Single transistor as level shifter

[austnais]

Hi guys!
Yes, the question is in the title! Is this socially accepted for real circuits (safety-critical)?

I'm translating both from 5V to 24V and 5V to 15V with one discrete NPN-transistor.
Please, if you have something to say regarding this topic, speak! It can be anything from other ways to do it, improvements, draw-backs and so on,
Thank you,

[T3sl4co1l]

- 5V logic cannot be directly connected to the base. Use a current limiting resistor (and usually a B-E resistor as well, to keep it fast).
- The output won't pull up to full +V, due to the LED.  This might not be a problem (is there another pull-up on the connection?).
- The output is not short, overvoltage or reverse protected, so I don't think I would call it "safety critical".
- What kind of "safety critical" application do you have in mind?  I sincerely hope you aren't doing something like medical life support or mission-critical, and asking questions to the internet about it.  I would seriously suggest spending real money talking to consultants about "safety critical" systems.

Tim

[austnais]

- 5V logic cannot be directly connected to the base. Use a current limiting resistor (and usually a B-E resistor as well, to keep it fast).
- The output won't pull up to full +V, due to the LED.  This might not be a problem (is there another pull-up on the connection?).
- The output is not short, overvoltage or reverse protected, so I don't think I would call it "safety critical".
- What kind of "safety critical" application do you have in mind?  I sincerely hope you aren't doing something like medical life support or mission-critical, and asking questions to the internet about it.  I would seriously suggest spending real money talking to consultants about "safety critical" systems.

Tim

Thank you T3sl4co1l for your reply!
Sorry! I should have included more of the circuit...
-There is of course a base-resistor (but it is outside the snapshot)
-There is also a separate pull-up on the 24V side, so no worries.
-There is also a current-limit resistor on the output, which actually goes to the base of an PNP.

Safety-critical was wrongly put, or wrongly understood. What I mean is robust. For example, is it more robust than using a dedicated chip?
Don't worry, you wont see this in a pacemaker or any medical... 

[mstoer]

I am assuming you wanted it to be unidirectional?  I mostly do level shifting for I2C communications (bidirectional) and use logic level MOSFETs for it.   Nothing wrong with a unidirectional BJT level shifter if that's all you need though.

[Audioguru]

Please post the entire schematic with the resistors and PNP transistor so we can see if the "separate pullup" prevents the LED from lighting and if there is anything that turns off the PNP.

[austnais]

Please post the entire schematic with the resistors and PNP transistor so we can see if the "separate pullup" prevents the LED from lighting and if there is anything that turns off the PNP.

Thanks for reply, but there is no reason to post the rest. The pullup is connected in parallel with the led and the resistor. The whole circuit is tested and works as intended. All I wanted input on was is if this is the typical way to do it.

[T3sl4co1l]

Alright, then yes, that's the correct way to do it.

Tim