eccles- Jordan 3 is labelled 4 and 

 Eccles-Jordan 4-8, and so on. The total count is the sum of the figures 

 beside the lighted lamps. Thus, a straightforward scale of 16 operates 

 according to the following table : 



Count No. 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 



1st E-J (Ones) 

 2nd E-J (Twos) 

 3rd E-J (Fours) 

 4th E-J (Eights) 



Continuous line = 'Circuit on" 



In order to count in decades the cycle has to be modified to this : 



Count No. 







1 



7 



8 







1st E-J (Ones) 

 2nd E-J (Twos) 

 3rd E-J (Fours) 

 4th E-J (Eights) 



Comparing the two tables it is clear that to count in decades it is necessary 

 to arrange that on the arrival of the tenth pulse the second Eccles-Jordan 

 is not switched to 'on' by the first, and the fourth circuit is switched to 'off'. 

 This may be achieved by the scheme shown in Figure 41.10. The fourth 



Eccles-Jordan circuit is arranged as a 'gate' for pulses passing from the 

 first to the second. The gate is open so long as the fourth circuit is 'off'. 

 Thus the first circuit controls the second in the normal manner until count 

 10, when it is unable to do so because the gate is closed at count 8; instead, 

 the first circuit output is used to operate the fourth, returning it to 'off' and 

 reopening the gate for the next counting cycle. 



648 



