Why not just use a driver chip like a 74hc244 or even a ULN2004 (in conjuction with the transistors)? Wouldn't this be simplier? Or do you have real estate concerns? Or is it just because you want to design it this way?

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This will never work. It is because of the intrinsic diodes to Vss and Vdd on the IO pins of CMOS chips. Even when the pin is in Hi-Z or input mode, the pin voltage cannot rise above Vdd + 0.7V or 4.0V in this case nor can it go more than 0.7v below Vss.
The 2 transistor method (Fig 3), modified with the extra base resistor, will work. Just invert your software/logic level to the pin in the chip if needed.
I wouldn't touch the zener idea with a 10ft pole. Think what will happen if the 12V line rises or spikes to 16V or higher for some reason.

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If in Hi-Z, the I/O pin allows enough current to forward bias the E-B junction of the transistor, then I'd say you're right, but I doubt that it does.

As a precaution, I guess one could put a diode on the output of the I/O pin to prevent that from happening.

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Please tell us how this diode should be fitted, pointing to the I/O pin or pointing to the PNP base?

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That could prove problematic, since the circuit will eventually go into an automotive environment. I don't want to regulate the 12v input, just take it directly from the car's power circuit (fused, of course).

I agree with others, that the two transistor solution is a bit parts-intensive, but it seems like the only way.

I tried NPN transistor between the load and the positive supply. That didn't work because the base current had to pass through the load resistance, when the circuit is "on". That caused the transistor to turn on only partially, unless I had a very low load resistance.

It seems to me, I need both transistors for a truly robust circuit, the NPN for voltage gain, and the PNP for current gain.

 

...Vehicle electrical systems, when the engine is running, have a "load dump transient" of several hundred negative volts.
...Bosch [among others] may have specs on the expected envelope for this "dirty" power.
...You'll need power conditioning at a system level for your circuitry; diodes, chokes, capacitors, maybe an IC regulator.

 

Allowing the transistor to turn on in high-Z state is not the issue. You don't want to exceed the max input voltage to the IC, which in this case would be 12V with a pull-up resistor.

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My poorly worded comment, I meant a breakdown (zener) diode from the input pin to ground to clamp the input voltage. This may be a circular argument, because if we believe the premise of the purpose of this type of clamp, then it would mean the transistor would never turn off. I would like to experiment some with this just to see what happens.

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Yes, I agree.

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"Everything that is done in the world is done by hope." -Martin Luther
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There are a couple of other two-transistor configurations that will work.

Attached Images

level shift.PNG (8.9 KB, 22 views)

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Ron

 

Look at the High Side power switchers from ST which have been designed for the car environment.

Makes life so much easier for switching larger currents.

If its only a small current you need to switch then go with the above suggestions

 

No, if the output is low, he is correct in saying that it will never turn off. The base is never more positive than the collector. What if you flipped the transistor in figure 2 upside down so that the emitter is connected to ground. then the base-emitter drop is dependant on the uC, not the source.

Try that.

 

oops, I think that'd make it inverting.

Just follow this example from wikipedia if you want the transistor on the high side of the load.

http://en.wikipedia.org/wiki/Common_collector

Why switch to pnp?

 

But the NPN emitter will always be about 0.7V lower than the base. With a 3.3V supply on the UC, the emitter will never go above about 2.6V, regardless of the voltage on the collector.

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They are MosFet Push-Pull outputs not bipolar transistors. When in Hi-Z mode the P and N channels MosFets are turned off but still connected between the Output, Vdd and Ground. MosFets have an intrinsic diode between the Drain and Source terminals that will conduct if the pin is driven above Vdd or below Vss by more than 0.7V.

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