TIMERS, COUNTERS AND RATE MEASUREMENT 



has devised a 'Woolworth' timer which converts the 50 cycle mains into a 

 train of pips occurring every 10 msec: only a handful of components is 

 used, as shown in Figure 41.2. The waveforms are sketched in Figure 41.3. 



Old 



250-0-250V 

 H 



S130 



001 ^F 



Output 



Figure 41.2 



At A there is 'raw' full-wave rectified a.c. Each time the potential here 

 reached the striking voltage of the regulator tube the tube fires and the 

 potential difference across it drops to the burning voltage, producing a 

 sudden rise of potential at B. B then follows A until the tube extinguishes 



Mains 



fV\ 



\r\r\ < 



♦ID m sec^ 10m s ec ♦ 



Point -4 



Point 8 



T Y 



Figure 41.3 



Point C 



Figure 41.4 



again, when it drops back to earth potential. This waveform is differentiated 

 by the 0-01 [x¥ capacitor and the 5k resistor so that only the transients are 

 preserved as positive and negative pips; the latter are removed by the 

 crystal diode. 



Variable frequency RC oscillators are frequently used for time marking. 

 It is then possible to choose a frequency so that the marker trace has the 

 appearance of Figure 41.4 rather than 41.1a. In this case the position of 

 the peaks of the waves is not difficult to measure off. A frequency accuracy 

 of 1 per cent may be expected from the cheaper commercial models, and 

 0- 1 per cent from the superior variety. 



For really accurate work recourse may be had to a valve-maintained 

 tuning fork or to a quartz crystal oscillator. The former is capable of 



644 



