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Thread: Open-collector question

  1. #1
    Senior Member Constantin's Avatar
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    Open-collector question

    Hi guys,

    Just wondering about the following. In the past, we've discussed how the DS18B20 is a open collector device, allowing you to power the thing at 5V and communicate at 3.3V (by using a pullup resistor on the data line to 3.3v). Based on the description of the DHT22 (see p.5 here: http://www.aosong.com/asp_bin/Products/en/AM2303.pdf), it appears that the AM2303 is no different. Thus, would it be possible to power the AM2303 at 5VDC but communicate at 3.3V by pulling up to 3.3V instead of 5V?

  2. #2
    Senior Member+ KurtE's Avatar
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    From your pdf file it looks like you could power it with 3.3v So I would assume your PU could be 3.3v

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    Senior Member Constantin's Avatar
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    Quote Originally Posted by KurtE View Post
    From your pdf file it looks like you could power it with 3.3v So I would assume your PU could be 3.3v
    Hi KurtE! Thanks for the reply. I'll try it out with a 3.2 and see what happens. Cheers!

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    I missed the discussion.why not power ds18b20 from 3.3v?

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    Senior Member Constantin's Avatar
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    Quote Originally Posted by Gibbedy View Post
    I missed the discussion.why not power ds18b20 from 3.3v?
    It may make sense based on your PCB layout and / or wire length. If the pull up is on the board then the voltage drop on the wire doesn't matter as much?

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    Sounds good.
    I connected ds18b20 at 3.3v (3 wire) over 10m of cat5 cable and it worked but was considering your method. Good to know it works.

  7. #7
    Senior Member Constantin's Avatar
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    OK, tested it tonight - the version of a DHT22 I used appears to be an open-collector device - it could be powered at 4.7V (or whatever the VIN is on a Teensy 3.0) while the pullup resistor for the data line was attached to the 3.3V regulated output from the Teensy 3.0. I used pin 13 and confirmed operation for the device at VCC=VIN as well as VCC = 3.3V

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