Below are the schematics for the 7490 TTL Chip Clock. After 10 years of not doing anything with the clock, I had to spend some time reverse engineering it a bit to understand it’s design. The original hand drawn schematics ( not published on this site ) were a bit off target as I deviated from them. These new ones that are published are accurate. They were hand drawn then scanned and imported into Gimp for cleanup.
Clicking on the images on this page will bring up a full sized version.
Gimp Drawing Cleanup
In Gimp basically the process is to use threshold to clean the “noise” out from the scan. Then cut more noise out using the cut tool and eraser. Then go over the lines with the pencil tool where they are faint. Finally, add text in. Yes, there are some CAD tools that could do this easier but, since I don’t do this kind of thing to often, Gimp works out nicely as I have had a lot of hours on it and can work quickly in it.
Slave Oscillator and Prescaler, dividing from 10MHz to 1kHz on the right hand board
This schematic shows all connections, including power and ground and is laid out to represent the component placement on the board. Looking at the 7490’s the pattern of the circuit is just like the example from How it Works, dividing frequency by 10 every time. The difference between the clock that I built and theirs is that I used a 10MHz source and they used 60Hz line frequency. Therefore I needed to use more divider stages. There is a shortcut, the 74390 chip. It is a BCD dual divider/counter. It could eliminate half of the stages as it divides by 100. I happened to have quite a few 7490s at the time that I built this clock and went with that approach rather than order 74390s. I would recommend using the 74390s though, less stages, less soldering, less power consumption being the benefits.
1kHz Divided down to Hours, Minutes and Seconds on the left hand side board of the clock
This schematic DOES NOT show all connections, power and ground are omitted, also no connects on the chips are omitted, plus the 7447 chips are there to show there location but the connections are not shown. Refer to the page that covers the How it Works information on how to connect 7490s to 7447s. It is laid out to represent the component placement on the board. However the board is inverted on the clock and the digit LEDs sit on the bottom of the board but inverted in order to read them.
I finally found the document that I based the 7490 clock that I built off of. Nothing like the process of moving to uncover lost items. The clock that I built had one main variation in that it did not use the 60Hz from the wall as a frequency reference. Instead it used a series of 7490 chips to divide down from 10 MHz to 1Hz to drive the clock portion. I did this with the foresight that at some point I was going to use a good crystal oscillator such as a TCXO, temperature compensated crystal oscillator or an OCXO, oven controlled crystal oscillator. These are readily available in 10MHz versions. Another plus is that WWV out of Fort Collins Colorado transmits on 10MHz using a signal derived from a cesium oscillator so the oscillator can be checked and calibrated ( mostly ) against their signal easily by using a shortwave receiver.
I was able to scan the original document and OCR it back to an electronic copy and published the text with the diagrams on this post. The original scan is at the bottom of this post. I downloaded it in 2007 from the How Stuff Works site. Unfortunately the well written article has disappeared from the Internet entirely or I would have just provided a link to it on this site. It has been put it up here for reference for anyone curious or wanted to build a clock out of 7490’s that use a 60Hz input. If this article reappears on-line, I would just link to it. If someone sees it somewhere, please let me know.
Here‘s a circuit diagram tor the power supply and time base.
As we saw in the article on electronic gates,the power supply is the most difficult part.
To create the rest of the clock you will need
At least four 7490 or 74LS90 chips
At least two 7447 or 74LSA7 binary to 7—segment converters
At least 20 resistors for the LEDs in the 7—segment displays ( 300 ohms would be
Some normal LEDs
At least two common-anode (CA) 7—segment LED displays (Jameco; part #172088 is
Breadboard, wire. etc.
The 7490 is a decade counter. Meaning it is able to count from 0 to 9 cyclically, and
that is its natural mode. That is QA, QB, QC and QD are 4 bits in a binary number, and
these pins cycle through 0 to 9, like this
You can also set the chip up to count up to other maximum numbers and then return to
zero. You “set it up” by changing the wiring of the R01,R02 R91 and R92 iines. If both
R01 AND R02 are 1 ( 5 volts ) an either R91 OR R92 are 0 ( ground ) then the chip will
reset QA, QB, QC and QD to 0. If both R91 and R92 are 1 ( 5 volts ), then the count on
QA, QB, QC and QD goes to 1001 (9). So
To create a divide-by-10 counter, you first connect pin 5 to to +5volts and pin 10
to ground to power the chip. Then you connect pin 12 to pin 1 and ground pins 2,3,6 and
7. Run the input clock signal ( from the timebase or a previous counter ) in on pin 14. The output appears on QA, QB, QC and QD. Use the output on pin 11 to connect to the next stage.
