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Is this correct to connect 5v outputs and 3.3v inputs ttl logic via an open drain transistor and an internal pull-up resistor on the digital input? I suspect the 5v part can influence to the 3v part through base -> collector -> pullup -> 3v power line.

(the transistor comes as a part of existing device, so doesn't cost anything for my solution).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ What sort of signal do you need this for? How fast? How long is the interconnect and is the environment electrically noisy? What exactly (part number etc) is the receiving IC and which pin or function of it? Can you tolerate the inversion present in this proposal? It's probably not the best way, but if the parts are already present there are many circumstances in which it will work fine, but without the circumstances being documented in your question no one can be more specific. \$\endgroup\$ – Chris Stratton Sep 14 '19 at 17:23
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If speed isn’t a big issue then using a BJT common emitter is fine. Be aware that this will cause a signal inversion and that may be enough of an encouragement to use a 5 volt to 3 volt resistor potential divider which, of course is simpler technically and won’t invert the signal. Speed of response can be a lot better with a resistor potential divider too.

You might also be able to get away with just a single series resistor if you don’t exceed the input current levels of the receiving chip.

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Translating down 5V to 3.3V can be done in several different ways:

  • Voltage divider
  • Diode
  • Series MOSFET
  • open-collector / open drain buffer (as you’ve shown)
  • 5V tolerant buffer

What you show can work, with a pull-up to 3.3V on the collector (it also inverts.) A FET like a 2n7001 or BSS138 would eliminate the need for the base resistor.

The series MOSFET approach yields the best performance if speed is an issue.

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  • \$\begingroup\$ Note the question specifies the receiver already has an internal pull-up, of course those are generally fairly weak which would limited speed. \$\endgroup\$ – Chris Stratton Sep 14 '19 at 17:22

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