COUNTERS 



High-speed counters or ''scalers'' 



These are important in biological work for applications which include 

 radioactivity measurements and the production of decade time marks 

 from a master oscillator. We begin by considering the hard valve 'scale 

 of two'. Consider the circuit in Figure 41.8: it is basically a symmetrical 



HT«- 



HT- 



HT- 



Figure 41.8 



Eccles-Jordan circuit with the addition of two diodes. Let point A be 

 normally maintained at HT+ potential and let V<^ be conducting and V^ 

 cut off. Then D<^ will be cut off and D-^ has both terminals at HT+. Now 

 let a negative pulse carry ^ to a potential below HT+ but rather higher 

 than K2 anode. D2, will remain cut off, but D^ will now conduct, taking 

 Kj anode negative and initiating the cumulative effect by which the circuit 

 triggers into the opposite condition — K^ cut off, Fo conducting. Because 

 the circuit is symmetrical it follows that a second negative pulse at A will 

 return the circuit to its original condition. Notice that the positive-going 

 back edge of the pulse A has no effect on the circuit, since the only valve 

 it could affect is the one which happens to be conductive, and it is isolated 

 from this because the relevant diode is cut off. 



HT+ 



u^ 



Figure 41.9 



Clearly either valve goes through a cycle of operations for every two 

 cycles of the driving pulse, and the circuit therefore 'counts down' on a 

 scale of two. It is not difficult to see that the necessary drive conditions 

 are met if A is fed from a tap on the anode load of one of the valves in 

 another, similar, scale of two. In this manner, chains of Eccles-Jordan 

 circuits can be made to count on the scale of two raised to any integral 



647 



