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I'd like to ask how this circuit works:

Circuit

I understand how it works in principle, but not technically. It's a USART voltage level shifter. It's supposed to divide 5V to 3.3V by a resistor divider, in the direction from microcontroller (on the left ATMEGA328), to RFmodule which works with 3.3V. But in opposite direction, a transistor has been used which I do not understand. Arrow above R8 resistor indicates 5V.

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Consider what happens when the TX on the 3V side goes high and low.

If its High, Vbe < 0V (emitter is at 3V and base is at the voltage of the 4k7 and 10k divider, ~2.2V), so no Base current, and the transistor is off. R8 pulls the other side to 5V.

When its Low, Vbe ~=0.7V, the transistor is on, and the TX is able to pull the other side low.

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  • \$\begingroup\$ Vbe is never 3.3V. The base is held at 2.245V when Uart_TX is high, and the base is at approximately 0.7V when UART_TX is low. T2 is simply a non-inverting buffer and level translator. \$\endgroup\$
    – Jack Creasey
    Jun 7, 2018 at 17:55
  • \$\begingroup\$ @JackCreasey -- thanks for pointing that out. I wrote that in a rush. I'll add it to the answer. \$\endgroup\$
    – Wesley Lee
    Jun 8, 2018 at 0:15

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