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Step 2Driver
Drivers connect a cathode (a number inside the tube) to ground, causing the connected digit to glow. Driver choice is always interesting. Three options are prevalent in nixie tube design. First, ten NPN transistors (one for each cathode/digit) can be connected to the tube pins. A microcontroller pin connected to the base of each transistor is used to light the appropriate digit. This driver is seen in Jeff's nixie watch – notice all the surface mount mojo in the picture of the PCB. To learn more about using discrete transistors with nixie tubes, check out this excellent tutorial by Jason Harper:
( http://home.earthlink.net/~jasonrandharper/NixieTransistors.pdf )
David Forbes uses a Darlington driver IC in his nixie watch. This is a modern IC that contains 8 transistors, and is similar in function and use to discrete transistor design discussed above. As I understand, this IC can't cope with the full voltage from the nixie cathodes. To get around this problem zener diodes are placed on each input pin to clamp the voltage at an acceptable level. I believe David used this approach, but I can't seem to find a citation. Assuming he did use these diodes, it added 16 tiny tiny surface mount zeners to the design.
The third way to drive nixie tubes is to use an antique BCD (binary coded decimal) chip. These chips are designed to drive nixie tubes and have no problem coping with the voltages involved. Four pins on the BCD chip receive input from the microcontroller while 10 additional pins connect to the 10 nixie cathodes. The input pins are interpreted as a binary number and cause the corresponding cathode pin to be grounded. For example, if all four pins input pins on the BCD are low (representing 0/0/0/0 in binary, 0 in decimal) then the BCD pin corresponding to '0' will be grounded, causing the '0' cathode in the nixie tube to light. Common BCD chips include the 74141 and KD155 Soviet equivalent.
The Soviet KD155 was chosen for this design. These chips are cheap (~0.50 from tube vendors and Ebay) and require no additional components. The down side is that these old chips only come in a DIP-16 package. I think thats OK – I really like Russian lettering on the chip and consider it as much a curiosity as the nixie tube itself. It takes a little extra space, but adds additional novelty and eliminates oh-so much surface mount mojo.
( http://home.earthlink.net/~jasonrandharper/NixieTransistors.pdf )
David Forbes uses a Darlington driver IC in his nixie watch. This is a modern IC that contains 8 transistors, and is similar in function and use to discrete transistor design discussed above. As I understand, this IC can't cope with the full voltage from the nixie cathodes. To get around this problem zener diodes are placed on each input pin to clamp the voltage at an acceptable level. I believe David used this approach, but I can't seem to find a citation. Assuming he did use these diodes, it added 16 tiny tiny surface mount zeners to the design.
The third way to drive nixie tubes is to use an antique BCD (binary coded decimal) chip. These chips are designed to drive nixie tubes and have no problem coping with the voltages involved. Four pins on the BCD chip receive input from the microcontroller while 10 additional pins connect to the 10 nixie cathodes. The input pins are interpreted as a binary number and cause the corresponding cathode pin to be grounded. For example, if all four pins input pins on the BCD are low (representing 0/0/0/0 in binary, 0 in decimal) then the BCD pin corresponding to '0' will be grounded, causing the '0' cathode in the nixie tube to light. Common BCD chips include the 74141 and KD155 Soviet equivalent.
The Soviet KD155 was chosen for this design. These chips are cheap (~0.50 from tube vendors and Ebay) and require no additional components. The down side is that these old chips only come in a DIP-16 package. I think thats OK – I really like Russian lettering on the chip and consider it as much a curiosity as the nixie tube itself. It takes a little extra space, but adds additional novelty and eliminates oh-so much surface mount mojo.
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Apr 2, 2009. 6:22 PMjforb
says:
The schematic for David's watch is on his web page. Find the Nixie Watch page, look for the Docs link on the left, find the User Manual, look at the last page...
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