Tube in a Tube Clock

The clock described on this page is the result of both events, a simple single digit nixie clock with some interesting features, but without any difficult components. This clock first of all demonstrates that it is possible to use standard, low-voltage, general puspose, small-signal transistors like the BC550, as nixie drivers. Since I can imagine that it will be a
Tube in a Tube Clock - schematic

surprise to most people, that a transistor with a BVceo of 50V can be used in a 200V application, I have gone to quite some length to explain it, starting from basic semiconductor theory. The high voltage supply for the nixie in this clock is directly taken from the mains. This works perfectly, but it has some serious implications with respect to safety. The first section on this page deals with this issue; read it carefully! However, the clock is completely safe as long as a completely isolating case is used, and as long as no metal parts of the circuit can be touched while it is connected to the mains. The circuit for this clock is so small, that an elegant glass test-tube could be used as a case! For safety reasons, the mains on/off switch, in this case a simple cord switch, is used to (besides switch the clock on and off), set the time. How this works is explained later on. By minimizing the power consumption of the controller circuit, it additionally was possible to feed the low-voltage controller circuit directly from main, without the need for a transformer! How it works is explained in the section Dropper capacitor power supply. The last point concerning this clock that needs introduction is the ambient lighting feature. On the backside of the clock a three color LED is mounted which produces an endless sequence of random colors (Fig. 1). This produces in the evening and at night a nice glow on the wall behind the...

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