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The 74ALS device that you found was unable to drive the output LEDs is operating more than likely correctly - the output high drive current is only specified at uA in the data sheet. LS TTL logic high output is extremely weak and can only source very weak current out.
In real world, the LED would be quite dim. However, what you have is not a real world scenario, that is just how your emulator works. Rds on is dropping the rest of the voltage internally. Nevertheless, you should add an appropriate limiting resistor - about ohms or so - to limit the LED current to about 4mA. Given that the 'ALS and 'HC have active-low outputs, that's a change you should do for both circuits anyway.
Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. The circuit with 74HC is working, but with 74LS it is not, why? Ask Question. Asked 1 month ago. Active 1 month ago. Viewed times. SamGibson What we can tell you is: there's a reason one has "HC" in the name, the other "LS". THat defines voltage levels and driving capabilities.
Please look up the datasheets of your two ICs yourself and determine if they're not simply working as expected. Active Oldest Votes. Compare this with the 74HC - it can drive a high output current of 4 mA. Andy aka Andy aka k 15 15 gold badges silver badges bronze badges. Justme Justme The question is, why does it work with the CMOS version? Sign up or log in Sign up using Google.
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