The diode avoids that the programmer has to power up the whole circuit. This configuration has been adapted to allow for the in circuit programming of the PIC thanks to the ICSP (In Circuit Serial Programming) connector. The microcontroller power supply is done via a diode and a 6 V line. This particular value has been chosen because it is easy to obtain a reference signal of 1 Hz with some simple divisions in the microcontroller prescaler and firmware. The PIC microcontroller clock is at 2.2768 MHz. In fact, in the first prototype I used some prehistoric 2N1990 (obsoleted at least thirty years ago), which however work as expected. In reality, the 470 kΩ and 220 kΩ resistances makes this circuit more tolerant against some poor performance transistor. My advice is to use eight transistors able to handle at least 200 V between emitter and collector. The main task of the software is to light up a Nixie tube at once, while putting the BCD code of the number to be shown on the lines controlling the 7441. Eight NPN transistors are used to handle the signals to be applied to the anodes of the four tubes. The microcontroller controls the Nixie tubes using multiplexing and the 7441 driver. It is definitively possible to find something much better. I used for the inductor a small toroidal inductor taken from an anti-EMI filter. A simple hack I adopted is to use three common 1N4148 diodes in series, to obtain a very fast diode, able to handle the currents needed. A standard rectifier diode is done for rectifying 50 Hz and would not work here, where the frequency is much higher. Even if the output current is always below 5 mA, it is very important that the diode used is sufficiently fast and bale to handle the voltages around it. The output signal of the oscillator controls a power transistor which acts on an inductor. Here is the first version of the circuit:Ī rectangular wave oscillator, obtained with half of a common NE556 (in reality, a NE555 would have been enough), is used in a basic step-up circuit. In particular, it handles in the same time the hour calculation and the multiplexing of the Nixie tubes. This circuit is quite simple, since all the complicated stuff is done by the PIC16F84 microcontroller. The overall power consumption is around 250 mA. However, it is clear that even if the current provided by the converter is small, this circuit is not for beginners: 170 V can bite very hard! Moreover, the circuit presented in this page is not fool proof. This is for me a very dangerous idea and I suggest not to use this solution. Some realizations you might find on the Internet suggest to obtain the 170 V directly by rectifying the 110 V AC mains (in the United States). This choice is motivated by the need of obtain a rather safe circuit and without a special transformer. Inside the circuit, there is a simple voltage converter able to obtain the 170 V indispensable for powering the Nixie tubes, from a low voltage (6 ) stabilized supply. Several defects of the first prototypes have been corrected and a printed circuit board is available. If you like this project, you might read also A nixie clock, version 2.1 on my site. In the same multimeter where I found the Nixie tubes, I also found several 7441 drivers, which allows to obtain a simple circuit. It is obsolete, but it is easy to found one of those laying in the junk box (at least, this was my case, it would not be very difficult to adapt everything to a newer microcontroller). To keep everything as simple as possible, I decided to use a microcontroller and in particular the good old PIC16F84A. We thus decided to build a clock using those tubes. Browsing the Internet, a lot of information about those display is available, since they have a definite vintage charm. Tearing apart an old broken multimeter, I got in my hands some of those tubes (the exact model was TAF1317A) and with a friend we tried to find a way to use them. By applying a sufficient voltage between the anode and one of the cathodes, the selected cathode glows thanks to a yellow/orange-ish electrical discharge in the gas. The latter are shaped following the numbers one wants to represents. They are neon discharge tubes where there is a common anode and several cathodes. Nixie tubes have been put on the market in the mid 1950's by Burroughs.
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