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Without getting into too much detail: For my project I've been provided exclusively 74LS chips, but I need either a BCD to decimal decoder, or a decimal counter like the CD4017 for control signals. The 4017 is very convenient because it already has decoded outputs which can be used as signals for other input pins on a circuit. Is there a similar chip that can fan out to at least 2 to 3 74LS input pins? I don't want to run into current issues mixing 4000 and 74LS.

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  • \$\begingroup\$ The johnson counter + driver (4017) was originally design as a pre driver for incandescent lamp displays. If you want a LS equivalent, but only for logic level, an 8 bit shift register is used. Like a 74ls595 \$\endgroup\$ – David Mikeska Apr 23 at 14:35
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Maybe I'm not understanding your question, but can it be an actual 74LS-series TTL device (and not just a "74LS-friendly"🤗 CMOS)? If so, then 74LS145 is a BCD decoder.

Source: Wikipedia's List of 7400-series integrated circuits.

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  • \$\begingroup\$ Yes in fact that would be convenient to keep all the chips in the same family. \$\endgroup\$ – BobaJFET Apr 22 at 22:33
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This is a request for a recommendation, which is generally off-topic here, but I'll try to provide a generic answer for replacing CD4xxx chips with LS or HC compatible ones.

Replace CD40xxx with 74HC40xxx. If you actually require LSTTL compatible input levels, then add a "T". So we would get 74HCT4017.

Then we can look to see if such a chip exists, and voilĂ , yes it does.

If it didn't and a 74HC40xxx exists, you could consider adding pullups to the LSTTL outputs.

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    \$\begingroup\$ The decoded outputs just need to be able to fan out to at least 2 or 3 inputs on the 74 LS IC's \$\endgroup\$ – BobaJFET Apr 22 at 22:23
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    \$\begingroup\$ @BobaJFET If you read the datasheet (a highly recommended task) you can see it's guaranteed LSTTL 400mV output level whilst sinking 4mA, which is 10 400uA LSTTL loads or 2 old-school 1.6mA TTL loads. \$\endgroup\$ – Spehro Pefhany Apr 22 at 22:25
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There is nothing in the 74LS world which is a fairly direct equivalent of the 4017. The closest would be the 74LS390. This will provide 2 decades of decimal counting, but you'll need another chip to get the last 2 bits provided by a 4017.

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If you are looking for something very close to the CD4017 in the LS family, I'd probably consider using one section of the 74LS90 -- dual decade counters followed by the 74LS42 -- BCD decoder.

The result will be an inverted sense, so if you need it you can include a couple of inverter packages. But that's up to you.

There are other options within the 7400 series LS family that may also work. The above aren't the only similar devices (74LS390 and 74LS445, for example.) But this gets the point across.

It will probably require at least two chips from the LS family, I think.

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  • \$\begingroup\$ Now you have me searching the datasheets to see how 74LS145 and 74LS42 differ. They have the same pinout... On 'LS42 all outputs are high for invalid input; on 'LS145 all outputs are low for invalid input. \$\endgroup\$ – Theodore Apr 23 at 12:45
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    \$\begingroup\$ @Theodore Later numbered chips were developed later. Earlier chips (numbered with smaller numbers, as a general but not perfect rule) tended more towards "open collector" outputs, a complete lack of tri-state capability, and less overall complexity. I provided an answer here that may be interesting in terms of an historical perspective. \$\endgroup\$ – jonk Apr 23 at 16:44

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