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.
More on the 7414