Single Meter Clocks

  
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An alternating display for my version 1 nixie clock led me to think of new logic designs for switching signals to the display. I took the design of my version 1 nixie clock, tossed out the 4013 flip flop and two 4017 counters, and redesigned the hours section and the reset circuits. A 4081 (quad 2-input AND gate) was used instead; a single gate serves as a Set/Reset
Single Meter Clocks - schematic

bistable, another gate basically takes in the signal when the hours reach 13 and goes HIGH to reset the hours, and the last two gates serve as buffers (one for the 60Hz line). There were several modifications mainly to drive a single meter (the meter used is a Heathkit "Antenna Impediance Meter" or a 0-100uA meter). The outputs of the 4017s were converted to analog signals using a homebrew DAC with a chain of transitors and resistors. First, a 500K pot was used and wired with one transistor in series. Tune the pot to one digit on the meter, record the resistance of the pot, retune the pot to the next digit, record the resistance, and so on. A resistor ladder was created from the recorded resistances to convert the digital signals to analog signals. For the meter to display 0, the resistance is open and the meter outputs nothing. For the meter to display 1, the signal has to go through the entire resistor ladder (total of about 520K), for 2, it goes through less resistance, and so on until the least resistance causes the meter to display 9 (maximum). The clock circuits are always on so there are always four analog signals being produced. A 74HC4851, a cool 8-channel analog switch mainly for audio, is used to switch one of the four analog signals to the meter. A separate 4017 and a free running 555 oscillator set to be variable from around 1/4Hz to 1Hz is used to switch the channel inputs on the 74HC4851 to allow the display...



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