Can a 555 Timer Serve as a Level Translator? Let's Find Out!

I couldn't resist - I tried an experiment with a couple of the 555s I've got here. This is with a 50mR thick-film resistor between the part GND pin and the board ground. The yellow trace is the voltage across the resistor, the blue trace the output of the timer. I've done this on a piece of pcb material, so NOT a plug-in breadboard, but the part is in a socket and I'm probing with a 10M probe (might have been better with a piece of 50R co-ax).

This is a Fairchild bipolar part which might be from 1986. It's brutal. As the output rises, it gets to 30A for a few nanoseconds before limiting and falling. (The local decoupling is two 10uF tants and a 1uF ceramic - possibly not a very good combination.) It's a poor test, though, because the ground has lifted to 1.5V. I haven't shown the other end of the waveform, but, curiously, there's no shoot-through when the output drops.

This next is the TLC555 part. Here the shoot-through is worse at the end when the output comes down again, though it's nothing like as bad as the bipolar part. Something that's interesting is that there's a very short period before the output goes up when the chip is taking a little under an amp (perhaps that's a more limited shoot-through when the driver stage for the output MFETs starts transitioning) - by starting a fair current moving, it looks like it improves the situation when the output actually starts to move.

I think all that I'm showing here is that the shoot-through current will depend very much on what the decoupling is and how it's arranged. It's all very messy to interpret - the decoupling caps will be interacting, there's enough inductance to play a part, and my probing isn't going to truly reflect what's there. I think I've ended up with more questions than answers, but it's looking fairly certain that there IS a shoot-through current there.

This is with just the sense resistor and the other channel off, with its probe disconnected and the input set to the GND setting. It's more or less the same.

I'd doubt that the shoot-through current itself is actually bipolar. I suspect if the sense was better, with a much lower voltage drop, so that it was affecting the chip much less, and far less inductance hanging around, and the probing was better, it would disappear (but I can't really explain it, as it stands, other than that it feels to me like I'm looking at the lead inductances doing their thing).

Possibly, the breadboard may even help a little. With some impedance in the supply wires and a decoupling cap directly across the pins, it maybe helps isolate the supply system momentarily as the cap is doing the heavy work.

This is with exactly the same test set-up, but with a KIA555P part in the socket (looking it up, it seems to be a Korea Electronics Corporation part). About 20ns of shoot-through for this part.

So, it looks like I was wrong and the sense resistor is doing fine.