Quote:
Originally Posted by julie_m
My concern with the circuitry around T1/T2 is more with T1 possibly not turning off fully when the Raspberry Pi is sending a logic 1; although I decided to trust the original design, at least for the first version I tried in BCP. I did take the slight liberty of adding a Zener diode across R2.
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Yes, I couldn't see how this would ensure T1 would be turned-off / there wasn't > 3.3V applied to the RPi (even at low-currents, which SiriusHardware had found still lifted the RPi's GPIO's)
If they'd used a P-Ch FET, with Vgs threshold > 5 - 3.3 = 1.7V, then that might have worked better and wouldn't get current flow from +5V rail. Vgs thresholds are rather variable, but drain current would be quite low at threshold and T2 would keep output at gnd. As long as Rds(on) is quite low at 5Vgs, then that would have worked OK.
If the design is built as shown, then might be interesting to measure what voltage levels you get in practice.