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en:projects:nixie-clock:power_supply [2011/02/19 14:50] alex |
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| The next step in driving the tube is the tube switching. Four tubes means 4 anodes to control and 48 cathodes (each tube has 12 cathodes, 10 numerals plus 2 dots at the bottom left and right). One method is to connect all 4 anodes to power and switch all the cathodes. However, 48 transistors is a bit much. I'm going for compact here, remember? Also, the microcontroller I'm using has only 28 pins. And if I do a two-board design for compactness, I want to minimize the interconnections between the two boards. More wires = more work. And more wires take up more space. One method to knock out transistors and control lines at the same time is to use 4 to 10 line decoder chips for the digits. It just so happens that there is a very special chip called a 74141 that was designed specifically to drive the cathodes of nixie tubes. The output pins are high-voltage, open-drain transistors. Perfect. Only one problem, they stopped making them in the 80s, so they're impossible to find. Fortunately, I managed to get my hands on the Russian pin-equivalent part, the K155ID1. And since I naturally want to use as few of these hard-to-find chips as possible, I also opted to switch the anodes of the tubes as well. That means 2 transistors per tube to switch the anode plus one decoder and two transistors to switch the cathodes. One decoder and 10 transistors, much more efficient than 48 transistors. Also, it only takes 12 control lines to control a 6 tube display and controlling a 4 tube display involves simply ignoring 2 of those. Future expandability! Also, connecting the LED anodes to the control lines and adding one more transistor for the LED cathodes allows control of the LEDs under the tubes with only one more IO pin. This sets the minimum number of interconnect lines at 16 -- 6 tube select lines, 4 binary digit select lines, 2 decimal point control lines, 1 led control line, ground, 5 volt logic supply, and high voltage tube bias. Perfect! | The next step in driving the tube is the tube switching. Four tubes means 4 anodes to control and 48 cathodes (each tube has 12 cathodes, 10 numerals plus 2 dots at the bottom left and right). One method is to connect all 4 anodes to power and switch all the cathodes. However, 48 transistors is a bit much. I'm going for compact here, remember? Also, the microcontroller I'm using has only 28 pins. And if I do a two-board design for compactness, I want to minimize the interconnections between the two boards. More wires = more work. And more wires take up more space. One method to knock out transistors and control lines at the same time is to use 4 to 10 line decoder chips for the digits. It just so happens that there is a very special chip called a 74141 that was designed specifically to drive the cathodes of nixie tubes. The output pins are high-voltage, open-drain transistors. Perfect. Only one problem, they stopped making them in the 80s, so they're impossible to find. Fortunately, I managed to get my hands on the Russian pin-equivalent part, the K155ID1. And since I naturally want to use as few of these hard-to-find chips as possible, I also opted to switch the anodes of the tubes as well. That means 2 transistors per tube to switch the anode plus one decoder and two transistors to switch the cathodes. One decoder and 10 transistors, much more efficient than 48 transistors. Also, it only takes 12 control lines to control a 6 tube display and controlling a 4 tube display involves simply ignoring 2 of those. Future expandability! Also, connecting the LED anodes to the control lines and adding one more transistor for the LED cathodes allows control of the LEDs under the tubes with only one more IO pin. This sets the minimum number of interconnect lines at 16 -- 6 tube select lines, 4 binary digit select lines, 2 decimal point control lines, 1 led control line, ground, 5 volt logic supply, and high voltage tube bias. Perfect! | ||
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