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.