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I have a circuit with 8 sensors connected to a shift register (CD4021BE), The shift register is powered by 3.3 V and the sensors outputs 5 V on the signal wire when triggered.

Is it allowed to connect the 5 V signal wire directly to the inputs of the CD4021BE or do I need something like a level shifter?

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  • \$\begingroup\$ What does the datasheet for your CD4021BE tell you? \$\endgroup\$
    – brhans
    Commented Feb 23, 2023 at 22:22
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    \$\begingroup\$ this can directly be answered by opening the datasheet, and looking at the table that says "Maximum Ratings". You will find that the input voltage must not exceed the supply voltage by more than 0.5V (typically, different manufacturers' chips differ ever so slightly here) \$\endgroup\$
    – Marcus Müller
    Commented Feb 23, 2023 at 22:23

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As the comments have pointed out, you can find anything relating to the electrical characteristics and limitations directly from the datasheet. In this case, you need to find the maximum voltage rating on the input pins of the CD4021BE.

Typically, absolute max ratings have their own section or table on datasheets, and they're usually found within the first few pages. The snipping below shows what you need:

Maximum Ratings page 3-39 from Texas Instruments CD4021B datasheet

(Image source: Texas Instruments - CD4021B datasheet)

So, if your VDD is +3.3 V, then the max you can supply to the inputs is +3.8 V.

You'll notice that the range for VDD is up to +20 V, so if you have the +5 V available, you can simply use the +5 V as VDD. If that's not possible, then there are plenty of other solutions that are easily available to find online since it's a very common problem.

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