Part 1 - The 'Magic Eye' Tubes

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Magic eye tubes were originally designed to be used as a radio tuning assist - a shadow would be present as in figure 1 if the radio was not properly tuned; however, with a strong signal, the shadow would disappear nearly entirely. This makes sense when analysing the fundamental operation of a magic eye tube.

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As with most vacuum tubes, there are some internal 'building blocks', in this case, there is a filament, a cathode, a control grid, an anode plate, a deflector plate, and a target plate. Running through the purpose of each building block below:

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• The Filament - The filament is used purely as a heat source and takes the form of a resistive length of wire. Since these tubes feature indirectly-heated cathodes, the filament does not even sport a special emissive coating, it literally just gets hot.

• The Cathode - The cathode has a thermally emissive coating on it and will emit electrons when heated by the filament. The advantage of having the cathode independent of the filament is the ability to set them at completely different potentials. The filament can be left electrically 'floating' and directly driven from a transformer (as was often the case), whereas the cathode can be given a potential with reference to circuit ground. This also allows the filament to be driven with DC or AC since there will be no 'gradient' effect (often observed in directly-heated cathodes in Vacuum Fluorescent Displays) due to voltage being dropped across the length of the filament.

• The Control Grid - The control grid acts, as it does with standard triodes, so as to inhibit the flow of electrons emitted from the cathode in an electronically-controllable manner. By applying a negative potential on the grid with respect to the cathode, electrons (which are also negatively charged) will be repelled since like charges repel each other.

• The Anode Plate - The anode plate is, again, equivalent to that found inside a triode or other amplifying tube. It serves to attract electrons towards it by operating at a high positive potential with respect to the cathode. Since opposite charges attract, the negatively-charged electrons will accelerate towards the anode.

• The Deflector Plate - The deflector plate is essentially a duplicate anode, but acts so as to 'bend' the stream of electrons by attracting them off to the side instead of directly upwards. This is similar to how old CRT devices worked (TVs, monitors, oscilloscopes, etcetera).

• The Target Plate - The target plate is also held at a high positive potential relative to the cathode; however, the target is covered in a phosphor that fluoresces when struck by electrons. The glow created is more of a jade green than the cyan of more modern VFDs.

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From this, we can see that by reducing the amount by which the control grid is negative with respect to the cathode (i.e. reducing how much it inhibits the flow of electrons), the electrons will be deflected by a lesser angle, since they pass by the deflector in a shorter amount of time and acceleration/velocity is related to time. Translating this to positive logic, if one was to bias the cathode to 5V, then applying 0V to the grid would cause the maximum deflection angle and hence the maximum shadow angle, producing a result similar to that in figure 1. On the contrary, if one was to apply 5V to the grid, the minimum deflection angle would be achieved and the shadow would virtually disappear. Of course, the range of grid voltage will be specified in the tube's datasheet and may not be 5V.

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So, now that the basics of magic eye tubes have been established, work can begin on designing a circuit to sweep the shadow 'open' and 'closed'. As a heads up, I shall be using the EM80 magic eye tube for this which is a side-view tube with a particularly pleasing shadow pattern, in my opinion at least. Figure 2 below gives a good impression of what the shadow pattern looks like.