Tag Archives: Schmitt Trigger

7490 TTL Chip Digital Clock Schematics

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.

TTL 7490 BCD

7490 TTL decade counter chip in action

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.

Circuit Diagram

7414 and 7490 Diagrams: Click on to enlarge

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.

More on the 7414



24 Hour Digital Clock Gets a Better Timebase

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.

7414 Schmitt Trigger Hex Inverter on Breadboard

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.

OCXO to 7414 to TTL Clock
OCXO to 7414 to TTL Clock: Signal path is via yellow clip leads.

Schematics for the 7490 TTL Chip Clock

Previous post in this series

Next post in this series