To create a divide-by-6 counter, you first connect pin 5 to to +5volts and pin 10
to ground to power the chip. Then you connect pin 12 to pin 1 and ground pins 6 and
7. Connect pin 2 to pin 9 and pin 3 to pin 8. Run the input clock signal ( from the
timebase or a previous counter ) in on pin 14. The output appears on QA, QB and QC. Use the output on pin 8 to connect to the next stage.
Creating the Second Hand
Knowing all of this, you can easily create the “second hand” of a digital clock. It looks like
In this diagram, the top two 7490s divide the 60-Hz signal from the power supply down by a factor of 60. The third 7490 takes a 1-Hertz signal as input and divides it by 10. Its four outputs drive normal LEDs in this diagram. The fourth 7490 divides the output of the third by 6, and its three outputs drive normal LEDS as well. What you have at this point is a“second hand“ for your clock, with the output of the second hand appearing in binary. If you would like to create a clock that displays the time in binary, then you are set! Here isa view of a breadboard containing a divide-by-10 counter. a divide-by-6 counter and a set of LEDs to display the output of the counters in binary.
Displaying the Time as Numerals
If you want to display the time as numerals, you need to use the 7447. Here is the pinout
of a 7447, as well as the segment labeling for a 7—segment LED.
You connect a 7447 to a 7490 like this
Provide +5 volts on pin 16 and ground on pin 8 to power the 7447 chip
Connect QA, QB, QC and QD from a 7490 to pins 7, 1, 2 and 6 of the 7447,
Connect 330-ohm resistors to pins 13, 12, 11, 10, 9, 15 and 14 o0 the 7447,and
connect these resistors to the a, b, c, d, e, f and g segments oo the 7-segment
Connect the common anode of the 7-segment LED to +5 volts
You will need to have the pinout for the specific LED display that you use so that you
know how to wire the outputs of the 7447 to the LEDs in the 7~segment device. ( Also,
note that the 7448 is equivalent to the 7447 except that it drives common-cathode
displays. Ground the common cathode of the LED in that case.)
V0u can see that by extending the circuit, we can easily create a complete clock. To
create the “minute hand’ section of the clock, all that you need to do is duplicate the
“second hand” portion. To create the “hour hand“ portion, you are going to want to be
creative. Probably the easiest solution is to create a clock that displays military time
Then you Will want to use an AND gate (or the R inputs or the 7490) to recognize the
binary number 24 and use the recognizer to reset the hour counters to zero.
NOTE: You can dispense with the and gate and simply wire the “2” line QB of the
hours-tens counting 7490 and the “4” line ( QC ) of the hours-ones counting 7490 and
connect BOTH to the same reset line (R1 or R2 )respectively on each of the hours 7490s.
In this manner when a 24 count occurs a reset is applied to both R1 and R2 on both hours
chips simultaneously, resulting in a reset to zero for both.
The final piece you need to create is a setting mechanism. On a breadboard, is is easy
to set the clock – just move the input wires to drive higher frequency signals into the
minute-hand section of the clock. In a real clock, you would use pushbuttons or switches
and gates to do the same thing.
If you happen to take your bedside clock or watch apart, one thing you will notice is that
there are probably not 15 TTL ICs inside. In fact, you may not be able to find a chip at all
in most modern clock or watches, all of the functions of the clock (including the alarm
and any other features) are all integrated into one low-power chip (in a watch, the chip
and display together consume only about a millionth of a watt.) That chip is probably
embedded directly into the circuit board. You might be able to see a blob of black plastic
protecting this chip. That one tiny chip contains all of the components we have discussed
About 10 years ago I built a clock built out of 7490 TTL decade counter chips. It was based off of an article that I found on How Stuff Works, that I could no longer find, but I have it available here. I also have posted the schematics for the version of the clock that I built that uses a 10MHz timebase and not the 60Hz one that the How Stuff Works version of the clock uses. Schematics for the 7490 TTL Chip Clock
The clock was in my shop for a number of years, then it got packed away when I moved in 2013. Meanwhile, In 2016 built a well made 24 hour clock from a kit from MTM Scientific, Inc that has a TCXO and drifts only a few seconds between the times of the year that it needs adjustment for daylight savings time. It has a nice bright readout and works well in my bedroom. But, it also got me thinking about the TTL clock again and I thought that one might be worth revisiting.
