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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.

Null
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jakubM
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    https://electronics.stackexchange.com/questions/296879/logic-level-converter-using-transistors/297092#297092 – G36 Jun 07 '18 at 16:56

1 Answers1

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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.

Wesley Lee
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  • 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. – Jack Creasey Jun 07 '18 at 17:55
  • @JackCreasey -- thanks for pointing that out. I wrote that in a rush. I'll add it to the answer. – Wesley Lee Jun 08 '18 at 00:15