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.
While in the process of working on a digital clock that I built in 2007, I had to take another look at how a 7490 decade counter works in order to remind myself. I rigged one up with some LEDs to count 0 to 9, showing the count in BCD form on the LEDs.
The oscillator used to drive the 7490 counter is one stage of a 7414 Schmitt Trigger inverter. Using a 47 microFarad capacitor and a 15K resistor, this gives a slow enough pulse to watch what is going on at a calculated period of 0.588 seconds. It is basically running the 7414 as a relaxation oscillator, charging and discharging the capacitor through the resistor repeatedly. Being an inverter the output of the 7414 will be high when the input is low and vice versa, so it is always either charging or discharging the capacitor. The waveform, even though I don’t have a scope to put on it, at the capacitor must be a sawtooth I imagine.
Video of 7490 Counting
The 7490 is both a divide by 2 and a divide by 5 counter. To get a divide by 10, the divide by 2 at Q0 is fed into input B. Outputs Q0,Q1,Q2 and Q4 are weighted by 1,2,4 and 8 respectively. I have the laid out in descending weight from left to right on the board. All set and reset lines are grounded. It will freely run from 0 to 9 and around and around. These lines could be fed into a 7447 7 segment display driver and that would drive digits like a real clock if desired.
The small LED on the left is connected to the output of the 7414 and blinks at the output frequency of it, the jumbo LEDs count from the output of the 7490 and the tiny LED to the right is just strapped from power to ground with a dropping resistor to show that the board is powered. There are also two capacitors 0.1 and a 0.01 microFarad strapped from power to ground to bypass an high frequency switching that appears on the power rails, always a good idea to bypass, this is a good idea on every chip if possible. This eliminates high frequency noise on the power supply rails. Also grounding unused inputs on the 7414 or any chip that you might use is a good idea to prevent noise and erratic behavior of the circuit.
Powered at 3.5V
The usual 5V power supply was tied up running the TTL Clock I built while testing it, so for this setup I grabbed a 3.5V supply. The TTL circuits seem to run fine at this voltage, the only thing is that the time constant is probably a bit shorter for the capacitor to charge and discharge.
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
The last post on this clock built out of 7490 TTL decade counter chips gave a bit of back story on it and the initial troubleshooting of it’s timebase drift and noise issues that caused extraneous counts to occur. Now that the noise is figured out and solved, it is time to do something better than a plain crystal with a trimmer 7400 ( NAND Gate ) oscillator for a timebase.
Driving the clock with an OCXO
The goal was to allow the clock to be driven by an oven controlled crystal oscillator (OCXO), which would surpass the performance of the oscillator on the board. The on board oscillator can be easily pulled off it’s frequency and slaved to an external source. By using a 7414 Schmitt Trigger Hex Inverter, it is possible to take the low level sine wave output from the OCXO and convert it into a digital signal capable of entraining the on-board oscillator. By using an external master, I can pull the master OCXO off if I need it to calibrate anything else, meanwhile the clock keeps running off of it’s board oscillator. Most of the time the OCXO is not being used and can just remain with the clock.
The hookup is pretty simple, I breadboarded it and found out the biasing one of the inputs of the 7414 mid range using a pair of 33K Ohm resistors in series from +5V power to ground with the input to an inverter stage on the 7414 in the middle at 2.5V, feeding in the OCXO signal, capacitively coupled and then feeding the output of that stage into another inverter on the 7414 for a buffer, I was able to get a clean square wave at 10MHz. This I coupled to the board oscillator using a 370 Ohm resistor. The resistor is more or less a protection in case I touched it to something that I shouldn’t and I don’t want too much current to be able to flow in either direction. I also temporarily drove an LED via a 370 Ohm resistor off of the same output as a check that I was actually getting output, the LED is lit at half the brightness when it is excited by a square wave.
Initial testing with the breadboard shows that the clock now tracks time very well as compared to another clock I have that is synced to the WWV 60KHz signal.