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Thread: Digital Input - BAT54S

  1. #1

    Digital Input - BAT54S

    One project I'm going to use BAT54S to protect the digital inputs.

    Question on the pull-up voltage. Any reason to use 3.3V, 5V or 12V?

    I have 3.3V currently on the pull-up, and if I put 5V (or 12V) on the input, the output to Teensy is 3.587V (I'm assuming the chip can handle 3.3V +/- 0.3V)
    The examples I have come across are for Arduino that handles 5volts, and they have 12v for pull-up.
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  2. #2
    Junior Member
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    @MolsonB

    Hmm could you specify which Teensy you use?
    Some can only handle 3.3V, others are 5V compatible.
    About the 3.3V =/-0.3V you are going 9% over the maximum voltage. I would suggest not going higher than 5%

    Regards, Otto

  3. #3
    Senior Member PaulStoffregen's Avatar
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    I suppose you're only interested in receiving a signal with this circuit, and only a rather slow one?

    The 10K series resistor means your best LOW output would be half the power supply.

    As an input, the 10K and 0.1uF means you're probably not going to be trying to receive anything more than about 1 kHz bandwidth. Probably useful for mechanical switches, maybe not so good for receiving serial data or anything else that changes rapidly.

  4. #4
    Teensy 3.2 and 3.6

    Yes it's just for on/off switches. I do believe I have a few on/off sensors that float at 5v. But I wanted to make sure down the road that it can handle 12v floating if the sensors change.

    Ah that may of answered my own question. If I have a pull up resistor attached to 3.3V, and the sensor floats at 12v, then the strong pull-up would make that sensor float at 3.3v which could mess it up internally.

    Okay so follow question. The zener clamping diodes, causes the max volts on the digital inputs to be Vcc +0.3V.

    Is that okay for teensy, getting 3.584V? Isn't that how it's done inside the chip for a weak protection, clamping zener diodes?

  5. #5
    Bump, wondering if this is acceptable protection circuit? Reading the data sheet, VCC + 0.3V seems okay.

    Quote Originally Posted by MolsonB View Post
    Okay so follow question. The zener clamping diodes, causes the max volts on the digital inputs to be Vcc +0.3V.

    Is that okay for teensy, getting 3.584V? Isn't that how it's done inside the chip for a weak protection, clamping zener diodes?

  6. #6
    Senior Member
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    can you not simply stay between (0 a 3.3V)?

  7. #7
    How do you do that with zener diodes? If the input volts are over 3.3V, it will clip to the power rail. Schottky diodes forward bias at about 0.2 V

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