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Electronic Switches

Author: J.B. Hoag

It is possible to use electron tube circuits instead of hand-operated switches or motor-driven devices to connect alternately first one and then another input signal to a common circuit. As an example, we may consider the simultaneous observation of two different wave-forms on a single cathode-ray screen. Now there are special cathode-ray tubes with two and even three separate beams (and deflecting plates) in a single glass envelope operating onto a single fluorescent screen. With a switch, however, a single-beam tube may be used to compare two wave-forms by alternately switching the vertical deflecting plates from one input to the other in rapid succession. This is called double-tracing. If the switching is accomplished slowly, each wave-form appears as a dotted line, as in Fig. 30 M, whereas, if the switching occurs with sufficient rapidity, the traces appear to be essentially solid lines.

Fig. 30 M. Double tracing on a single-beam cathode-ray tube

The circuit diagram of a particular electronic switch is shown in Fig. 30 N.

Fig. 30 N. An electronic switch

A sinusoidal wave applied at S is amplified and then clipped by the twin diode 6H6. Thus positive half-cycle square-waves appear across AC and negative half-cycle square-waves across BC. The 6C5 tubes on the right are biased at or beyond cutoff. When the input from 6H6 makes 1 conductive, a signal at S1 will pass on to the output terminals. When the input from 6H6 makes 2 conductive, a second signal at S2 will go to the output but, since 1 is non-conducting at this time, the signal at S1 does not go out. Thus, signals at S1 and S2 are passed on, alternately, and at a rate of succession determined by the frequency of the switching control S.

In other electronic switches,1 multi-grid tubes are used instead of the 6C5 tubes at the right of Fig. 30 N, so that the input signals from S1 and S2 can be injected on separate grids from those which un-bias the tubes. Also, a multivibrator circuit is sometimes used instead of the clipper-tube to generate the square-waves for un-biasing the switching tubes.

1 See Review of Scientific Instruments, April 1941.

Last Update: 2010-11-21