Hello to everyone. I need some help and I don't find it anywhere.

I've using some 7490 IC for counting purposes, with a simple 555 circuit for clock pulses. Everything was fine.
Yesterday I had to replace one of the 7490 counter, so I used a 74LS90 instead, because I had no other. Surprise!! the counter didn't work!! and I changed no other thing.. I'm really confused..

Where the 7490 worked, this new 74LS90 doesn't... I tried everything in my breadboard but don't have any idea why this happends. The counter does not count correctly, and changes state not only in 1 to 0 clock transition, but also in 0 to 1. Counting seems random..

I tried 3 74LS90, and 3 7490; all 7490 worked, none of the 74LS90 did. I bought each one separately. Maybe the 74LS90's are defective??

I REALLY aprecite some help.
Greetings.

Of course, the 74LS90 chip is designed for maximum current levels approximately half of those used with the 7490, but the primary difference -- the one probably causing your troubles -- is the minimum pulse widths on the resets. The 7490 calls for a minimum 15ns pulse width, while the 74LS90 requires 30ns pulses for resetting. Here is a TI datasheet that covers both chips...

Thanks for answering.

ChrisP, I don't thnik it is a reset problem, as I ground both reset inputs and don't use them now, and the pulses coming from the 555 are about 1 Hz.

k7elp60, I didn't try to use a bypass capacitator.. the power supply is +5V stabilized. I'll try adding the 22uF capacitator between V+ and GND, but.. why use a 0.1 ceramic capacitator in parallel?? Wouldn't the 22uF be enough??

I'll try adding the 22uF capacitator between V+ and GND, but.. why use a 0.1 ceramic capacitator in parallel?? Wouldn't the 22uF be enough??

Electrolytic capacitors have a higher internal resistance than other types - this can cause 'slow' response when working at high frequencies.

The 0.1u ceramic has a lower internal resistance, and is better for 'catching' the high frequency transients.

do I have to use a bypass capacitor before every IC??
Or just at the input, where power supply??

I'm going to use a LM7805, so I though on using a capacitor between its common and input pins, and maybe, another one after it, between Output and common pins... I'm I doing well?? Is still necesary a bypass capacitor before the other IC??

Thanks

I don't know if I understud exactly the last thing you said. So should I use a small ceramic capacitor close to each TTL?? Is it that?
What happends if it is not close to them?

I'm gonna make the circuit on two universal board (soldering), so the 7805 is on one of them, along with 5 TTLs, and the other one containing another 8 TTLs. Initially I was going to use only an output capacitor along with the 1n4001 (where the 7805), and planned no other. I though that using a good filtering at the output, it was not necesary to put other capacitors along the rest of the circuit...