Building a digital clock is a great project with a rewarding climax. The 6-digit clock described here, besides being a great conversation piece, will make an outstanding piece of furniture for your desk or living room.
Digital clocks, list most accurate instruments, are expensive. However, since the advent of IC's and with advances in the state-of-the-art a digital clock can be a reality. This clock can be build for about $250.00 with six digits as described here. For the more economically minded, a four digit model can be made for considerably less, leaving out IC10 and IC8, V5 and V6. A little scavanging (like we all seem to be masters at), and a well-stocked scrap barrel can also do wonders in saving the bank roll.
The Nixie tube readouts can be remoted. This gives the builder the utmost in freedom of design. Two of the clocks on the cover are good examples of this technique. The black desk-set clock was originally a desk radio that was redesigned and converted into a six-digit clock. Its electronics are packaged separately and can be unobtrusively secured to the side of a desk. The cramped space dictated the use of rectangular Nixie tubes type B.
The red clock started life as a bed lamp. After a trip to a junk shop it was resurrected and now is a four-digit clock with the electronics mounted behind the bezel. A friend of mine packaged the readout for his clock in a roll-on deodorant can and mounted it on the neck-piece of a broken goose neck lamp. He packaged the electronics in the base. It's a real Buck Rogers masterpiece that fits well into his modern home furnishings. A trip through a second-hand junk store or your local drug store will give you many great ideas for a great new and original clock design.
The two most important ingredients in my lock, the IC's and readouts, were chosen very carefully. THe IC's are the heart of the clock I was especially careful here. I use SN7490N counters and SN7441N Nixie tube drivers made by Texas Instruments. They have good noise immunity for a 5V IC and can be counted on for reliability and long life. The Burroughs B-5750S Nixie tubes were chosen because they were originally designed for a calculator market as a high pressure tube and they are the best available when one digit must remain on for any length of time. (The 0 in the first digit is on for 9 hours at a time, the 1 is on for 3 hours.)
How it works
Clock operation can be broken down into basic subsystems - the 13 o'clock reset circuit, the basic counting circcuits, the timing circuits and, of course, the power supply.
The power supply is a simple half-wave rectifier that provides pulse input tot he timing circuits. Also provision is made through transistor Q5 and the 6.3V Zener diode for 5.6 volt Vcc. Also B+ is tapped from the 117 Vac input and converted to +170 Vdc by the voltage booster D9, R16, and C1. Capacitor C12 is installed across the secondary to remove line transients on the input pulse (see Fig. 1).
Timing circuit
The half-wave 60-Hz input to the timing circuit is shaped to positive square wave pulses by Q4 and associated circuitry (see Fig. 5 and Fig. 3). The resulting 60-Hz square wave is applied at pin 14 of the divide by 10 counter, IC 12. These counters count on the negative going side of the square wave. Output from IC 12, a 6 pps square wave, is applied to pin 14 of IC 13, a divide by 6 counter. IC 13's output is a 1 pps square wave which is applied to pin 14 of IC 11. The counter is wired in a divide by 10 configuration. It resets when 9 is displayed and the square wave goes negative. The counter outputs standard 8421 BCD which is applied to readout driver IC 10. Basically IC 10 is a BDC to decimal decoder driver with output transistors that can handle enough current to directly drive the Nixie tube. It provides the first seconds digit for a 0 through 9 readout. When the 9 is displayed on the first Nixie tube, V6, codes D and A (see logic table) go high as the counter resets to zero and D goes low. IC 9, the divide by six counter, is pulsed. The BCD output of this counter is decoded and displayed on V5, the second significant digit in the clock. This cycle repeats with IC 9 counting once for each 10 counts of IC 11.
When IC 9 reaches 5, logic codes C and A are high. C (pin 8) is connected to the input pin (14) of IC 7 which is the first digit of the "minutes" section of the clock. IC 7 and its associated decoder driver, IC 6, are wired in the same configuration as the 0-9 indicator in the seconds second (IC 11, IC 10 and V6). Operating IC 7 and IC 6 cause 0-9 minutes to be displayed on V4. When 9 comes up, logic D and A are high and cause an input to be felt at pin 14 of IC 5, the 0-5 portion of the minutes indicator. (Remember, the count occurs as the pulse goes toward the negative in these counters.) The minutes 0-5 is wired exactly the same as the seconds 0-5 section and operates in a similar manner. When 5 is displayed on V3 and logic C and A is high, the C is felt on pin 14 of the divide by 10 counter, IC 3. This counter counts once each time IC 5 resets and is wired similarly to IC 7 and IC 11 with the one important difference described in the next paragraph.
Reset circuit
The problem in the hours circuit is that V2 must count from 1 o'clock to zero as V1 indicates 1 for 10 o'clock. V2 must then count 1 and 2, then reset to 1 o'clock. This is done by wiring decoder driver IC 2 so it thinks it is displaying a 0 on V2 when it is actually displaying a 1. When it thinks it is displaying a 1, it actually showing a 2 and so on until it displays a 0 which the decoder driver thinks is a 9. After the zero is displayed the counter resets to zero, logic D and A are high causing flip flop IC 1 to change state and turn off Q1 and turn on Q2 which extinguishes the 0 displayed on V1 and lights the 1. (See Fig. 2).
As V2 counts to a 3 (would appear as 13 on hours indicators V1 and V2) logic B goes high which causes pin 2 of IC 2 to go high. Simultaneously current flow through D2 is routed from B to E of Q3 causing pin 2 of IC 1 to go low and reset the flip flop. The result is that 0 lights on V1, pins 2 and 3 in IC 3 are high and IC 3 resets to 1. The count cycle 1-0 begins again for the second half of the 24-hour day.
Construction tips
Assembling this project can be extremely complicated because of the number of ICs and associated components. To simplify procedures a bit I strongly recommend using IC sockets. There are two reasons. First and foremost, if you apply too much heat to an IC lead, chances are you'll zap the IC and when you consider you will be soldering upwards of 180 IC leads your odds are short on a perfect job. Second and of paramount importance from an operational standpoint, is the fact that IC characteristics will vary somewhat and a quick switch of ICs may solve a problem that otherwise might have to be corrected by a value change in the associated circuitry.
Tables are provided for both component to component and point-to-point Vector board wiring. Either method is effective. However, if you are not an experienced builder with many "projects" under your belt I'd suggest using the Vector board chart as it is the simplest and most straightforward.
Follow the steps in the order listed and you will be home free after a few evenings of enjoyable labor.