One issue with the 24 hour clock that I built from TTL chips is that it occasionally required adjustment of a trimmer capacitor to keep the oscillator running in time. I also noticed that occasionally it would flake out completely and get extra counts, causing it to run fast by about 1.5%. Sometimes just restarting it would fix the issue, it was a mysterious. It first occurred when I moved it to a different spot on the work bench after initially getting it put together and pinned to a wood board. My first thoughts were that I had a flaky chip. At one point I had accidentally connected the clock to a 12V power supply and blew out one chip and I speculated that there might be others that were working but damaged. Or it could have been a bad solder joint, with all the points of connection that was certainly plausible.
The Real Issue
One thing that I didn’t have access to in 2007 that I have in hand now is a frequency counter. When I built the clock my frequency counter was missing. I had it packed away somewhere in 2003 and only found it years later (2013) as it was packed in a box that made no sense at all. But, it would have been handy at the time to track down where I was getting these extra counts. In early 2017 with the frequency counter in hand, I was able to quickly determine that it was not only extra counts, but noisy extra counts as the lower digits on the counter were fluctuating. Something was ringing or going into an oscillation, that was my initial thoughts. I relatively quickly suspected a buffer chip, 7400 inverter, that I had added to provide some reference outputs at 1MHz,100KHz, 10KHz and 1KHz. The chip was the only one on the oscillator and divider board that I had tacked on after the clock was working, so it seemed likely that it might be part of the issue. Plus, it was the only one that I had neglected to put a bypass capacitor across from power to ground. Removing power from the chip solved the problem, good counts, a perfect multiple of the 10MHz clock were now coming out of the board to drive the dividers on the second board with the digits. The chip might not be worth using, if I need lower frequency references I can always take another oscillator I have and use that.
Calibrating the 10MHz crystal oscillator by tweaking the trimmer, against a 10MHz OCXO ( Oven Controlled Crystal Oscillator ) and then watching it keep time for a few days, it now holds reasonable time.
The Next Plan
The next plan for this clock is to drive it with the 10MHz OCXO to overcome the limits of the simple 7400 Inverter TTL chip 10MHz crystal oscillator with a trimmer capacitor. This simple oscillator is limited, temperature will make it swing along with any proximity of metal or whatnot that changes the frequency. If you touch the crystal or any part of the circuit around it, I imagine the frequency is pulled off target.
By feeding it with the closed box OCXO, powered through a decent regulator, frequency variations due to temperature changes, voltage changes and capacitive changes due to proximity of conductors will be minimized. It will be interesting to see how stable the clock can be.
Also, it would be nice for this thing to finally wind up in a decent box and be powered by something other than a 12V to 5V regulator tacked to a spare breadboard.
I plan on writing more on this as the project unfolds, along with the schematics, when I find them, all hand written and might be lost when I moved, but I might just have to recreate them!
It starts with a 10 MHz 7400 inverter oscillator and divides down using 7490 decade counters set up to divide by 6 or 10 as needed. Some AND/OR logic appears in the design as well to provide a pseudo WWV time code, 1kHz second ticks, minute and hour marker. This output is provided as an amplified audio output. This is done using a small 1 stage transistor amplifier driving the 2 inch speaker, with series resistor to limit volume. A 1/8in jack is provided as well for driving a larger speaker. The marker is also able to modulate a 1MHz output for a test signal. Three modes of output are provided, 1MHz carrier, 1MHz modulated with steady 1kHz signal and 1MHz modulated with pseudo WWV time signal. This signal and 100kHz,10kHz,1kHz are provided as buffered outputs for off the board use.
The display itself is an array of dual 7 segment common anode 0.75 inch elements, with appropriate 7-segment drivers. Setting is via 3 pushbuttons. Two provide speedups of seconds 1000X and 10X to roll the clock ahead faster than real time, a third button is a halt button for syncing with another clock source. A 10Hz ‘heartbeat’ LED is provided for debugging purposes. This is connected at the junction between both boards.
One board is the oscillator and divider to 1000Hz and the test outputs (1MHz,100kHz, 10kHz and 1kHz. Pseudo WWV 1MHz and audio) and the other board divides down further and has the display and the drivers for the 7-segment LEDS.(3/2007)