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I'm sorry if my question is too specific to the 74HC logic family, but as the answer may vary by family, I place myself on the safe side.

The 74HC family runs from 2 to 6 volts or 4.5 to 5.5 volts as nominal voltage according to Wikipedia.

I will probably be using 4.5 - 5.5 volts chips, as those are the easiest to find.

The problem is, while my project will possibly be powered from a 5 volts power supply, I would like it to also run on batteries.

For my needs, I think I should use 3 AA batteries in series, which output will output 4.5 nominal volts.

But, as the batteries discharge, there will be drops in voltage across the circuit. As 4.5 volts is already on the low edge of the 74HC logic family, my project will definitly go below that.

Is the use of voltages of 4.5 volts or lower still "ok" for the ICs, at the cost of increased propagation delay ? Will it affect the lifespan of the ICs ?

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    \$\begingroup\$ Don't look at Wikipedia. Look at the datasheet from the manufacturer whose chips you intend to use. They will state the minimum voltage there. \$\endgroup\$
    – Transistor
    Oct 14, 2017 at 20:53
  • \$\begingroup\$ This is something you check in the data sheet for each IC to be used. Also, you need to consider that three new alkaline AA's will total to over 5.1 volts, while three rechargeables could put you down around 3.3 volts at the end of useful service. Of course discharged batteries don't automatically turn off your circuit - your circuit has to do that or it may misoperate. And overdischarged rechargeables may not then be chargeable. \$\endgroup\$ Oct 14, 2017 at 20:54
  • \$\begingroup\$ Will it affect the lifespan of the ICs ? In the datasheet there will be a table showing maximum ratings. If you stay within those ratings then lifetime will not be affected. Usually the supply voltage max rating -s -0.6 V to 6V meaning you can use 4.5 V or lower. Even 1 V will not break the chip, the chip might not work but that is not what maximum ratings are for. Then you need operating range. \$\endgroup\$ Oct 14, 2017 at 20:57
  • \$\begingroup\$ Could @Bimpelrekkie make it an answer so I can accept it ? \$\endgroup\$ Oct 14, 2017 at 21:00
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    \$\begingroup\$ It is the HCT (notice the 'T') chips that are designed for 4.5-5.5V. That is so they can interface directly with TTL logic. The HC chips are designed for 2-6v. \$\endgroup\$ Oct 14, 2017 at 21:14

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The information you want can be found in the datasheet.

I take Texas Instrument's SN74HC74 as an example.

The IC cannot be damaged (lifespan is not affected) if you do not violate the Absolute maximum ratings.

See table 6.1 There it states that the supply voltage range is -0.5 to 7 V. The - 0.5 V volt should give you a hint: this is the maximum rating, it says nothing about the chip working or not. Obviously at a supply voltage of -0.5 V the chip is not going to work. But it will not be damaged either.

So a 4.5 V or lower battery voltage will not damage the chip.

In table 6.3 we find the operating conditions. Now we learn that the supply voltage should be between 2 V and 6 V for proper operation.

Below 2 V there are no longer any guarantees. Your chip might work even at 1.5 V. But all should work at 2 V.

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74HCxx operates over a wider voltage range than standard TTL, which should make 4.5v work fine. Consult the manufacturers spec sheet for exact details. HC family is not technically TTL compatible although in practice they usually work fine within certain limits. For full TTL compatibility you need HCT.

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